Experimental Characterization of Pressure Distribution, Bandwidth, and Range of Motion in a Hybrid Elbow Exosuit

1 Experimental Characterization of Pressure Distribution, Bandwidth, and Range of Motion in a Hybrid Elbow Exosuit Silvia Maccaferri1, Ali KhalilianMotamed Bonab2*, Lorenzo Sterzi2, Antonio Frisoli2, Cristian Secchi1and Domenico Chiaradia2 Abstract—Wearable soft exosuits are emerging as lightweight and compliant solutions for upper-limb assistance in daily life and rehabilitation. This work presents the experimental characterization of a cable-driven hybrid elbow exosuit that combines rigid and soft components to deliver assistance while preserving comfort and mobility. Characterization addressed three domains: pressure distribution under interfaces, humanin-the-loop bandwidth, and range of motion. Results show that the dual-cable architecture and compliant cuff design maintain interface pressures below reported discomfort thresholds, while the system achieves a bandwidth of 1.37 Hz, sufficient to track natural elbow dynamics in daily activities. Range of motion testing confirmed preservation of functional joint mobility. These findings demonstrate that the exosuit can provide effective assistance without compromising usability, establishing a foundation for future rehabilitation and augmentation applications. I. INTRODUCTION The growing prevalence of work-related musculoskeletal disorders, combined with demographic aging trends and escalating rehabilitation requirements, has positioned assistive wearable robotics as a critical engineering discipline with expanding technological importance [1]. Rigid exoskeletons provide precise control and high torque transfer capabilities [2], yet practical deployment is hindered by fundamental issues: joint misalignment, disrupted natural biomechanics from added inertia, and bulky designs affecting user comfort. The introduction of exosuits made of soft materials addresses these critical limitations, utilizing the inherent structural integrity of the human body to transfer forces between body segments, allowing the development of lightweight and compliant devices [3]. Within this field, elbow exosuits have emerged as particularly promising devices for assisting both impaired and healthy individuals [3]. Recent research has demonstrated the potential for elbow exosuits to reduce muscle fatigue, improve endurance, and provide meaningful assistance in activities of daily living [4], [5]. Despite these advances, key challenges remain. Existing designs often struggle to balance force transmission efficiency with user comfort, as high assistive forces can generate localized pressure at the human–robot interface. Moreover, there is still a limited systematic characterization of elbow exosuit mechanical transparency, pressure distribution, and impact on natural joint mobility. 1Department of Sciences and Methods for Engineering (DISMI), University of Modena and Reggio Emilia, Reggio Emilia, Italy. 2Mechanical Intelligence and the Department of Excellence in Robotics & AI, Scuola Superiore Sant’Anna, Pisa, Italy. ∗[email protected] The work was supported by the Next Generation EU project ECS00000017 ‘Ecosistema dell’Innovazione’ Tuscany Health Ecosystem (THE, PNRR, Spoke 9: Robotics and Automation for Health). Fig. 1: Hybrid elbow exosuit, consisting of forearm and upper arm cuffs, back interface, hand orthosis and shoulder harness. To address this gap, the present work reports the experimental characterization of a hybrid elbow exosuit that combines soft textiles with 3D-printed compliant cuffs to enhance comfort, distribute forces more evenly, and improve transmission efficiency. The exosuit was evaluated in three domains: interface pressure distribution, human-in-the-loop bandwidth, and range of motion. By quantifying these aspects, the study provides a comprehensive assessment of how the device interacts with natural human dynamics. II. METHODOLOGY A. Exosuit Design 1) Actuation Unit Design: The actuation system comprises a brushless DC motor (Cubemars AK80-9) driving a precision pulley with dual cable architecture for load distribution. Highstrength stainless steel tendons are routed through low-friction Bowden cable sheaths to provide flexion assistance. 2) Exosuit Unit Design: The exosuit consists of anatomically contoured 3D-printed compliant cuffs for the upper arm and forearm, each featuring dual anchor points to distribute assistive forces and reduce peak interface pressures. Antimigration components include a hand orthosis (AS-N-02, Reh4Mat) and shoulder harness (Master-03, Reh4Mat) that prevent cuff slippage and distribute loads to the torso. 3) Sensor Integration and Control System Framework: The system integrates multi-modal sensing through load cells 2025 I-RIM Conference October 17-19, Rome, Italy ISBN: 9788894580570 10.5281/zenodo.17629820 187 2 10-2 10-1 100101 Frequency [Hz] -5 -3 -1 0 1 3 5 Gain [dB] -3 dB cutoff BW = 1.38 Hz Model (gain) Experiment (mean) 0 kg 0.5 kg 1.0 kg 1.5 kg 2 kg 2.5 kg 3 kg 0 4 8 12 16 20 forearm anchor forearm bottom upper arm anchor upperarm bottom Shoulder flexion Shoulder abduction Elbow flexion Wrist flexion Radio-ulnar Deviation 0 53 107 160 0 47 93 140 0 53 107 160 0 40 80 120 0 20 40 60 wo/ exosuit w/ exosuit (a) (b) (c) Fig. 2: Exosuit characterization results: (a) Bode plot of the estimated transfer function demonstrating a bandwidth of 1.37 Hz, (b) Pressure distribution measured at top and bottom anchor points for forearm and upper arm, and (c) Comparison of range of motion between the two experimental conditions. (LSB205, Futek) for force monitoring and inertial measurement units (MTi3, Xsens) for kinematic estimation. The control architecture employs a collocated admittance controller with gravity compensation and intention detection in the outer loop, and velocity control in the inner loop. B. Exosuit Characterization 1) User Comfort and Range of Motion: User comfort was evaluated in five healthy participants by measuring pressure distribution at the anchor points using pressure sensors (A502, Tekscan) placed between the skin and cuffs, as interface pressure is a direct indicator of user discomfort [6]. Participants held their arm at a 90◦angle while receiving varying levels of gravity assistance ranging from 0 to 3 kg in 0.5 kg increments. This configuration maximized gravitational forces on the forearm to assess maximum pressure conditions. Range of motion assessment compared joint mobility between unassisted and exosuit-assisted conditions across multiple degrees of freedom including shoulder flexion, shoulder abduction, shoulder intra/extra rotation, elbow flexion, wrist flexion/extension, and wrist deviation. 2) Human-in-the-loop Exosuit Bandwidth: System bandwidth was assessed to ensure adequate performance for natural movements through sinusoidal trajectory tracking tasks with 0.5 kg gravity assistance. participants followed reference trajectories with amplitude covering typical elbow ranges for daily activities across frequencies from 0.05 Hz to 1.5 Hz, corresponding to velocities of 18-540 deg/s. Bandwidth was calculated using the RMS ratio of measured to desired elbow angles, with a second-order system fitted to experimental data using MATLAB System Identification Toolbox. III. RESULTS 1) User Comfort and Range of Motion: Pressure measurements revealed baseline values of approximately 5 kPa at both forearm and upper-arm anchor points without assistance. Maximum pressures occurred during 3 kg gravity assistance, remaining typically below 20 kPa, reported in previous studies as the threshold for discomfort [6]. Range of motion analysis showed slight reductions across all joints, maintaining functional mobility for daily activities. 2) Human-in-the-loop Exosuit Bandwidth: The humanexosuit system demonstrated a bandwidth of 1.37 Hz at -3 dB gain, corresponding to a peak velocity capability of 493°/s. This performance indicates adequate capability for tracking typical daily movement patterns and partial coverage of workrelated tasks, confirming the system’s suitability for natural human movement assistance within the design specifications. IV. CONCLUSIONS This work presented the characterization of a cable-driven hybrid exosuit for elbow assistance. 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