Design and optimization of the stationary lower limb gait rehabilitation exoskeleton

8 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ Design and optimization of the stationary lower limb gait rehabilitation exoskeleton Nodirbek Kimsanboev, Andijan branch of Kokand University Department of computer engineering and digital technologies, Andijan, Uzbekistan. [email protected] Javlonbek Rakhmatillaev, Andijan State Technical Institute Department of automation machine-building production, Andijan, Uzbekistan. [email protected] Umidjon Takabaev, Andijan State Technical Institute Department of information technologies, Andijan, Uzbekistan. [email protected] Zafar Juraev, Andijan State Technical Institute Department of Biomedical Engineering, Andijan, Uzbekistan. [email protected] Abstract: This paper examines the mechanical design and optimization of a stationary lower limb exoskeleton for paraplegic patients undergoing indoor gait rehabilitation. The prototype integrates frame segments, joint modules, and physical interfaces. The study first analyzes global and regional demand for exoskeletons to define patient needs, gait phases, and design criteria. A systematic methodology, based on literature reviews, guides the design and optimization process using CAD modeling and simulation software to test motion cycles and validate performance. The paper concludes with recommendations and technical guidelines for future exoskeleton development and research. Keywords: lower limb exoskeleton, mechanical design, stationary exoskeleton, rehabilitation robotics. 1. Introduction Mobility impairment is a major challenge in rehabilitation medicine. Around 2.41 billion people with various causes (not just spinal cord injury (SCI)), who lose some or all lower limb functions, need rehabilitation worldwide. Traumatic SCI affects 12.06 to 61.6 people per million in Asia, indicating a significant impact on the population [1-2]. The global market of lower limb rehabilitation exoskeleton robotics is projected to grow from $2.5 billion in 2023 to $6 billion by 2030, driven by advances in technology and increasing awareness of rehabilitation benefits. However, despite these promising market trends, significant accessibility barriers persist, with only one in ten people having access to the assistive technologies they require [3-4]. The need for advanced rehabilitation technology in Uzbekistan is especially high. Official statistics indicate that people with disabilities account for 2.1% of the population, or approximately 693,900 9 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ individuals. According to the World Health Organization's (WHO) assessment, an estimated 9 million individuals in Uzbekistan may require assistive rehabilitation services, representing about 25% of the national population [5-6]. Uzbekistan began developing lower limb exoskeleton rehabilitation robots later due to a weak foundation but has made significant progress and faces challenges in accessing advanced rehabilitation technology, mainly due to high costs and exoskeleton systems often requiring large investments that most healthcare facilities cannot afford. In recent years, the applicability of these devices has notably improved. On February 27, 2024, presidential decree of Uzbekistan No. PQ-88 was issued to improve support for socially disadvantaged people in Uzbekistan and advance the "Uzbekistan-2030" strategy. The decree expands available prosthetic-orthopedic and rehabilitation devices from 18 to 30 types. Stationary lower limb exoskeletons are a promising option for advanced rehabilitation in resource-limited settings. Compared to mobile versions, they offer benefits like lower cost, simpler design, safer controlled environments, and easier training for healthcare staff [1-2]. Locally designed and manufactured exoskeletons can contribute to addressing the rehabilitation technology gap and developing technical expertise. Such initiatives may reduce costs, encourage culturally relevant approaches, facilitate maintenance, and strengthen healthcare and engineering capacity. Local development can also provide a foundation for future advancements in exoskeleton technologies and robotics-based rehabilitation. This study aims to mechanically design and optimize a low-cost and locally manufacturable lower limb exoskeleton for paraplegic patients with SCI in Uzbekistan. This study presents a comprehensive mechanical design methodology for stationary exoskeletons, which includes requirements analysis, mechanical component design, CAD modeling, and simulation validation using advanced engineering tools. This work supports rehabilitation and serves as a model for local innovation in assistive devices. By validating locally designed exoskeletons, it promotes wider access to advanced rehabilitation technology and helps reduce global health disparities in mobility. The paper is organized as follows. The second section shows the review results and related works; the third section describes the design of the lower limb exoskeleton prototype. The fourth section presents the CAD and Simulink model of the lower limb exoskeleton. The last section explains the conclusion and future work of this research. 2. Review and related works This section presents a literature review conducted to identify and examine the primary functionalities and components of lower limb exoskeletons. 2.1. Search strategy The literature review of this study utilized the Patient/Element/Result (PER) search strategy to achieve an evidence-based classification. We designed this approach to formulate research questions that identify relevant information based on existing experience and reported outcomes. - P: Patients, including patient categories and related parameters, use lower limb exoskeletons for rehabilitation. - E: Proposed exoskeleton elements: frames, joints including DOF, plane, simulation software, and other aspects. - R - Research outcomes for lower limb exoskeletons. The search strategy was implemented in the following databases: Springer Nature, IEEE Xplore, and Google Scholar. A total of 19 articles were selected to identify the main elements of the exoskeleton. Lower limb exoskeletons reviewed in the literature, stated in Table 1, are currently under research and are not yet commercially available. Ongoing research focuses on various areas, including sensory devices, new design and materials, and control strategies. Table 1 introduces that reviewed lower limb exoskeletons incorporate multiple DOF, from 3 to 12, in the sagittal and frontal planes. 10 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ Table 1. Review of the lower limb exoskeletons These devices often use advanced CAD software like SolidWorks, Fusion 360, ADAMS, and Autodesk Inventor for precise modeling. The findings describe recent developments and current trends in lower limb exoskeleton mechanical design, noting a growing focus on customizing solutions for patient groups and rehabilitation objectives. 2.2. Requirements for patient category The proposed exoskeleton is intended for medical rehabilitation of individuals with paraplegia resulting from SCI. Individuals with paraplegia experience symptoms including loss of sensation, movement, reflexes, and other functions; mobility in the lower limbs is completely or partially lost due to spinal cord injury (SCI), while upper body movement is typically preserved. The mass of patients in this category should not exceed 60–90 kg, and their height should be between 170 and 190 cm. When using exoskeletons, the ability to walk at a minimum low speed is not required. 2.3. Degree of freedom, Range of Motion The number of DOFs and the number of powered DOFs are key exoskeleton features. The exoskeleton prototype features 6 DOFs in the sagittal plane as a multi-joint system [2-3]. The hip joint links the pelvis and femur, permitting flexion/extension rotation [11] with an extension of at least 30° and flexion up to 140°. The knee allows flexion to 125° and hyperextension to 5°, while ankle flexion/extension rotation [7,17] is actuated within a 20° range of motion. Joint velocity, torque, and power requirements are defined based on gait data from a healthy individual walking at 1.0 m/s. The hip, knee, and ankle are the three joints of the lower limb. Every joint has the freedom to move in its designated DOF in the sagittal plane. The design of leg exoskeletons can differ based on implementing actuators in these joints [18]. 2.4. Types of exoskeletons Multiple gait rehabilitation platforms incorporate lower-limb exoskeletons to regain users’ walking movements. This study evaluates the primary functions of lower-limb exoskeletons employed in gait rehabilitation, with emphasis on bodyweightsupported, treadmill-based, and stationary (fixed) subsystems intended for indoor environments (Fig. 1). Fig. 1. The primary features of indoor gait rehabilitation system The proposed prototype of the exoskeleton is an immobile lower limb device that provides body weight support and includes a fixed or stationary component for bilateral use. 11 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ 3. Methodology for mechanical design of proposed prototype Gait rehabilitation devices are designed to improve users’ well-being, drawing on mechanical, biomedical, and electronic engineering. The design process in these disciplines uses key questions at each stage to guide prototype development [19], and the development of next-generation exoskeletons requires the adoption of innovative hardware paradigms. The mechanical blocks of lower limb exoskeletons are modular components like frame segments, joint modules, actuator assemblies, and transmission elements. Each part has a distinct function, and modularity allows for easy customization, maintenance, and upgrades without redesign. Modular mechanical blocks improve manufacturing efficiency, reduce costs, and support scalability for wider clinical and industrial use [19]. Mechanical design features are categorized as either user-centered or device-centered. Our prototype was developed entirely around device-centered features, shown in Fig. 2. The device-centered features are grouped into three main categories: structural and frame design, joint module design, and physical interfaces, and these elements are interconnected and work together for optimal performance. This device-centered method for the mechanical design of the proposed lower limb exoskeleton emphasizes the importance of starting with a robust design before moving on to real-world deployment, along with control strategies that ensure an effective, safe, and user-oriented exoskeleton system. Fig. 2. Device-Centered Features 3.1. Frame design The 6 frames—main, support, handle, links, foot, and control frames—of the proposed stationary lower limb exoskeleton represent the core structural backbone and provide the foundation for all other mechanical blocks. Their design is important because it guarantees overall system strength, rigidity, and ergonomics while maintaining wearability and user comfort. Material selection for frames is a balance between achieving sufficient mechanical strength to withstand operational loads and minimizing weight to reduce user fatigue. Common materials include aluminum alloys, carbon fiber composites, and advanced polymers, each offering specific benefits relating to rigidity, weight, and manufacturability. 3.1.1. Mainframe The mainframe of the developed exoskeleton is the primary structural component that forms the backbone of the device. The exoskeleton has a rigid tubular or rectangular metal mainframe that ensures stability and safety during the training process. The mechanical design for mainframe of the prototype exoskeleton It is responsible for supporting all other mechanical, electronic, and functional parts of the exoskeleton, providing necessary rigidity and strength to withstand user weight, applied forces, and dynamic loads during operation, acting as the mounting base for joints, actuators, and control systems, and ensuring stability, safety, and alignment of the exoskeleton with the user’s body. The mainframe’s design also allows for ergonomic fitting and mobility, crucial for assisting or augmenting human movement. 3.1.2. Support frame The support frame of the developed exoskeleton constitutes a foundational biomechanical interface between the user’s body (shoulder, lumbar, chest, and knee) and the exoskeletal structure. The support frame redistributes body weight to reduce stress on lower limb joints and muscle effort, helping users with limited strength or mobility maintain posture and move safely. This is essential for precise alignment of joint axes and transmission of mechanical power from actuators to limb segments. By optimizing 12 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ load distribution, it increases comfort and lowers the risk of soft tissue injuries and pressure ulcers. The frame uses advanced alloys or composites that balance rigidity, lightness, and biocompatibility for reliable performance and user safety. Support frames consist of shoulder support, lumbar support, chest support, body-weight support, arm support, knee support, and tubular stands for support elements. Adjustable shoulder, lumbar, and chest supports attach to the vertical uprights for precise positioning to accommodate different patient sizes and anatomies. 3.1.3. Links The primary two links (separately on two sides: right and left) in the developed exoskeleton are rigid structural elements that function as artificial limb segments, mechanically connecting the joints and actuators within the device. Links serve as the fundamental transmission elements, conveying forces and moments generated by actuators and joints to the corresponding segments of the user’s limbs. By interconnecting rotational and translational joints, links create a kinematic chain that emulates the natural biomechanics of human motion, enabling controlled, anatomically accurate articulation of the exoskeletal system. Additionally, these two rigid links form the left and right segments of the exoskeleton’s lower limb structure, providing mechanical connection and support for bilateral movement assistance. 3.1.4. Handle frame The handle frame of the developed exoskeleton refers to the subsystem comprised of elongated, rigid elements designed for manual user interaction and stabilization. Handles are mounted bilaterally on each side of the exoskeleton frame, serving as ergonomic hand grips for the user. Their primary functions include stabilizing the upper body, facilitating postural control, and aiding in safe body weight transfers throughout rehabilitation exercises. By providing a secure gripping interface, the handle frame significantly enhances user safety and comfort during assisted movements. In advanced models, handles may be equipped with integrated physiological monitoring devices—such as a pulse oximeter sensor—permitting real-time assessment of user vital signs (e.g., blood oxygen saturation and pulse rate) during rehabilitation. This additional functionality enables continuous health monitoring and data collection, further optimizing adaptive therapy protocols and ensuring patient safety within rehabilitative environments. 3.1.5. Foot frame The foot frame of the developed exoskeleton comprises an anatomically contoured footplate that accommodates and stabilizes the patient’s feet during assisted movement and rehabilitation. Integrated adjustable foot straps are utilized to fasten the feet securely to the plates, ensuring immobilization and minimizing unwanted relative motion between the user and the exoskeletal device. This configuration enhances kinetic coupling, optimizes load transfer, and maintains precise alignment of the feet for effective gait rehabilitation. The combined use of plates and fastening straps is essential for user comfort, safety, and biomechanical efficiency during exoskeleton operation. 3.1.6. Control frame The control frame of the developed exoskeleton is the integrated subsystem responsible for real-time command, monitoring, and adaptation of device functions. This assembly includes a monitor, actuators, sensors, and a control panel, which collectively form the core of the exoskeleton’s intelligent interface. This part uses smart software and hardware to improve therapy results, customize help with movement, and record how well the system is working, making it the essential technology for smart rehabilitation and mobility support. 3.2. Joints modules design Joints in lower limb exoskeletons serve as critical articulating points that replicate or assist the natural motion of the human hip, knee, and ankle in Fig. 3. Their mechanical characterization often divides them into rigid and compliant types, each offering distinct advantages depending on use cases. Rigid joints [20] generally allow fixed degrees of freedom, closely mimicking human anatomical constraints, and providing precise motion control during rehabilitation or assistive walking. 13 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ Fig. 3. Joints of prototype for lower-limb exoskeletons Compliant joints incorporate elasticity or variable stiffness components to absorb shocks, accommodate misalignments, and enhance safety by reducing transmitted forces during unexpected impacts [17]. The hip joint, anatomically a multi-DOF ball-andsocket joint, is often simplified to rotational DOFs about flexion/extension and abduction/adduction axes in exoskeleton designs. The knee typically incorporates a hinge joint permitting primarily flexion and extension with very limited rotational DOFs. The ankle joint is more complex, but it is frequently modeled with two primary DOFs enabling dorsiflexion/plantarflexion and inversion/eversion motions [10]. An alternative joint proposal involves introducing kinematic redundancy by increasing degrees of freedom with additional revolute (rotational) or prismatic (linear displacement) joints. Each type provides one degree of freedom. These joints enable kinematic redundancy at various levels [19]. Fig. 4 shows that each joint is an active joint (motorized/controlled), and each provides one DOF, allowing rotation in a plane. The three joints represent the hip, knee, and ankle, with angles labeled as  1,  2, and  3. Segments between the joints (l1, l2, l3) correspond to the thigh, shank, and foot. The dashed lines indicate the orientation of each limb segment as determined by the joint angles. Table 2. Gait data for joints Table 2 presents gait data for lower limb exoskeleton joints—the hip, knee, and ankle— including joint angles (ℎ𝑖𝑝:  1, 𝑘𝑛𝑒𝑒:  2− 𝜋/ 2, 𝑎𝑛𝑘𝑙𝑒:  3), all of which are active [20] and require a power supply to be moved, revolute joints. The table lists each joint's torque (  1,  2,  3), mechanical power (𝑤1, 𝑤2, 𝑤3 calculated as Power = Torque × Angular Velocity), and speed (𝑣1, 𝑣2, 𝑣3 𝑖𝑛 𝑑𝑒𝑔𝑟𝑒𝑒𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑). Fig. 4. Alignment of the joints These variables are crucial for analyzing joint movement and actuation, designing efficient exoskeletons, and monitoring joint force, speed, and power during walking. The structure and joint design have been addressed, leaving one final component to be considered within the mechanical design of the lowerlimb exoskeleton. 3.3. Physical Interface The final device-centered aspect pertaining to the lower-limb exoskeleton's mechanical design is the physical interface. These interfaces serve three primary functions: transmitting forces between the user and the robotic system, ensuring precise alignment of the exoskeleton with the body, and facilitating comfortable physical interaction [8, 19, 22]. These elements transmit energy from the device to the user, acting as their connection. Physical interfaces [20-21] are important because they complete the energy transfer process. The physical interface components of the proposed prototype that connect with the user’s body are as follows: 14 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ - Cushioned pads or braces are positioned at the waist, thighs, and lower legs to ensure comfortable contact and enhance support. The device includes adjustable shoulders, lumbar, and chest supports that can be modified in height and depth to fit the patient’s body dimensions. These supports help maintain the patient in an upright position. - Horizontal rods and mechanical linkages designed for effective force transmission and controlled joint movement. - The device features multiple adjustable elements to facilitate accurate placement and alignment of the device with the user’s anatomy. In summary, the physical interface must support, align, and comfortably interact with the user to ensure effective and safe use of lower-limb exoskeletons or rehabilitation devices. 4. CAD design and Simulation 4.1. CAD design The proposed stationary lower limb rehabilitation exoskeleton was designed using CAD (Computer-Aided Design) in SolidWorks, as illustrated in Figure 5. The patient stands in an upright position within the device, held by the adjustable vertical body support system in height and depth. Fig. 5. CAD design of the proposed exoskeleton The vertical support system includes shoulder, lumbar, knee, and chest support; additionally, body weight is mounted on the frame at the level of the patient’s waist, shoulders, and chest. To position the patient upright in the exoskeleton, the shoulder, lumbar, and chest supports are moved on the tubular stands and fixed at a chosen height and depth corresponding to the patient’s size and build. The patient’s feet, placed on the footplate, are fastened by foot straps to prevent involuntary leg movements in certain conditions, and wrists are fixed by straps for the same reason. Each exoskeleton lever contains hip, knee, and ankle actuators with rotation sensors. The actuators’ rotation sensors gather information about the number of steps and distance traveled. This information is saved in a database and displayed on the monitor. The actuators are controlled by a specific program recorded in the control panel. This program creates stepping movements by moving the exoskeleton levers. Knee support is provided for patient safety. For continuous visual monitoring of the patient’s condition, a pulse oximeter sensor is installed on the handle. During lower limb movement, signals from all sensors are sent to the computer control system, which, according to the control program, monitors the patient’s physiological state and assesses rehabilitation activities. Particularly, sensors on the upper and lower legs and feet collect information about the patient’s physiological state and transmit it to the control panel. This system operates according to a specific program that generates stepping movements and allows adaptation of training to the patient’s individual needs. 4.2. Simulation model The bilateral lower limb exoskeleton is designed using Simscape Multibody (MATLAB), featuring a symmetrical structure with comprehensive joint modeling for rehabilitation and assistive applications. This model demonstrates sophisticated biomechanical modeling principles aligned with current exoskeleton development practices in Fig. 6. The Simulink model uses a tree-based kinematic structure, with the World Frame acting as the absolute reference coordinate system. Each leg implements an identical 3-DOF serial kinematic chain: - Right Leg Chain: World → Main_Frame → R_Hip_Joint_Revolute → R_Upper_Link → R_Knee_Joint_Revolute → R_Lower_Link → R_Ankle_Joint_Revolute → R_Foot_Plate 15 “Al-Farg‘oniy avlodlari” elektron ilmiy jurnali ISSN 2181-4252. Tom: 1 | Son: 4 | 2025-yil "Descendants of Al-Farghani" electronic scientific journal. ISSN 2181-4252. Vol: 1 | Iss: 4 | 2025 year Электронный научный журнал "Потомки АльФаргани" ISSN 2181-4252. Том: 1 | Выпуск: 4 | 2025 год https://al-fargoniy.uz/ - Left Leg Chain: World → Main_Frame → L_Hip_Joint_Revolute → L_Upper_Link → L_Knee_Joint_Revolute → L_Lower_Link → L_Ankle_Joint_Revolute → L_Foot_Plate This Simulink model provides 6 total DOF (3 per leg) in the sagittal plane for gait rehabilitation applications. This simulation represents a wellexecuted implementation of lower limb exoskeleton modeling and provides the next steps: advanced control algorithm development [23], biomechanical validation, and clinical application research. Fig. 6. Simulink model of proposed lower limb exoskeleton in Simscape Multibody The simulation results demonstrate joint motion patterns that closely align with normal human gait characteristics, and the trajectory data reveals proper inter-joint coordination and physiologically appropriate motion ranges for all three joints. Fig. 7(a) corresponds to the hip joint flexion and extension with characteristics matching normal gait patterns; the range of motion is ±1.5 to ±2.5 units (approximately 30-45° total range). The trajectory amplitude is the largest. Fig. 7(b) represents knee joint flexion and extension with distinctive biphasic characteristics; the range of motion is ±0.5 to ±1.0 units (approximately 15-30° total range). The trajectory amplitude is medium. The smallest amplitude trajectory is shown in Fig. 7(c), and it corresponds to ankle dorsiflexion and plantarflexion with subtle but critical motion; the range of motion is ±0.2 to ±0.5 units (approximately 5-15° total range). Fig. 7. Position of left leg element 5. Conclusions and future work This paper reviews and designs stationary lower limb exoskeletons for gait rehabilitation, especially for paraplegic patients with SCI. The proposed prototype has distinct mechanical features that set it apart from previously existing categories of exoskeletons: - The system provides full upright support and stabilization features and elements to ensure perfect safety. - Bodyweight-supported system improvement. - The system ensures the availability of settings for individual patient parameters and needs. - The system can fully automate and control it in real time. The prototype will be developed for experimental testing to assess the interaction between the exoskeleton and its user, as well as to evaluate the control system’s performance. Future work will include comprehensive forward dynamics analysis with applied joint torques, inverse dynamics computations to determine required actuator forces, kinematic analysis of foot trajectories along with workspace assessment, and stability analysis to enhance balance and prevent falls. 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