Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery

Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery JOSÉ MANUEL VEGA-CEBRIÁN,Department of Computer Science and Engineering, Universidad Carlos III de Madrid, Spain ELENA MÁRQUEZ SEGURA,Department of Computer Science and Engineering, Universidad Carlos III de Madrid, Spain MARÍA FERNANDA ALARCÓN,Department of Rehabilitation, University Hospital Getafe, Spain TOMÁS BONINO COVAS,ESITEF, Spain LARA CRISTÓBAL,Department of Plastic Surgery, University Hospital Getafe, Spain and Department of Medicine, Faculty of Biomedical Science and Health, Universidad Europea de Madrid, Spain ANDRÉS A. MALDONADO,Department of Plastic Surgery, University Hospital Getafe, Spain and Department of Medicine, Faculty of Biomedical Science and Health, Universidad Europea de Madrid, Spain ANA TAJADURA-JIMÉNEZ,Department of Computer Science and Engineering, Universidad Carlos III de Madrid, Spain and UCL Interaction Centre, University College London, United Kingdom Designing technologies to support long-term rehabilitation is challenging, particularly when patients’ needs, capacities, and expectations evolve over extended periods of recovery. This article reports on a three-year Research through Design project exploring how wearable technologies might scaffold rehabilitation practices for individuals recovering from peripheral nerve transfer surgery. Working with a multidisciplinary team of clinicians, therapists, engineers, design researchers, and patients, we developed a series of minimalist smartwatch-based prototypes that provide real-time multisensory movement feedback designed to integrate into everyday rehabilitation routines with minimal setup and interaction burden. We contribute: (1) a set of exploratory wearable prototypes that demonstrate how minimalist multisensory feedback can support repetition-based therapeutic exercises; (2) a detailed account of the multidisciplinary design journey through which these prototypes emerged, highlighting the situated design knowledge that was generated; and (3) a discussion of key design qualities and tensions encountered, including minimalism, open-endedness, normative therapeutic goals, long-term engagement, expectation management, and the opportunities and constraints of building on existing Authors’ Contact Information: José Manuel Vega-Cebrián, [email protected] of Computer Science and Engineering,, Universidad Carlos III de Madrid, Madrid, Spain; Elena Márquez Segura, [email protected] of Computer Science and Engineering,, Universidad Carlos III de Madrid, Madrid, Spain; María Fernanda Alarcón, [email protected] Department of Rehabilitation,, University Hospital Getafe, Getafe, Spain; Tomás Bonino Covas, [email protected], ESITEF, Madrid, Spain; Lara Cristóbal, [email protected] Department of Plastic Surgery,, University Hospital Getafe, Getafe, Spain Department of Medicine, Faculty of Biomedical Science and Health, and Universidad Europea de Madrid, Madrid, Spain; Andrés A. Maldonado, [email protected] Department of Plastic Surgery,, University Hospital Getafe, Getafe, Spain Department of Medicine, Faculty of Biomedical Science and Health, and Universidad Europea de Madrid, Madrid, Spain; Ana Tajadura-Jiménez, [email protected] of Computer Science and Engineering,, Universidad Carlos III de Madrid, Madrid, Spain UCL Interaction Centre, and University College London, London, United Kingdom. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]. ©2025 Copyright held by the owner/author(s). Publication rights licensed to ACM. Manuscript submitted to ACM Manuscript submitted to ACM 1 2 Vega-Cebrián et al. device infrastructures. These contributions extend the understanding of how wearable technologies can be designed for complex rehabilitation contexts and point toward more adaptable, situated, and ethically attuned approaches to technology-supported care. CCS Concepts: •Human-centered computing → Interaction design process and methods;Interactive systems and tools; Ubiquitous and mobile computing design and evaluation methods;Accessibility technologies;•Applied computing → Health informatics. Additional Key Words and Phrases: Research through Design, Wearables, Embodied Interaction, Health, Rehabilitation, Nerve Transfer Surgery, Disabilities, Assistive Technologies, Smartwatch, Wear OS ACM Reference Format: José Manuel Vega-Cebrián, Elena Márquez Segura, María Fernanda Alarcón, Tomás Bonino Covas, Lara Cristóbal, Andrés A. Maldonado, and Ana Tajadura-Jiménez. 2025. Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery. 1, 1 (December 2025), 50 pages. https://doi.org/10.5281/zenodo.17903256 1 Introduction Designing technologies to support long-term rehabilitation is challenging, particularly when patients’ needs, capacities, and expectations evolve over extended periods of recovery. In this paper, we focus on neurorehabilitation in particular, a therapeutic process that focuses on facilitating neurological recovery from neurological conditions, injuries, or disorders such as stroke, and peripheral nerve damage [ 24 ]. Neurorehabilitation is typically addressed in a multidisciplinary way [ 24 ], including activities such as: physical therapy to improve strength, coordination, and motor control; occupational therapy, working with strategies to regain independence in daily life; and cognitive rehabilitation, to help restore the brain and nervous system’s capacity to adapt and reorganise (neuroplasticity) to compensate for lost functions or impaired abilities. Over the last years, advances in interactive and movement-sensing technology have opened new possibilities for rehabilitation and training, addressing issues such as demotivation or disengagement. However, finding a good balance between effectiveness, attractiveness and motivation is not straightforward in neurorehabilitation in particular [8], nor in technology-supported training in general [64]. In this paper, we focus on a co-design project which had the objective of supporting people facing challenging rehabilitation treatments, specifically, those who had undergone a nerve transfer surgery in their upper limbs. The project is a collaboration between Human-Computer Interaction researchers from Universidad Carlos III de Madrid and the Peripheral Nerve Unit of Hospital Universitario de Getafe. We engaged with a multidisciplinary team, including plastic surgeons, rehabilitation doctors, physiotherapists, occupational therapists, design researchers, engineers, and patients recovering from peripheral nerve transfer surgeries. To support the rehabilitation of these patients, we co-designed and iterated technology-supported activities involving wearable devices. In the project, we followed a Research through Design (RtD) approach: a mode of inquiry that uses the design practice and the design process as primary means of generating knowledge [ 28 , 33 , 131 ]. The multidisciplinary collaboration was set out to explore to what extent could wearable devices support the rehabilitation treatments of patients who had undergone peripheral nerve transfer surgery. For this, we planned and carried out several design activities, including interviews, observation sessions, co-design workshops with patients and medical experts, design and development of exploratory prototypes, and embodied explorations and discussions of such prototypes. Inspired by previous works in RtD [ 21 , 34 , 46 , 78 ], in this paper we embrace the complexity, nuances, uncertainty and temporality that characterised this design journey: we describe and engage with the findings and challenges at each one of the stages shaped and fed the next ones, but not necessarily in a straightforward or expected way. Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 3 This paper contributes with different forms of design knowledge to the design of technology-supported rehabilitation exercises in general, and in particular in the context of neurological rehabilitation: (1) A set of minimalistic smartwatch-based prototypes providing real-time movement-based multisensory feedback to support activities for peripheral nerve transfer surgery rehabilitation and beyond. We report on three stages of their development to facilitate a discussion about their role and evolution throughout the design process. (2) A nuanced account of our three-year long multidisciplinary design journey, foregrounding the knowledge created along the way [ 78 ] and not only the final artifacts. In this way, the takeaways we present include the challenges, limitations, and loose ends [ 37 ] that emerged in the process and could inform future work in this line or in similar application domains. (3) A discussion of several considerations regarding the design qualities of the prototypes in relation to the application domain and their stakeholders, along with further reflection on the nature, challenges, opportunities and tensions of such a design process. 2 Related Work Over the years, the HCI community has researched interactive technologies supporting physical rehabilitation and neurorehabilitation, including Tangible User Interfaces (TUI) [ 12 , 52 ]—such as e-textiles [ 18 , 47 , 48 , 87 ] and wearables like smartwatches [16,22,35,45,88]—and Virtual Reality (VR) movement-based games [10,32,128]. In the Health domain, TUIs have been employed for purposes such as promoting health, facilitating diagnosis, facilitating or improving the rehabilitation process, improving everyday life or practitioner work, and providing mental and social support [ 12 ]. Furthermore, rehabilitation is the second most targeted medical field in tangible design, which has been proven useful to facilitate or improve the rehabilitation process [ 12 ]. This is not surprising, given the close relationship between tangible technology and the moving body, which is at focus in rehabilitation. For the sake of scope, in this section we focus on designs and design processes supporting neurorehabilitation, especially of conditions affecting the upper limbs that share needs and characteristics with the rehabilitation of peripheral nerve transfer surgery, such as stroke, spinal cord injury, or chronic pain. 2.1 Sensory Feedback Sensory feedback on movement is increasingly being used in the context of physical activity and physical rehabilitation to motivate, inform and guide people. For instance, real-time auditory feedback can provide additional information on movement—such as movement trajectories or qualities—, to aid in movement execution, control and sensorimotor learning [ 14 , 90 ] or to support the reacquisition of lost motor capabilities [ 120 ], such as those following strokes [ 93 , 122 ]. Sensory feedback on movement can further address the underlying psychological barriers or needs that prevent people from engaging in physical activity or rehabilitation [ 82 ]. For example, in the Go-with-the-flow project [ 95 ], movement sonification provided information about the movement angle, start or end to help people build confidence in physical activity despite chronic pain [95]. Haptic feedback represents a flexible channel for conveying information, with vibration emerging as the most frequently used modality in this space [ 4 , 11 , 13 , 40 , 69 , 96 , 97 , 102 , 107 , 124 ]. Additional forms of haptic input used to support body awareness include thermal cues [ 19 , 50 ] and pressure-based feedback [ 51 , 113 ]. Vibratory cues are often applied to prompt posture correction [ 11 , 124 ] or to deliver instructional signals during movement [ 69 , 96 , 97 ], and they can also be designed to evoke a range of bodily sensations. For example, metaphor-driven vibration patterns Manuscript submitted to ACM 4 Vega-Cebrián et al. have been shown to elicit altered perceptions related to body shape, posture, size, or weight [ 104 ], or to modulate bodily experience in ways that promote body awareness and greater engagement in physical activity for inactive populations[ 60 ]. Such metaphorical cues can further support movement quality—for instance, by creating sensations of being “pulled” upward or “pushed” downward during squat exercises, which may help guide proper execution [94]. A recent line of research has also explored the potential of altered feedback—modifying the perceived body or body movement instead of providing accurate movement information—to address psychological factors related to physical activity and rehabilitation. This approach can, for instance, create the perception of a lighter and more capable body [ 103 ], evoke sensations of being “pushed” by sound or vibrotactile feedback [ 94 ], or alter the perceived weight of a body part to influence movement execution, such as reducing gait asymmetry in chronic stroke patients [ 36 ]. Employing real-time sensory feedback to movement can be a powerful design material to better support rehabilitation processes. 2.2 Interactive Technology for Neurorehabilitation Several types of tangible and interactive technologies have been designed and researched to support physical rehabilitation processes [ 12 ]. Many examples have combined these technologies with playful or game-like features to create Exertion Games (Exergames) for rehabilitation. For instance, some projects have focused on the affordances of Virtual Reality (VR) or Mixed Reality for this purpose. Exergames have been developed to support the rehabilitation of Spinal Cord Injury [ 80 ], stroke [ 9 , 56 ], or upper Limb rehabilitation in general [ 10 , 32 , 128 ]. Chiu et al . [17] , interviewed several physical therapists after playtesting a commercial VR game to evaluate the potential of these technologies to support physical therapy, providing a positive outlook on the possibilities of such games. Some of these works have involved an interaction not only with the VR system but also with robotic devices (arms or exoskeletons) [ 32 , 128 ] or with custom wearable sensors [ 56 , 57 ]. Relatedly, there is a strand of work that has focused on employing external computer vision sensors such as the Kinect or Leap Motion to estimate the position of the limbs and use them to act upon the games or experiences [ 27 , 30 , 31 , 86 , 89 ]. These technologies free the patient from wearing anything but involve setting up a place with a specific configuration to be able to leverage the sensing that they provide. All these Exergame works have indicated advantages of employing game-related features to support rehabilitation by promoting engagement and providing further anchor points for motivation. Regarding wearable devices, there is a line of work developing, testing and exploring custom e-textiles-based sensors for joint position estimation. For example, in the works of SeamSleeve [ 87 ], E-Serging [ 48 ] and ReKnit-Care [ 47 ] the researchers investigated several possibilities for embedding conductive seams within garments for motion detection. These e-textiles sensors and actuators remove the need for wearable integrated circuits or external computer vision sensors, and are promising for physical rehabilitation applications. They allow for completely custom-fit designs, which would also allow personalisation for the wearer needs. However, they tend to require specialized equipment that could make them challenging to replicate. 2.3 Smartwatches and Mobile Devices To address the feasibility of deploying and evaluating designs resulting from research into-the-wild, there have been some works reappropriating and employing more readily-available wearable technologies, such as smartwatches or mobile phones. These devices provide an assortment of available sensors such as accelerometers, gyroscopes, magnetometers, heart rate sensors, cameras and more, along with processing capabilities to extract and analyse movement data. For example, Go-with-the-flow [ 95 ] used a mobile phone worn in a belt to provide real-time feedback for chronic pain Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 5 patients based on motion sensor data, and Tannus de Souza et al . [105] leveraged the phone camera to perform computer vision-based motion tracking to provide gamified feedback. Nowadays, most modern smartwatches are geared towards physical activity and exercise uses. For example, they track activity and rest during the day, and can detect and provide detailed data for specific types of sports and exercises, including strength training movements, although not necessarily for the purposes of rehabilitation. Given their compact form factor and their compatibility with custom fabric straps that extend their wearability, these devices can become a very appropriate platform for exploring and prototyping wearable interactions, especially because some of these devices allow for the development and installation of custom software (apps)—the main ones being those based on Wear OS by Google and watchOS by Apple. By developing for these platforms, one can leverage the already-existing hardware and operating system to focus on prototyping the interactions. For us, this is especially relevant because custom hardware tends to be more expensive to create, control, modify, maintain and distribute [ 42 ], and in several contexts where resources are scarce—financially, timeor labour-wise—there is a need to consider how to design technologies within limits [20]. Based on the capabilities of programmable smartwatches, several works have explored how to use them to support physical rehabilitation. For instance, some works [ 35 , 88 ] have investigated the potential of smartwatches for the detection and evaluation of rehabilitation movements, with promising results regarding their level of accuracy. Gomes et al . [35] , analysed 43 works employing smartwatches and leveraging their in-device accelerometers, gyroscopes and magnetometers as inputs for different detection algorithms to achieve gesture recognition with high precision. For the case of arm rehabilitation, Samyoun and Stankovic [88] focused on detecting and providing feedback for the quality factors of rehabilitation exercises, such as missed repetitions and duration. In a similar line, to support real-time movement feedback for upper limb rehabilitation, some works have taken advantage of the pairing between smartwatch and mobile phone, using the former as a sensor array and controller and the latter as the data visualizer [ 15 , 66 ], game screen [ 16 ], or as another sensor array [ 22 ]. Others have studied the effects of providing feedback on activity levels across time in rehabilitation, showing promising possibilities and results [ 26 , 109 ]. Most of these works focus on the movement detection algorithms or the effects of the feedback on the patients, both in real-time or during longer periods. However, it is worth considering that, despite their ubiquity, there are several design decisions behind commercially available smartwatches and fitness trackers that prior work has problematised, as these tend to have a normative and healthist approach to activity tracking [ 99 ]. First of all, an argument can be made regarding how these devices extend a line of thinking that emphasises an “individual responsibility for health, well-being, and self-knowledge” [ 25 ] instead of providing or cultivating a more suitable social support system. In this sense, there is an assumption that tracking more data is better [ 67 ] as it would help the wearers to get a better understanding of what they do and how to change it [ 67 ]. Additionally, many of them demonstrate a level of opacity [ 129 ] in the measures they present to the wearer, such as number of steps [ 99 , 129 ] o stress scores [ 116 ], which might not always match the experience of the wearer [ 129 ]. This becomes more problematic when those measures are reductively taken as signs of fitness and health—more steps and less stress are better—without accounting for the real context and needs of the wearer [ 99 , 116 , 129 ]. For the case of activity tracking (e.g. in sports), there might be a mismatch between the data that are captured and that which would provide a better understanding of such activity for the wearer, such as “felt, emotional, and contextual aspects” [ 38 ] or broader temporal windows surrounding the activity [ 38 ]. When involving chronic conditions, tracking technologies could support more nuanced recordings of the activities and their context [ 38 ], while also considering that what the wearer might need to do is to limit their activity levels and balance exertion with rest [ 43 – 45 ]. To illustrate this point, Homewood et al . [45] investigated how commercial fitness trackers were reappropriated and (mis)used for the purposes Manuscript submitted to ACM 6 Vega-Cebrián et al. of pacing energy levels, identifying tensions between this and the intended use, while also demonstrating that the devices could technically support these alternative goals. 2.4 Designing Tangibles for Rehabilitation In this work, we take inspiration from several works describing co-design processes and discussing design insights for tangibles and wearables to support physical rehabilitation. For instance, Magnusson et al . [63] presented design recommendations for tangible interaction projects for stroke rehabilitation, where they include considerations for holistic activities—e.g. taking into account the social context of the stroke survivor, purposeful goals, balance between rest and activity, personalization, and others. Kelliher et al . [52] described a co-design process where they created modular objects for interactive therapy at home, focusing on the affordances of their shapes and their relation to everyday activities. Similarly, Balaam et al . [8] engaged in a participatory design project where they iterated personalised interactive devices to support a long-term engagement of the patients’ rehabilitation activities. These works involved stroke survivors as participants in their co-design activities, along with health personnel such as physiotherapists and occupational therapists, and family members in one case [ 8 ]. Beyond stroke, Loewen et al . [61] involved people with upper limb disabilities to co-design gaming wearables, and highlighted how there is a broad line of research on wearables for rehabilitation and not so much for non-corrective uses, such as play. These works exemplify the importance of involving as co-designers the people who are directly affected and will use the proposed designs. Other co-design projects have only recruited and collaborated with the health personnel experts in their application domain. For instance, Madapura Nagaraj et al . [62] recruited clinicians and mindfulness experts and derived design principles for mindfulness-based embodied tangible interactions for at home rehabilitation of stroke patients, bringing forward a holistic approach based on the practice of mindfulness. Cochrane et al . [18] carried out a couple of co-design workshops with occupational therapists to develop adaptive soft switches for youth with acquired brain injury. In the work by Turmo Vidal et al . [111] , minimalist technology probes were employed as design probes for the co-design of physical training activities for children with motor challenges. These works illustrate how involving the relevant domain experts can also be useful to derive rich guidelines and insights for the initial stages of a design that can be later evaluated by the targeted patients. 3 Foundational Knowledge In this section, we present key concepts on motor learning, peripheral nerve transfer surgery, and rehabilitation to provide the context for our project. 3.1 Implicit Motor Learning Classical rehabilitation exercises usually prescribe exercises in a direct and explicit way, i.e. clearly specifying the goal of the exercise, how to do the exercise, and the expected outcome [ 53 , 65 ]. For instance, a typical exercise might be indicated as “Flex the elbow until your hand touches your shoulder. Do 3 sets of 10 repetitions.” This approach has important caveats, such as the required proprioceptive skills in the patient for them to fully engage with the movement. Further, engaging with a repetitive movement action could be perceived as boring and might undermine motivation. An alternative and effective way of achieving motor learning is implicit, i.e. with limited conscious awareness of the facts and rules underlying the learned movements and no or minimal increase in verbal knowledge of movement performance [ 53 , 65 , 81 ]. This type of learning can be achieved by employing an external focus of attention, i.e. by directing Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 7 the learner’s focus to the effects of their movements on the environment, such as to an apparatus or implement [ 81 , 125 – 127 ]. For example, the instruction of “Scratch your right shoulder with your right hand” will lead to an implicit flexion of the elbow without a direct mention of the joint involved, nor the target action. In our work, we have focused on this approach for motor learning. 3.2 Peripheral Nerve Transfer Surgery and Rehabilitation Injuries and trauma in the arms do not only entail bone fractures or skin lacerations, but also commonly nerve injuries with dire consequences. Injuries in the peripheral nerve and brachial plexus are complex and usually present considerable functional and sensory impairments and pain [ 3 ]. In Europe, it is estimated that there are 300.000 new cases per year [ 3 ]. If one of these injuries prevents a muscle from receiving nervous signals (i.e., if the muscle is denervated) and the injury is not treated soon, the muscle can become atrophied and unable to recover its complete function [ 3 ]. To treat these injuries, there are surgeries such as primary nerve repair, repair with nerve grafts, nerve transfers, free functional muscle transfers and tendon transfers [ 3 ]. All require weeks for the repaired nerves to fully regenerate and provide stimuli to their target muscle [ 3 ]. In the cases of transfer surgeries, the body is surgically “rewired” and recovering the original motor function requires relearning how to perform movements. As an example, the Oberlin ulnar nerve transfer [ 3 , 77 , 100 ] is a common peripheral nerve transfer that is used to recover the flexion of the elbow after this type of injury. Losing the capacity to flex and extend the elbow can be very limiting. This loss might happen due to a nerve injury involving the musculocutaneous nerve, which innervates the biceps brachii muscle. This peripheral nerve transfer surgery consists of taking part of the muscular branches of the ulnar nerve and using it to reinnervate the biceps [ 3 , 77 , 100 ] (Fig. 1). Once the nerves regenerate and the biceps start receiving nervous impulses, these impulses originate from the motor commands corresponding to one or a combination of the movement provided by the ulnar nerve. In this paper, we focus on the rehabilitation for this kind of peripheral nerve transfer surgery. 3.2.1 Rehabilitation Goals after Peripheral Nerve Transfer Surgery. Previous work [ 3 , 100 ] has described three fundamental goals for the rehabilitation of any peripheral nerve transfer surgery, presented in Table 1and illustrated in Figure 2. In order to achieve these goals, patient education is key: they have to understand the procedure and what it is required for them to be able to recover. Goals 2 and 3 require also a progressive involvement in strength and precision training and development. Also, for the best achievement of these goals, it is not enough to rely on clinical sessions: it is critical to continue treatment at home as that is where the most improvements can happen. Depending on the severity of the initial injury, the time that passed before the surgery, and the complexity of the surgery, the level of rehabilitation that can be achieved varies considerably between patients. Ideally, the patients will be able to activate the renervated body part, eventually recovering at least some of the lost mobility. Only a few patients would recover it fully. 3.3 Physical Rehabilitation in the Public Health System of Spain There are several contextual factors that might challenge the ideal rehabilitation path. To better understand them, here we provide further context of the Public Health System of Spain, where this project is situated. 3.3.1 General Context and Challenges. For the case of plastic surgeries in general, they are performed in the hospital that corresponds to the area where the patient lives. If post-surgery rehabilitation is needed, it would also be performed in the same area hospital. For this, and depending on the type of surgery, a surgeon might prescribe instructions for the Manuscript submitted to ACM 8 Vega-Cebrián et al. a) b) c) injured nerve: musculocutaneous donor nerve: ulnar biceps brachii muscle Fig. 1. Oberlin Nerve Transfer Surgery. a) Injury in the musculocutaneous nerve preventing the flexion of the elbow. b) Peripheral nerve transfer surgery, using muscular branches of the ulnar nerve to reinnervate the biceps. c) The biceps is now activated using signals coming from the ulnar nerve and the elbow can be flexed. Goal 1: Preservation of the Range of Motion in the Elbow Joint Goal 2: Motor Activation of the Biceps by Involving Movements of the Donor Nerve Goal 3: Re-learning of the Original Movement Patterns Fig. 2. Main rehabilitation goals after Oberlin nerve transfer surgery rehabilitation, communicating them to the corresponding rehabilitation doctor. The rehabilitation doctor evaluates the state of the patient and prescribes a specific treatment to be followed, both in the hospital and at home. Depending of the situation, the in-hospital treatment can require a combination of physiotherapy and occupational therapy. The rehabilitation doctor communicates this treatment to the therapists and assigns a duration of the sessions—which might last from 30 to 90 minutes—, and a weekly frequency—daily, three, two or once a week—, during a given period. It is worth noting that, in general, in this country, patients might need to wait for several weeks for an appointment with the rehabilitation doctor—who sees from 10 to 20 patients per day—to then have assigned their physical or occupational therapy sessions. These appointments requires to balance the load and availability of the therapists, who treat between 15 and 20 patients a day. Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 9 Goal In Oberlin surgery (1) Preservation of the range of motion in involved joints. Preservation of the range of motion of the elbow by using passive motion therapy. (2) Motor activation of the renervated body part, by involving movements of the donor nerve. Activation of the biceps by using movements of the ulnar nerve (flexion of the wrist, flexion of the fingers, adduction of the wrist.) (3) Re-learning of the original movement patterns, by dissociating them from the movements provided by the donor nerve. Learning to flex the elbow without the movements of the ulnar nerve, and vice versa. Table 1. Goals of peripheral nerve transfer surgery rehabilitation in general and in particular 3.3.2 Peripheral Nerve Transfer Surgery Challenges. In the specific case of peripheral nerve transfer surgery, because it requires a high level of specialization and as of the writing of this paper, it can only be performed in specific hospitals of Spain. One of those hospitals is Hospital Universitario de Getafe, which has its own Peripheral Nerve Unit. Such a unit consist of a medical team—comprised of plastic surgeons, rehabilitation doctors, occupational therapists and physiotherapists—who are knowledgeable of the specific requirements of peripheral nerve transfer surgery rehabilitation and can provide appropriate care for before, during and after the surgery. Not only the specialized team from the hospital will perform the surgery, but also see the patients after the surgery and teach them the fundamentals and requirements of the rehabilitation. However, given the scarcity of this specialization, this means that many times, the patients who could benefit from peripheral nerve transfer surgery have to be sent to a hospital outside of their area to have the surgery performed there. If the patients live relatively close to the hospital, they might be able to get their rehabilitation treatment in there—this depends on the resources of the hospital, because, in principle, by treating patients from outside the area they would be reducing their availability for patients in the area. If the patients live in a distant area, or if the specialized hospital is saturated, they will be rehabilitated in their own area hospitals. However, it might happen that the receiving rehabilitation doctor and corresponding therapists are not too knowledgeable of the specific requirements of peripheral nerve transfer surgery rehabilitation. In either case, follow up sessions with the plastic surgeon or specialized rehabilitation doctor might take place. After some time of treatment, and depending on the level of impairment and initial injury, patients will get discharged from the rehabilitation treatment at the hospital. This could happen even if there is no noticeable improvement, or if the recovery has not achieved the expected level. In the follow up sessions, patients might receive further exercises to continue their treatment at home. 3.4 Design Drives In our design journey, there were design drives that guided the development of our prototypes: minimalism,openendedness, and generalisability. We present an overview of these principles here to then weave them throughout the account of our design journey (Sec. 6.) In general, with our prototypes, we were interested in exploring to what extent a minimalistic design could support the kinds of rehabilitation goals in our application domain. We were looking for a simple setup that would not require much involvement from the patients to use it. We reasoned that this would accommodate the needs of the therapists and Manuscript submitted to ACM 16 Vega-Cebrián et al. Fig. 4. Circle-based visualization in the Points prototype: Moving from the first position (left, green) to the second one (right, blue), the green circle gets bigger while the blue one gets smaller. When arriving to the blue position, its counter is incremented by one. Fig. 5. Basic workings of the Angle prototype For our purposes of on-clinic calibration and at-home rehabilitation, we designed three modes for the application: Configuration, Activity, and Log. In the Configuration mode, the rehabilitation doctor or therapists can set the target positions for the end and beginning of movement, the sensitivity, and the number of target repetitions for the exercise. Additionally, this mode benefits from a mobile “companion” app, where these parameters can be adjusted without having to interface with the smartwatch. Once the configuration is done, it can be disabled. Then, in the Activity mode, the patient can start an activity: a stopwatch begins counting time and the system counts and displays the number of performed repetitions. In both the Configuration and Activity modes, the real-time feedback is provided either when reaching the target positions (in the case of sounds and haptics) or always (in the case of the visualisation.) Finally, in the Log mode, one can look at a visualisation of the logged activities along with textual displays of the corresponding statistics (Figure 6) Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 17 Configuration Activity Log Fig. 6. Screens in the Angle prototype: Configuration, Activity, and Log 6 The Design Journey In this section, we draw inspiration from the work of Oogjes and Desjardins [78] and provide an account of the design events that shaped the design journey in this Research through Design project. This perspective justifies documenting design activities with stakeholders, conceptual pivots, and emerging insights as consequential components of the knowledge process, rather than treating them as background context. In this section, we report particular encounters [ 78 ] where different stakeholders came together with a generative intent (e.g. coming up with designs, or iterate on existing designs); moments, highlighting brief events when something significant in the design process occurs that reveal “under-considered relations” [ 78 ]; relevant pauses, [ 78 ] which in our case came often from intrinsic complexities in the rehabilitation process, access to patients, and availability of our multi-disciplinary team; and transitions, [ 78 ] between events, moments, and pauses; “when tasks are taken over, materials are changed, or prototypes or samples are left behind.” [ 78 ] We articulate "knowledge externalization and contributions" [ 78 ] emerging from these transitions, like relevant design features from former design concepts that remained alive in subsequent designs. In the spirit of providing a nuanced account of the design journey [ 33 ], we include in the key takeaways of each design activity not only the successful stories but also some aspects that did not work as expected [ 34 , 46 ] or that were left unexplored [ 37 , 106 ] and could be revisited and expanded in future work. These events are narrated from the point of view of A1, A2 and A7, the design and HCI researchers of the team. We came to this work with prior experience in movement, rehabilitation and health-related contexts, which informed our sensitivity to the embodied, affective, and ethical dimensions of designing for recovery and care. This background shaped our interpretations and interactions throughout the design process, influencing how we framed challenges, engaged with participants, and recognized the nuances of rehabilitation practice. A summary of these activities is provided in Table 2. Manuscript submitted to ACM 18 Vega-Cebrián et al. 6.1 Sensitising Designers We started our journey with a stage of sensitising: our intention was learn about the lives and rehabilitation treatments of patients of peripheral nerve transfer surgery. Out intention was to get a sense of the context where custom-designed interactive technologies could support them. For this, we organised interviews and observation sessions with patients. 6.1.1 Activity: Interviews and Observation. We (A1 and A2) conducted individual semi-structured interviews with eight patients, to get a sense of their injuries, their rehabilitation timelines, and the challenges that they had faced before and after their peripheral nerve transfer surgery. Additionally, we organised observation sessions during the therapy sessions (physiotherapy and occupational therapy) of three of the patients that were treated in Hospital Universitario de Getafe. These activities were carried out during a period of eight months, as the patients were being recruited during this time. 6.1.2 Key Takeaways: Initial Challenges and Opportunities for the Design Process. Each of the interviews was an encounter [ 78 ] between the patients and us, which started to challenge our expectations of how to proceed with the project, as described below. While conducting the interviews with patients, we started to notice many logistical challenges for working with them as “a population”. The first one had to do with their availability: some months we were able to recruit two or three patients, while other months there were none. In this sense, this period was filled with pauses [ 78 ] between interviews, which provided time to reflect on what could be an appropriate approach for the upcoming co-design activities. Speaking with the patients, we started to notice that even though they had their surgery performed at Hospital Universitario de Getafe, they were not necessarily treated there, or even lived close to the city, so further access to them could be complicated. Even though they had peripheral nerve transfer surgeries, they had them in different body parts, and some of them had tendon transfers performed along the nerve transfers. For some of them, this was not their first surgery to treat their injury, and for others, several months passed after their injury before they could receive appropriate treatment. These situations altered the treatment timelines that each one of them had. Additionally, they were in different stages of rehabilitation, were already discharged or were waiting for further treatment. For those that had periods without formal rehabilitation treatment, it was not clear what they could be doing or expecting in the meanwhile. Furthermore, several of them were not aware of the specifics and implications of the peripheral nerve transfer. In contrast, some of them were very motivated in their rehabilitation and did not seem to have any problems with following their treatment. In the observation sessions at the hospital, we noticed the patients were treated warmly and in a personalised manner by their assigned physiotherapists (PT) and occupational therapists (OT). The PT provided passive motion therapy, massages, electrical muscle stimulation (EMS), and the OT assigned custom activities to help increasing range of motion, coordination and strength, using an assortment of analogue materials. Even though the therapists could have several patients at the same time, they were able to rotate between them while also making sure that they all had appropriate feedback and exercises. From all these observations, we started to realise that even though the patients had undergone peripheral nerve transfer surgery, each one of them had very different requirements rehabilitation-wise. Initially, coming from an Interaction Design perspective, we imagined that having a population with the same type of surgery would provide a strong scope for the designs, and therefore, that the sensitising stage would reveal a clear direction to follow for the co-design activities. Instead, the project started to transition [ 78 ] from that general perspective, as now it seemed Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 19 appropriate to focus the initial designs on the individual needs of the patients. In any case, something that appeared clear was that the biggest opportunity for introducing a helpful technology design was in the periods when the patients did not have frequent rehabilitation sessions: we gathered that any meaningful impact would come from supporting the rehabilitation at home, as the therapy sessions at the hospital were very helpful and appreciated already. 6.2 Ideation Begins: Personalised Designs Once we had gathered enough information from the diversity of patients and needs, we advanced to what was the next stage in our plan: co-design with patients. Specifically, our intention was to involve patients, therapists, and interaction designers to arrive at design concepts that we could later analyse and implement. We decided to focus on the specific context of each patient, assuming that there would be emerging insights that could be extrapolated for a first prototype. 6.2.1 Activity: Co-design Workshop with Patients and Therapists. We organised a co-design workshop in a room of Hospital Universitario de Getafe, involving the participation of four patients (Px1, Px6, Px7 and Px8), the rehabilitation team from the hospital working with the Peripheral Nerve Unit (A3, ) and four interaction designers (A1, A2, and three other researchers from the lab). We arranged the group in four teams, each one with one patient, one therapist or doctor, and one interaction designer, therefore instigating an encounter [ 78 ] between the different stakeholders of the project. The objective of the workshop was to arrive at design concepts of wearable technologies that would support the patients on their specific rehabilitation needs. The workshop lasted for two hours. For organising it, we had to coordinate the availability of the medical personnel, which was very limited, with that of different patients. For instance, originally we had planned for an additional team but neither the patient nor the doctor could attend. We engaged in bodystorming for ideation. First, each team defined an application context based on the experience of the patient, focusing on a situation they found challenging in their day-to-day. Bodystorming followed, first as an exploratory and divergent activity and then with the objective of creating a low-fidelity prototype that could be shared with others. This prototype was documented along with the application context in sheets that we had prepared—these sheets asked for details regarding the patient, their context, and the design proposal. In the end, the teams presented their ideas between each other. 6.2.2 Key Takeaways: Diversity of Individual Needs, Sensory Rehabilitation and Textures, At-home Usage, Reminders. The diversity of patients and a focus on their personal needs led to a diversity of wearable design concepts (Table 3.) The team of Px1 created the prototype of an inflatable glove that would help with passive motion therapy to extend the fingers while also providing haptic stimulation through different types of textures. The team of Px6 envisioned a pulley system that would be installed at home and allow him to self-administer passive motion therapy for the mobility of his shoulder. The team of Px7 designed a glove that would provide sequences of vibrotactile stimuli in the finger for sensory therapy. In the case of Px8, they focused on the hand and wrist split that he had to wear with the objective of helping to extend his fingers. Px8 felt that the splint was cumbersome to wear, so the team designed a lighter version of it, along with “smart” features that would remind him of taking it off for resting. In the following, we highlight key aspects and commonalities that the workshop and the designs provided to us. Diversity of Individual Needs. The workshop confirmed the diversity of needs and the specificity of injuries and treatments that these patients had at their different stages of rehabilitation. We noticed that each design was on a different direction and even technical field, not necessarily within the scope of our project. For instance, the designs by Manuscript submitted to ACM 20 Vega-Cebrián et al. Px. Application Context Design Description Px1 Rehabilitation of range of motion and sensitivity in fingers to enable grabbing objects without dropping them. Glove attached to an inflatable ball with different textures in the surface. It would extend the fingers, simultaneously providing haptic stimulation. Px6 Passive motion therapy of the shoulder at home, to be done individually. Splint with pulley system attached to the room, to allow for pulling with the healthy arm and lifting the other in different directions. Px7 Recovering sensitivity and reducing pain in fingers. Gloves with moving textures to stimulate the skin in phalangeal areas in sequence. Px8 Improvement of a splint to support the extension of the fingers in activities such as typing. Light splint with a more elegant aesthetic and smart reminders for usage and rest. Table 3. Summary of the design concepts from co-design workshop with patients (Ac2.) Px: Patient. and for Px6 and Px8 had a strong focus on mechanical engineering and industrial design, while the ones by Px1 and Px7 leveraged mechatronics. Sensory Rehabilitation and Textures. The designs in the workshop were completely led by the patients, and to arrive at them, they identified their main need at the moment. For a couple of them (Px1, Px7), aspects of tactile sensitivity and its rehabilitation—sensory rehabilitation—were brought up. This was probably the case because they had noticed these issues, which were not treated as much as the mechanical and functional aspect. Even though we did not pursue this line on sensory rehabilitation and we left it as a loose end [ 37 ], we would like to highlight these ideas that had an emphasis on robotic tactile stimulation and the usage of different textures. This appears to be a fruitful line for further research, especially because it is not usually prioritized as motor rehabilitation, which was the focus of their rehabilitation and our project. At-home Usage. In any case, we noted that what the designs had in common was their intended context of usage: all of them were meant for the homes or workplaces of the patients, to support rehabilitation activities that they would want (or need) to engage frequently during the day. This confirmed what we had previously discussed in the interviews, that supporting the patients at their home would be an appropriate opportunity for our designs. Reminders. Additionally, we gathered some insights regarding more low-level aspects of interaction. For instance, all the design concepts envisioned using vibrations to provide interactive feedback and reminders, as they were perceived as more personal and less intrusive for others. The reminders were envisioned either to promote activity (Px6) or to encourage resting from the splint (Px8.) We took note of this aspect but did not explore it further, given that the main design direction was still to be decided. Need for an Alternative Frame for Ideation. With this workshop, we expected to find inspiration for our design. However, we realised after the fact that, by foregrounding the patient’s experience at different stages of rehabilitation, without another common focus in the workshop, we ended up with an assortment of ideas that were very personalised to the patient’s context while simultaneously not necessarily specific to peripheral nerve transfer surgery rehabilitation in general. We concluded that the ideation session was too open and in the end it did not provide a strong foothold for Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 21 exploratory prototypes. The encounter [ 78 ] that was the workshop became a moment [ 78 ] were we realised that our approach had to change in some way. 6.3 Towards a Goal-Oriented Design From the co-design session with patients, we gathered that we would need an alternative framing for ideation. In this sense, we started looking for applicable embodied core mechanics [ 68 ] in the context of peripheral nerve transfer surgery rehabilitation: desirable and repeatable movement-based actions that could serve as the basis of our designs, in the form of core actions that the patients needed to do as part of their rehabilitation. For this, a functional perspective, such as the one that is used in physiotherapy and occupational therapy, would be helpful. In a conversation with A6—an encounter [ 78 ] between the medical and design team—, we learned that the elbow flexion is the most important movement to rehabilitate in upper limbs, in general. In this sense, given a partial loss of function in upper limbs, the first target for a peripheral nerve transfer would aim to regain the elbow flexion, using the the Oberlin nerve transfer (Sec. 3.2.) This moment [ 78 ] started another transition [ 78 ] in our process, where we would now focus on a very concrete surgery with concrete core actions. Therefore, we started to follow following previous work [ 3 , 100 ] describing rehabilitation goals for peripheral nerve transfer surgery in general, but with a focus on the Oberlin nerve transfer [ 100 ], because of the concrete actions in play there. We decided to frame further designs on the rehabilitations goals these works [3,100] describe, focusing on the three that are discussed above (Sec. 3.2.1.) By focusing on the mechanics of the rehabilitation, we reasoned that it would be possible to eventually extrapolate our findings to other types of peripheral nerve transfers. Additionally, to further support the focus on movement, we decided to involve notions of implicit motor learning (Sec. 3.1), which we reasoned could aid in developing ideas for a shared frame of reference between patients and therapists, as previous works [114] had done before. When organising the following workshop, we faced a temporal dissonance [ 78 ]: because of the needs of the embodied sketching [ 72 ] methods we planned to use, and the depth that we envisioned was needed for each of the three rehabilitation goals, we anticipated that we would need a session two to three hours long. This was at odds with the time and availability of the medical personnel of the hospital, who had already gone through great efforts to accommodate the previous two-hour workshop. This situation was resolved with a transition [ 78 ]: for the new co-design workshop, we invited external experts, specialised in the rehabilitation of peripheral nerve injuries, specifically in the upper limbs (See above, Sec. 4.3.3.), who could also provide a fresh view to the problem. This co-design workshop took place two months after the previous one. 6.3.1 Activity: Co-design Workshop with External Experts. In the workshop, we had the presence of five external experts (OT2, OT3, OT4, OT5, PT3) and A4 as participant co-designers. We divided the workshop into two main stages: sensitising and co-design. The sensitising stage introduced participants to relevant theoretical and methodological aspects to inspire their design process, establishing a common vocabulary. We introduced our embodied design methodology and the goals of the workshop. We discussed key concepts of peripheral nerve transfer surgery and its rehabilitation (Sec. 3.2), which would guide the stages of the co-design session. Then, we introduced, physically engaged with and practised relevant concepts of motor skills pedagogy, such as explicit vs. implicit motor learning, to provide some inspiration for the designs: we presented and compared instructions such as “flex your right elbow until it reaches its maximum position” (explicit) with “touch your right shoulder with your right hand” (implicit.) For the co-design stage, we divided the group in two teams, separating those who worked in the same hospital: Team A had OT2, OT3, and A4, and Team B had PT4, OT4 and OT5, each one with a different facilitator (A1 and A2.) Manuscript submitted to ACM 22 Vega-Cebrián et al. 1A: River Crossing 1B: Feedback Wristband 2A: Virtual Drums 3B: Maze Tray 2B: Kinesthetic Blob 3A: Red Riding Hood Tray 3B': Magnetic Wristband Fig. 7. Design concepts from co-design workshop with experts (Ac3) We divided this stage into three sections, each based on a different rehabilitation goal. For each section, the objective was to develop a holistic home activity using wearable technology, and to document it through a video prototype that would be presented between the teams. The focus was on an activity because the idea was to build up on the key core mechanics for the given rehabilitation goal, turning them into something engaging by means of involving different design resources [ 118 ], such as objects, technologies and people. To achieve this, we engaged in a 15 min bodystorming, followed by a 15 min convergence activity leading to one design concept, which would be (1) implemented as a low fidelity prototype, (2) video recorded, and (3) presented to the rest of the group. 6.3.2 Key Takeaways: Design Concepts, Wearability, External Focus of Attention, Multisensory Feedback, Generalisability. Design Concepts. From this goal-oriented co-design workshop with experts, we obtained a total of six design concepts, two per rehabilitation goal. See Table 4and Figure 7for a summary of the concepts. Wearability. We expected the participants to propose some of the activities based on mainstream wearable devices such as smartwatches, or straightforward form factors such as gloves. Instead, they proposed devices and objects in other shapes and locations, some of which would even challenge the definition of wearable. For instance, the participants envisioned devices worn on the upper back (Designs 1B and 3B), on the shoulders and feet (above and below) (Designs 1A, 1B, 3B) and on a belt (Design 3B), but also embedded in objects that were not worn but rather held for an amount of time: drumsticks (Design 2A), a moving blob (Design 2B) and a weighted tray (Designs 3A and 3B). All of these had in common that they would be on the patient only while the activity was performed. Therefore, they would not be permanently worn or held, challenging a perspective of always-on devices. Here we found an interesting aspect of minimalism, in that the devices would serve their purpose of supporting an activity, and not more. External Focus of Attention. We found it relevant that in most designs, the position of the wearable device or the focus of attention of the whole activity was not necessarily the body part under rehabilitation. For instance, the activity Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 23 Goal ID Design Name Description (1) Preservation of the Range of Motion in the Elbow Joint 1A River Crossing The patient simulates crossing an imaginary river while sitting on a wheeled office chair. They must keep their forearm on a table, encouraging elbow flexion as they “walk”. The setup includes an elbow flexion sensor and motion sensors in the feet to track the movement and generate splashing sound effects. 1B Feedback Wristband A wristband worn in the recovering forearm, sensing the position of the forearm relative to the arm, and wirelessly connected to devices worn in shirts or shoes. The design provides visual feedback in the wristband—changing its brightness and colours—, and haptic feedback—pleasant vibrations or massages—from the location of the other devices, such as the lower neck, shoulders or feet soles. (2) Motor Activation of the Biceps by Involving Movements of the Donor Nerve 2A Virtual Drums A virtual drum kit responding to a drumstick with sensors: it plays the drums, emits lights, and vibrates when doing a motion that corresponds closely to the one that would activate the biceps—previously identified during consultation. When the motion is far from the desired one, the drums sound distorted. The patient can also play along a song that they enjoy. 2B Kinesthetic Blob A “blob” of a malleable material with kinesthetic memory which can move and passively guide the hand along different directions. It supports identifying the movement combination that activates the biceps during consultation, and practising it at home by following its haptic feedback: it encourages or blocks movements depending on their closeness to the desired one. The blob is connected to a social app tracking the patient’s progress. (3) Re-learning of the Original Movement Patterns 3A Red Riding Hood Tray Video game based on the Red Riding Hood story, where she brings biscuits on a tray to her grandmother by traversing a path. The controller is a weighted tray that has to be tilted and moved in a specific manner, flexing and extending the wrist independently of the elbow. The tray counteracts the flexion of the wrist and fingers that naturally emerges when patients want to flex the elbows after Oberlin nerve transfer. 3B Maze Tray A tray with a maze that is solved by tilting it, providing multisensory feedback when the end of the maze is reached. 3B’ Magnetic Wristbands A wristband and a belt with a sensor in the middle of the sacrum area, plus the haptic feedback devices in shoulders, neck and insoles from Design 1B. Here, the goal is to align the wristband with the belt, therefore implicitly promoting the desired movement—extension of the elbow simultaneous to an adduction of the wrist. When successful, a pleasant massage is provided by the haptic devices. Table 4. Summary of design concepts from co-design workshop with experts (Ac3) Manuscript submitted to ACM 24 Vega-Cebrián et al. focus was on “crossing the river” (Design 1A), the virtual drums (Design 2A), the “blob” (Design 2B) or the tray or screen (Designs 3A and 3B). Furthermore, in Design 1B, the haptic feedback was provided on the back, shoulders or feet, not on the arm. This was coherent with the implicit motor learning strategy practised and bodily explored in the sensitizing part of the co-design workshop. Even if the concept was not consciously known by the participants, they found this strategy valuable and were able to reflect it in their designs. Multisensory Feedback. The design concepts presented the use of multiple sensory modalities to provide feedback: visuals (Designs 1A, 1B, 2A, 3A, 3B, 3B’), sounds (Designs 1A, 2A, 3A, 3B), and haptics (Designs 1A, 1B, 2A, 2B, 3B’). Only in the case of Design 1B, the focus was on a single modality: haptics. From this, we gathered that it would be appropriate to incorporate two or three sensory modalities in the prototypes we would develop. Generalisability. The participants took into account that the designs would need to be adapted to different ranges of motion, dimension of body parts and the specifics of each patient’s surgery. Therefore, in principle they would be flexible enough to be adapted to other application domains. 6.4 Implementing Initial Prototypes: Flex, Drums and Maze The co-design workshop with experts provided rich insights and inspiration from which we could derive a first version of exploratory prototypes. Following our design drives, we looked at how we could design a minimalistic technology that could be used to build and support holistic activities such as the ones that the participants had designed. For this, we looked into connecting the concrete core mechanics from the activities to the functionality of the technology we would design. Therefore, we focused in each of the three rehabilitation goals (Sec. 3.2.1), and designed and developed one initial prototype per each: Flex (Goal 1), Drums (Goal 2) and Maze (Goal 3.) See Sec. 5for a detailed description of the prototypes. Developing the prototypes consisted on creating a custom app for Wear OS that could be installed on commercial smartwatches. We recognised that the design concepts throughout the process were rich in potential placements and interaction capabilities, which invited to explore custom hardware designs and solutions. However, given our focus on minimalism and on Interaction Design, first we wanted to explore the affordances of already-existing hardware to support these goals. Because custom hardware tends to be more expensive to create, control, modify, maintain and distribute [ 42 ], we saw in the smartwatches an available, small, flexible, and wearable platform that could be subverted for our purposes. Developing on this platform would allow us to leverage the already-existing hardware—which could be altered through custom straps to support alternative placings of the device and different needs for different bodies [ 98 ]—, to focus on prototyping the interactions. Additionally, previous works [ 35 , 88 , 88 ] had indicated the potential that custom apps for smartwatches have for the detection and evaluation of rehabilitation movements, which became an useful precedent especially when being involved on a medical context with strong requirements of safety. A1 developed the applications with feedback from A2 and A7. Analysing the design concepts from the workshop, writing about the process, designing possible activities based on them, and developing the prototypes, took approximately four months. Once we had a first version, we organised participatory embodied sketching [ 72 ] sessions with our participants to collect their feedback and iterate the prototypes before involving the patients again. 6.4.1 Activity: Participatory Embodied Sketching. We (A1 and A2) organised and led five participatory embodied sketching sessions with different subgroups of the medical participants (Table 2): encounters [ 78 ] between the medical practitioners, designers and prototypes aimed at an initial assessment of the latter. In these sessions, for each prototype Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 25 we: (1) showed how it worked and invited the participants to use it and experiment with it; (2) asked for feedback on the prototype appropriateness to support the given rehabilitation goal; and (3) provided some time for free embodied explorations about alternative rehabilitation applications the prototype afforded. While this third step was originally planned by us—as we were considering a possible transition [ 78 ] towards a broader rehabilitation domain—, it naturally emerged from the onset of the session on the initiative of our participants. The five sessions lasted one hour each and were carried out within one month. They induced a rhythm [ 78 ], where the repetition of the same session structure and similar responses created a sense of being on a fruitful direction. 6.4.2 Key Takeaways: Data Logging, Dual Design, Sensitivity. In general, the participants confirmed that the prototypes were heading in a useful direction, and provided helpful notes regarding how to improve them according to their point of view. Here we describe key aspects that led to the next iteration of the prototypes. Multisensory Feedback. The participants commented on the different sensory modalities and stimuli provided for feedback. For the case of Flex, some (A3, OT1, PT1, PT2) concurred that the two percussive sounds worked effectively— these two samples we kept for the rest of the journey. However, in Drums, several participants (PT2, OT2, OT3, A4) mentioned the difficulty of differentiating the drum kit sounds—“How do I know which one [position] is it? Are the sounds different?” (PT2)—, and suggested using other more recognisable sounds, such as notes (OT3), numbers (A4), a motivational voice (OT3) or animal sounds (A4, OT2). More semantically-charged sounds were also suggested for Flex and Drums, such as alerts to signal when one is crossing from the safe area (PT5) or trumpets or other “success sounds” when achieving something (PT5). Given the long-term nature of the rehabilitation of peripheral nerve transfer surgery, and the challenges of supporting motivation over time [ 8 ], we were cautious about implementing these sorts of motivational affordances from the start. Nevertheless, for the next iterations of the prototypes, we decided to further curate the sound samples to increase their clarity and distinctiveness. Additionally, for further ideas, OT5 suggested the idea of sonifying when approaching (and not only reaching) the goal in Flex or Drums to build up anticipation and increase motivation. A4, PT5 and A3 commented on providing feedback as well when one is exceeding or pushing beyond that target to reward that effort. A4 and A3 saw it as a positive reinforcement for some exercises, while PT5 as some kind of warning for situations when the patient has to stay within certain limits to avoid further injury. Regarding haptic feedback, when trying out Drums, both PT6 and PT2 suggested using different vibration patterns for each one of the four positions. This is a direction that we did not pursue so far. In general, the visuals were deemed adequate: the colour feedback when reaching the target positions in Flex and Drums was clear, and the game-like animation in Maze, using basic shapes, was engaging. We observed that the real-time feedback provided by the player ball in Maze enabled a fluent coupling between the expected and performed movements. In this sense, these observations became moments [ 78 ] when we realised that the visuals were underutilised in Flex and Drums: in principle, they could provide more information—for the patient and the therapist—for the movement within target points, increasing the transparency [ 129 ] of the interaction. Additionally, we reasoned that we could use them for showing even further information regarding aspects that emerged in the sessions, such as data tracking, dual design and sensitivity. In further iterations, we followed this line of development. Data Logging. To support the achievement of the rehabilitation goals and track their progress, the participants reflected on the worthiness of implementing data logging and a visualization of such data. “What is valuable here is the possibility of logging the activities, otherwise this is the same as the [Nintendo] Wii we have” (OT1). The participants Manuscript submitted to ACM 32 Vega-Cebrián et al. Initial Point Between Points Final Point Flex Points Angle Fig. 9. Comparison of the visual feedback in Flex, Points and Angle. relation to the prescribed activity. This enabled a transition in the design focus, where we started developing real-time visualisations to aid in this regard. For instance, the circle-based visualisation in Points provided to the therapists a visual explanation of why someone might not be reaching the position they thought they were reaching, assisting the wearer in finding the target position again if needed. Then, the transition [ 78 ] towards a simplified measurement accounting for a single angle only in Angle, also brought with it a change in the visualisation, which became a literal representation of such angle. As commented by the medical personnel (OT1, PT1, PT2), this visualisation was very clear. In general, because we were working within the limits of the smartwatch screen size, we were prompted to implement visualisations that were simple and clear, aligned with the design drive of minimalism. Another relevant addition to the visual aspect of Angle, was the usage of real-time numerical indicators for different quantities according to the mode: measured angles in the setup mode, and number of repetitions and activity duration time in the activity mode (Fig. 6.) In this sense, the numerical values added an additional layer that on one hand made the design less open-ended but on the other helped to provide a common language between therapists and patients. 7.1.3 Haptic Feedback. We focused on developing the audiovisual feedback because of its potential for providing a shared frame of reference for patients and therapists. For the haptic feedback, we implemented a simple approach of Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 33 triggering a single and mild vibration when reaching the target points. In future work, the hardware in the device could be leveraged for rich vibrotactile interactions, exploring different intensities or patterns as others [ 60 , 94 , 96 , 97 , 104 ] have done. Further exploration in this line would tie back to the initial designs (Ac2) proposing sensory rehabilitation: in these rehabilitation processes (and in our designs) the focus is on the motor rehabilitation, but there is a sensorial and haptic aspect that could be further explored and supported. Work such as the haptic design toolkit by Zhou et al . [130] could provide the technical means and ideation tools to further explore that direction. Alternatively, a possible way to use haptic feedback through commonly available technologies would be by leveraging the interconnection with other devices—such as the mobile phone paired to the smartwatch—and providing the vibrotactile stimuli from their location in different body parts. This would echo a couple of design concepts (1B, 3B’) from the co-design workshop with experts, where vibrations were placed on the back, the shoulders or the feet in response to the movement of the elbow. In this regard, further work would be needed to evaluate the effects of the feedback on body parts other than the ones in rehabilitation. Here, we wish to highlight our experience with haptics when testing our Angle prototypes with Px6 and Px9, as it could inform further work in this line. Across the sessions, we noticed that the haptic feedback that we had implemented was not effective: neither of them could feel it, because of low or no sensitivity in their forearms (and the prevalence of pain, in the former case.) We did not consider that this could happen, and its detection became a moment [ 78 ] of realisation: even though we had gathered the insights on sensory and tactile rehabilitation from the initial co-design workshop (Ac2), these made us think more about the fingers having trouble feeling textures, and not about other body parts unable to feel vibrations. Therefore, we overlooked this aspect and naively implemented the vibrations without further consideration. In this sense, future co-design workshops could explore to what extent vibrotacticle feedback from the devices could be meaningful and effective, taking into account the sensorial diversity of patients such as ours. 7.2 Implications of a Minimalist Design for Rehabilitation During our design journey, we explored to what extent a minimalistic design based on smartwatches could support the kinds of rehabilitation goals in our application domain. Here we discuss insights regarding the suitability and limitations of such an approach. 7.2.1 Degrees of Minimalism. Across our design process and successive prototype iterations, the emerging feedback and needs continuously pushed us to expand our prototype’s feature set, placing sustained pressure on our intended minimal approach. However, the drive to keep these new features within limits was still present and shaped their final forms. For example, the visual feedback provided by Flex and Drums was overly minimal, offering no indication of the user’s position relative to target points, which in turn caused confusion in the participatory embodied sketching sessions (Ac4.) In the subsequent iterations of Points and Angle, we introduced visualisations that conveyed this information while preserving a simple and clear aesthetic. Similarly, our initial prototypes focused on the use of a single device. However, feedback concerning dual design and normative goals prompted us to build on the existing connection between the smartwatch and its paired mobile device. Nonetheless, instead of developing a fully-fledged companion application, we opted for developing a minimal mobile app as a proof of our dual-design concept. Interestingly, there was one aspect of our designs that ended up being reduced: the number of possible target positions for the rehabilitation exercises. Flex and Drums featured similar interactions, differing in the number of target positions (two and four respectively). Hence, they evolved into Points, where one could choose the number of target positions Manuscript submitted to ACM 34 Vega-Cebrián et al. for the rehabilitation exercise (from one to four). We reasoned that this would, in principle, provide more degrees of freedom and adaptability. However, in our preliminary testing, we observed that dealing with more alternatives was less intuitive and less generative for the experts. In this sense, a more constrained approach, working with two target points only, was more generative. Additionally, an arbitrary number of target positions would bring a considerable amount of technical complexity when implementing the data tracking, so the more minimalistic two-point setup (Flex) was maintained for the new Angle prototype. This choice allowed us to implement features that would be challenging to develop for an arbitrary number of target positions, such as repetitions counting, the linearisation and visualisation of the current position with respect to the target points, or logging of the differences between the actual and the target ranges. By limiting the number of target points to two, we were able to deepen the exploration of what could be done with them. Another aspect in which we had to balance complexity was the implementation of the dual design. Given that the activities that the therapists perform with the patients before their exercises tend to have more complexity (e.g. they might need to evaluate, measure, calibrate the situation and activities for the patients), there was a need for the technology to support these practices. This increased complexity, at least in the Configuration mode of the Angle prototype, which included certain features and controls, such as an adjustment of the sensitivity threshold and the setup of target repetition counts. These features were kept constrained and minimal, to maintain some degree of minimalism (Figure 6.) In contraste, the Activity mode of Angle remained as minimal as possible. For the case of Px9, this approach seemed to be adequate: he got slightly overwhelmed observing the configuration procedure in the Angle prototype and showed resistance to using the device because he “would not be able to use it” (Px9.). However, once we disabled the Configuration mode and introduced the system through a brief tutorial co-facilitated by the rehabilitation personnel, he resolved he would use it and was indeed able to use it without any problems. We recognise that providing a simplified patient-facing version while reserving a more complex controls for therapists introduce certain tensions, particularly in relation to power dynamics. Prior works has emphasised the value of users calibrating their own movement space [ 95 ], or to personalise their devices [ 61 ]. In contrast, our approach positioned calibration as a clinical task, enabling medical staff to define the desired movement of range without offering the patients the opportunity to further adapt their devices. While this workshop aligned with the immediate rehabilitation setting, we contend that meaningful participation, control, and self-determination are central to equitable design. In this light, we see significant value in future work that more actively explores how patients might take part in calibration and configuration processes. More broadly, we suggest considering how degrees of minimalism can be thoughtfully distributed across different stages or instances of a design to support both clinical needs and user autonomy. 7.2.2 Multiple Fits. As mentioned above, our intention was to develop minimum-viable prototypes that could support multiple rehabilitation goals in our application domain. The initial apps—Flex, Drums, Maze—, each targeted supporting specific activities grounded in the design concepts generated during the co-design workshop with experts (Ac3). Subsequent versions—Points first, followed by Angle—drew selectively from these earlier ideas to support rehabilitation activities in a more general way. Even though Angle was originally envisioned for elbow flexion and extension against gravity, its simple mechanics and interaction allowed it to be adapted to other motions. During patient testing, we observed that Angle’s minimal features enabled the prototype to support multiple rehabilitation goals and movements: for Px9, the prototype supported his active motion therapy to strengthen the biceps while helping him dissociate biceps activation from hand movement —the last goal from the nerve transfer rehabilitation process. For Px6, who no longer required passive motion therapy, but was in the early stages of generating Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 35 contractions after reinnervation, we adjusted the angle detection of the prototype to facilitate exercises focused on elbow extensions parallel to gravity. Because these movements occurred in a different plane than anticipated in the original design, detection was inherently less reliable. In practice however, both the elbow extension and the accompanying forearm rotation occurred simultaneously during the prescribed exercise, meaning that our indirect measure, although not anatomically precise, still provided a consistent and distinguishable signal for progress within that specific task context. Hence, the measured angle reflected this composite movement rather than elbow extension alone. Also, the core behaviour—providing feedback and counting repetitions when reaching two target positions—remained useful in supporting the exercise routine. 7.2.3 Potential Cheating. An interesting implication of the simple movement detection methods used in our prototypes, was that they prompted remarks about the possibility of “tricking” or “cheating” the system by doing different movements than the expected or desired ones. For instance, Px6 told us he noticed he could trigger the position detection by just rotating (supinating) the forearm without executing the full elbow extension on the table. However, his reflection “I would be tricking myself if I just did that [the incomplete exercise]” (Px6) – reveals a more complex relationship than mere deception. Some therapists (OT1, PT1, PT2) also explored how detection could be adapted or bypassed via compensatory motions in the first iteration of the prototypes. Rather than treating this behaviour purely as a “bad” or unwanted outcome, we contend that it points to deeper issues of agency, motivation, and even playfulness [ 73 ], and these in turn carry important design implications. In the domain of gamified and sensor-based rehabilitation, the slippage between target movement and detected movement is well-documented: technical systems often struggle to distinguish true compensatory motion from correct motion, and efforts to eliminate “cheating” purely via increased sensor fidelity or algorithmic detection have limitations [ 5 ].Further, in games and play research, behaviours that might appear as “cheating” (e.g. taking shortcuts, finding unintended workarounds) are something re-conceived as part of the user user experience, reflecting users’ playful explorations of the system, or actions supporting motivation, or reflecting their desire to assert control [ 6 , 29 , 55 , 58 , 73 , 121 ]. Because our prototypes employed minimalistic detection (rather than detailed biomechanical monitoring), the risk of “cheating” was intrinsic to the design. However, this is not unique to minimalist systems—commercial fitness and exergaming platforms based on rich sensing (e.g. Wii, Kinect, mobile exergames, and wearable trackers) are also routinely “cheated” or exploited due to sensing and design limitations [ 83 , 123 ]. We argue that rather than purely eliminating this risk, designers should recognise how such behaviours reveal meaningful aspects of user interaction. In our case, the fact that Px6 chose not to exploit the shortcut – because he judged it would undermine his exercise goals– suggests an alignment between personal agency and the system’s simple detection. The minimal design therefore supported an element of "fit" between patient intention and system behaviour, even if the detection was not anatomically precise. In short, the "cheat-able" nature of minimal detection should not be dismissed as a flaw alone, but can be viewed as an indicator of where user agency, system transparency, and motivational design converge. By recognising these tensions, designers of rehabilitation technologies can better integrate minimalism with meaningful user control and motivation. 7.2.4 Reappropriation of Available Technologies. We used commercial Wear OS smartwatches as a development platform because they offer an available, small, flexible, and wearable platform that could be reappropriated for rehabilitation purposes. Rather than treating them as watches, we approached them as powerful general-purpose embedded devices whose existing hardware – sensors, processor, battery, screen, wireless connectivity – could be re-directed toward therapeutic interaction. To support this reframing, we employed ready-made velcro straps that facilitated alternative placings of Manuscript submitted to ACM 36 Vega-Cebrián et al. the device. This allowed for quick adjustments to different bodies and movement capabilities [ 98 ], and also diminished the visual and fuctional association with a consumer wristwatch. Beyond convenience, the choice to work with commonly available technologies carried a more strategic intention: grounding the design in widely distributed, affordable, and easily replaceable hardware increases the plausibility of future uptake outside our study settings. Rehabilitation devices frequently struggle with limited accessibility due to cost, specialised maintenance, or availability [ 41 ]. By contrast, consumer wearables are already part of everyday technological infrastructures, which positions them as promising hosts for lightweight rehabilitation tools [85]. At the same time, we acknowledge a tension embedded in this choice: while our prototypes were intentionally minimal, the underlying system—Wear OS—is not. Even when the resulting app is simple and straightforward, patients must still engage in certain system-level interaction, such as the swipe gesture to launch apps or adjust settings. Although these interactions resemble Android and may therefore be broadly familiar [ 39 ] , they nevertheless introduce ancillary complexity that sits somewhat at odds with a minimalist design ethos. To mitigate this, we “cleaned” the device interface prior to deployment—re-arranging icons, and grouping or removing unused apps or features, reducing notifications, and establishing a streamlined sequence of steps for starting and ending each training session. We also provided a lightweight visual guide so that patients could follow an optimal, consistent workflow. However, this approach raises more fundamental questions about what it means to design minimally on top of non-minimal infrastructures. Rather than viewing this solely as a constraint, we consider it an opportunity to reflect on the creative and practical value of working with existing technological ecologies rather than treating custom hardware as the ideal. Highlighting another angle of the minimalism approach, we contend that by using Wear OS as a development platform we are leveraging the availability of devices running it as potential hosts of the app, therefore minimising or avoiding altogether the considerable expenses that creating, controlling, modifying, maintaining and distributing [ 42 ] custom hardware involves. Building on ideas of salvage computing [ 20 ], and repurposing and appropriation practices in HCI and health (e.g. [ 54 , 92 ]), we argue that using already-existing devices can itself be understood as a form of design minimalism: reducing new material production, leveraging the embedded knowledge and familiarity users already have with certain technologies, and enabling adaptations that are more feasible and sustainable over time. Finally, design research on wearable health technologies has highlighted the importance of considering wearables not only as medical instruments but also as accessories that intersect with the wearer’s bodily, social, and aesthetic preferences [ 74 ]. Our use of velcro straps and alternative placements, combined with interface simplification, reflects a modest step in this direction—seeking configurations that fit into people’s lived environments and bodily histories, rather than imposing a single “correct” way of wearing or interacting with the device. In summary, our use of Wear OS is not simply a pragmatic shortcut but part of a broader stance on minimal intervention and maximal re-use. Instead of investing in expensive, fragile, and often proprietary custom hardware pipelines—each of which entails design, manufacturing, certification, maintenance, and distribution—we leverage a pre-existing, globally available device infrastructure. This opens pathways for future research and development that can scale more readily and invites others to build upon what is already widely accessible. 7.3 Open-endedness and Normative Goals In all the design concepts from the co-design workshop with experts (Ac3), we observed the participants foregrounded the need for the activities and devices to be open-ended enough to support adaptations and variations within the same application domain, including adaptations to different ranges of motion, dimensions of body parts, and the specifics of Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 37 each patient’s surgery. The designs took into account that even with the same type of nerve transfer, there could be differences regarding the movements of the donor nerve that activate the biceps. The rehabilitation would become even more complicated when considering other injuries that the patient might have suffered too. In the co-design workshop with experts (Ac3), our objective was to design concrete activities for the specific kind of rehabilitation in our project. Still, we hoped that the resulting design insights could have potential for broader application domains, i.e. other types of nerve transfers and other rehabilitation applications. In this line, we observed that from the outset, emerging design concepts and activities organically developed exhibiting a degree of independence from the constraints of our specific application domain. Moreover, during the participatory embodied sketching sessions (Ac4), the experts promptly and intuitively proposed variations and potential extensions of the designs for other rehabilitation cases, even when explicitly instructed to momentarily set those ideas aside. This speaks to the flexibility and adaptability skills that OTs and PTs tend to have, which help them develop ad hoc treatments for their patients by reappropriating a variety of objects at their disposal. In this sense, the designs had the capacity to support these cases, confirming the design drive of open-endedness we had from the start. In our initial exploratory prototypes, a high degree of open-endedness was implemented by not directly displaying “right” or “wrong” assessments, but rather providing open audiovisual output, echoing the concept of non-judgmental interfaces [ 108 ]. For instance, the sounds played by Flex and Drums and the colours shown in their screens were chosen to be neutral, open for interpretation by the therapists and patients. Additionally, the target positions were not pre-programmed, and instead they could be determined in the moment given the current context and sensations of the patients. In Maze, a final count of avoided and collided obstacles is shown, but there is no message regarding “winning” or “losing” the game. In general, the open-endedness in the designs was related to the capacity of the devices to measure quite general movement aspects: beginning and end points, orientations, and speed/acceleration, which allowed their appropriation to support multiple other exercises. These comments on open-ended designs mirror the findings on previous works with minimalist technology probes that have been reappropriated for multiple application domains beyond rehabilitation [69,70,110–112,114,115]. 7.3.1 Data Tracking in a Medical Context. Even though the open-endedness of our designs was deemed appropriate, there were several suggestions regarding the implementation of normative goals that would contrast with it. Given the medical context and the needs of the project, we integrated most of them into our designs, while attempting to keep the spirit of open-endedness where possible. In this sense, we implemented explicitly quantitative measures—angle ranges, duration, number of repetitions—and the tracking of these data. Because of the challenges in following up with the patients, the medical doctors reasoned that recording data of the exercises done at home could provide them with a better understanding of the rehabilitation process. Similarly, the therapists reflected that for the patients, having a log of the activities they had done could be a source of motivation. This is in line with the general trend of employing motivational affordances to support challenging processes—such as physical rehabilitation—, and we noticed that it was actually reflected in tracking technologies and rehabilitation equipment that medical staff used already. Within this context, and to facilitate the communication between the different stakeholders of the project, it made sense to incorporate the quantitative measures and goals. For this, we needed to add some extra complexity to the application: we started displaying the data that was already being acquired for the interactivity in the prototypes—such as the rotation angles of the device—while also implementing a recording of other relevant variables, such as the duration of the activity and number of repetitions. The latter implied redesigning the usage flow of the app, dividing it into the three modes: Configuration, Activity, and Log. Even though all these Manuscript submitted to ACM 38 Vega-Cebrián et al. features can be considered conventional and expected within a given app to support physical activity or rehabilitation, we contend there is much potential in designs that do not implement these measures, as illustrated with our previous prototypes, Flex, Drums and Points.. In the usage of Angle by the patients, we could observe multiple overlapping purposes and uses of tracking, as discussed by Ancker et al . [2] . For instance, the tracking and its visualization was useful for sense-making [ 2 ] between the patients and us as designers and doctors: it allowed us all to see the levels of activity consistency and map it to the accounted experience. In the case of Px9, he was very consistent in doing their exercises in two sessions per day. When he did not have the device, he created his own log on paper, which he showed to the rehabilitation doctor and designers. When asked about his consistency and commitment to self-track, he said he was hopeful that by following the routine he will improve considerably. In this sense, he was using tracking for goal-checking [ 2 ], using the data to assess his progress towards the rehabilitation goal. Unfortunately, given that he did not feel he was improving, he also expressed a sense of hopelessness “I also ask myself this, why do I keep doing this if I’m not getting better” (Px9). In the case of Px6, he recognised that he was aware that we as designers and doctors would be able to analyse the logs, and that made him push himself further “so that he didn’t come with an empty log” (Px6). He did more exercises when having the prototype than when not, according to his own report. Even though he acknowledged that doing the exercises was better for his goals, it seemed that in his case the tracking was primarily for the doctor [ 2 ], who would be the recipient of the data. Similarly, another instance in which the patients considered that the tracking was for the doctor [ 2 ] was when they jokingly referred to the device as a snitch, that would reveal to the doctor the truth of their activities. Again, even though they recognised that performing the prescribed exercises would be beneficial for them, we contend that this degree of irony surfaced some tension and mismatch between their own personal goals and those of the treatment or the doctors. These comments and reflections by the patients regarding the prototype as a “snitch” echo previous work discussing the moral valence of tracking and medical data [ 2 ]: within a medical context, it is easy for the data to become a vessel for moral judgments, where those who track themselves and show in the tracking a bigger commitment could be considered “better” (by themselves, or the medical personnel) than those who do it less. Therefore, introducing a logging device within this paradigm can produce a kind of antagonistic relationship: the device would become a “judge”, or at least an auxiliary to a (moral) judge. However, research has shown that even when technologies "call out" or expose user actions, this dynamic can be playfully reframed by users and transformed into motivational resources. In their work on Playification, Márquez Segura et al . [73] observed precisely this phenomenon: during early trials of the PhySeEar system, the patients and the physiotherapist spontaneously joked about the device acting as a "judge" and a "tattletale," adopting a humorous stance toward this externalized authority. Rather than correcting or suppressing this interpretation, the authors leveraged this emergent meaning—designing interactions that embraced teasing, provocation, and playful competition. This design move is explained by Altarriba Bertran et al . [1] as “chasing play potentials”: recognizing and amplifying naturally occurring moments in which people reinterpret technological constraints or frictions as opportunities for play [ 1 ]. Seen through this lens, a device framed as a “snitch” or “tattletale” need not reinforce moral pressure; instead, such playful resignification can support agency, engagement, and motivation in rehabilitation contexts. These perspectives also open space for reconnecting playfulness with more reflective and non-judgmental approaches to logging and feedback. For example, the affective diary [ 101 ] foregrounds personal meaning-making and emotional expression rather than performance evaluation, offering a way for people to engage with their bodily data without feeling assessed or corrected. Similarly, non-judgmental interfaces [ 108 ] explores how to design systems that explicitly Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 39 avoid moralizing framings, enabling users to encounter their data without implicit expectations of compliance or discipline. Complementary, discussions on data feminism such as the ones by Gómez Ortega and Vincenzi [38] , invite designers to question whose values and interpretations shape data practices, and to cultivate forms of logging that acknowledge embodiment, context, and situated lived experience. Bringing these strands together, we suggest that future rehabilitation technologies could combine playful resignification (as seen in Playification) with non-judgmental, holistic, and context-sensitive approaches to data. Such designs may help to alleviate the moral valence [ 2 ] often attached to tracking—where data becomes a proxy for commitment or compliance—while at the same time offering richer avenues for patients to understand themselves and be understood by clinicians. By embracing both the playful and the reflective dimensions of tracking, rehabilitation technologies can move beyond the paradigm of the device as a “judge” or “snitch,” supporting instead forms of engagement that are motivating, and meaningfully aligned with patients’ lived realities. 7.3.2 Gamification and Motivational Affordances. In line with the experiences described above—where patients sometimes framed the prototype playfully as a “snitch” or external judge—some therapists suggested, after trying our prototypes in their different stages, that such dynamics could be extended through explicit gamification layers. Their proposals included adding motivational and reward-like audio and tactile cues—instilling soothing or pleasurable sensations, such as the haptic feedback in the co-design workshop with experts (Ac3)—, motivational messages, and illustrated characters. Some therapists related some of these ideas to other interactive technologies they had tried in the past in therapy sessions, such as commercially available biofeedback technologies, or reappropriated videogames. These suggestions reflect a broader trend in rehabilitation leveraging game elements to sustain motivation or engagement over taxing routines (e.g. painful, or long-term). Other therapists, however, felt that the minimalist feedback already provided by our prototypes was sufficient: OT1 reasoned that, while it might not necessarily “exciting,” it would likely be effective for most adult patients. Given that these technologies were meant to support a long-term rehabilitation process, we approached additional motivational features with caution. Designing effective motivational affordances in such settings can be notoriously challenging [ 8 ] (see below, Sec. 7.4.1), and poorly aligned gamification can unintentionally amplify feelings of judgment, obligation or failure. In keeping with aspects discussed in the previous section, where playful re-signification emerged organically, we preferred to first understand the engagement, dynamics, and motivation that might grow from the minimal design itself, rather than overlaying predefined strategies. A potential direction to explore in the future, would build on the playful activity-centric design concepts emerging in the co-design workshop with experts (Ac3), such as the crossing of an imaginary river or hitting fictional drums in space. Both are compatible and could be supported by our Angle prototype, which points to being a flexible host for future playification layers. But implementing them fully requires bespoke spatial and narrative stagings, which posed barriers in both directions of deployment: at home, environmental setup might be impractical for long-term independent use; and in clinical rehabilitation, tightly allocated session time limits therapists’ capacity to prepare such settings. Given these real-world constraints, we focused on exploring minimal feedback that minimised interactiona and environmental overhead for both patients and therapists first. 7.4 Challenges and Opportunities in Technological Interventions for Health Our design journey surfaced several challenges and opportunities—that we did not anticipate—for the insertion of technologies such as ours in long-term rehabilitation context. Manuscript submitted to ACM 40 Vega-Cebrián et al. 7.4.1 Designing for Long-term Treatments. Because the prototypes were intended to support home use and incorporated explicit data tracking, the design evolved toward a long-term deployment horizon. Designing technologies for long-term rehabilitation demands sustained engagement with users’ values, contexts, and personal motivational drivers. Balaam et al . [8] offer influential guidance for this endeavour, outlining four lessons for interactive systems supporting extended therapeutic routines: (1) Help people articulate what motivates them; (2) Balance between work, duty and fun; (3) Support motivation over time; and (4) Understand the wider social context [ 8 ]. Our design journey sought to move in this direction. We engaged patients early in a co-design workshop (Ac2) that emphasised self-articulation of therapeutic goals and motivations and understanding of the wider social context (aligned with a couple of points by Balaam et al . [8] ). Although the emerging designs did not materialize in current prototypes, they informed our broader stance: developing a minimal yet adaptable intervention layer that could integrate into everyday routines, leverage familiar device infrastructures, and support repetition-based therapy without imposing unnecessary learning or setup burden. In doing so, we aimed to create prototypes that were not only technically feasible, but also ecologically compatible with long-term rehabilitation workflows, aligning with Balaam et al . [8] ’s emphasis on context, pacing, and sustainability of motivation. However, minimalism could also risk long-term repetition fatigue: a small, fixed palette of auditory, color, or haptic cues may become monotonous, reducing motivational impact over time. Thus, therapists’ proposals —providing rich and flexible palettes that could be personalised for the interests and sources of motivation of the patients—should not be read as superficial embellishment, but as pointers to potentially interesting motivational affordances to explore in the future. In parallel, rehabilitation logging systems often capture activity events but not very relevant context, particularly disabling experiences like pain and fatigue, or particular pacing needs, or reasons for not performing a prescribed exercise. In our case, Px6 ’s experience illustrated this gap: although pain levels could be recorded on days he performed exercises, the interface provided no way to register pain’s levels on non-activity days. This is particularly relevant because pain was often the main barrier to doing the exercises. In this sense, our activity-centric logging system is only capturing the overt actions, overlooking the equally important dimensions that manifests as non-activity. This limitation aligns with critiques from self-tracking and disabilityor chronic-illness-focused research, which argue that clinically relevant data are routinely incomplete because systems lack space to express absence, context, or constraint. E.g. Gómez Ortega and Vincenzi [38] reflect on fitness tracking technologies generally missing the possibility of tracking pain or chronic conditions and their impact in physical activities. Another important design aspect to consider in the context of chronic conditions, such as our application domain is pacing so that one does not exceed their limits [ 43 , 45 ]. For instance, in the last session with Px9, he reported that he had started feeling some pain that he had not felt before. Discussing it with A3, we realised that it could have been that he was tired after pushing himself very hard to do extra amounts of repetitions. In this sense, having a non-judgemental logging interface was not enough: additional safeguards to prevent overexertion could be crucial to implement too. Singh et al . [95] reflected on the importance of providing these notifications during the activity to avoid the triggering of further pain after an enthusiastic exercise session. Designing for a longer time frame beyond a single activity, Homewood et al . [44] leveraged the constant tracking of activity data from commercial smartwatches to then reinterpret it through more a fitting and sensorially rich experience. Manuscript submitted to ACM Co-designing Minimalist Wearables to Support Physical Rehabilitation after Peripheral Nerve Transfer Surgery 41 Further refinement of the logging capabilities of the prototypes could take into account a more holistic activitycentered design design, considering that activity is both exertion and rest, and logging relevant contextual elements, like feeling fatigue, pain, or caution. Previous work can inspire in these directions, e.g. [43,44,49]. 7.4.2 Centring on the Patients. Our early explorations and co-design activities with patients recovering from nerve transfer surgery (Ac1 and Ac2) revealed a far more heterogeneous set of needs, rehabilitation trajectories, and resulting designs than we had anticipated. We initially hoped that focusing on such a specific clinical domain would lend itself to more design convergence in the patients’ proposed designs, but instead, each participant’s rehabilitation stage, goals, bodily capabilities, and daily context differed substantially. This divergence underscored the complexity of the domain and clarified why a single, highly prescriptive solution would be unlikely to serve all or many patients. As discussed in the previous subsection, these insights informed our stance toward developing a minimal yet adaptable interaction layer—one capable of fitting into different everyday routines and rehabilitation scenarios without requiring extensive setup or learning. In this sense, the co-design work shaped not a unified prototype, but an understanding of the conditions under which a generalisable tool could support long-term rehabilitation. Given the trajectory of our design process, the prototypes that followed were primarily shaped by the perspectives of medical practitioners—therapists and doctors—and not by the patients. This approach aligns with prior rehabilitation research in which early prototypes are grounded in expert knowledge to ensure clinical relevance[18,62,111]. We contend that this approach resulted in designs that support and extend the dynamics that already exist between medical practitioners and patients, where the former—based on their expert knowledge—design and prescribe activities for the latter, adapting them to the patients’ needs and taste. Our designs were meant to support these kinds of activities through the design qualities that we have discussed above—minimalism, multimodality, and a degree of open-endedness. At the same time, opportunities remain to bring patients more centrally into future design stages. Although challenging, we consider it would be very valuable to balance highly personalised designs—such as those of the participants in previous works [8,61]—with generalisability and scalability, and access to non-designer patients. 7.4.3 Differences in Expectations. For some of the patients recovering from nerve transfer surgery, especially those who faced significant impairment before the procedure, rehabilitation is often palliative. The expectations may be modest – aiming for slight improvement, stabilization, or simply avoiding deterioration. These realistic prospects are typically communicated by clinicians before and after the surgery, as discussed by the medical doctors with us. Against this backdrop, introducing our technology – intended only to support the already-existing treatment – can nonetheless reshape how patients interpret their rehabilitation trajectory. Even though our prototypes were never framed as tools that would produce additional recovery, they may have been perceived as offering an alternative or enhanced possibility of improvement. This risk is amplified by the pervasiveness of tools that emphasise individual responsibility for health, goal attainment, and normative improvement, sometimes suggesting that increased effort or disciplined tracking will produce a “better” outcome [ 25 , 99 ] This manifested in the case of Px9. Motivated by the technology, he performed more repetitions than prescribed, hoping this extra effort might accelerate his recovery. In the final sessions, however, he expressed disappointment at not perceiving any improvement, despite his increased activity. This frustration was likely intertwined with the timing of his discharge from regular hospital rehabilitation therapy. 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