Can We Redesign a Shoulder Exosuit to Enhance Comfort and Usability Without Losing Assistance?

TL;DR

This study investigates whether a redesigned soft shoulder exosuit (Soft Shoulder v2) can improve wearability and user comfort without sacrificing assistive performance. By shifting assistance from the coronal plane to the sagittal plane and implementing new anchoring to reduce torso constraint, the authors compare v2 against the original v1 in eight healthy participants using static, dynamic, and functional tasks, with EMG, kinematics, and user feedback as outcomes. Results show that while both versions provide effective support, v2 enhances forward arm positioning, increases transverse plane mobility, reduces compensatory trunk motion, and significantly improves comfort and perceived interaction, all without increasing muscle demand. These findings support targeted, user-centered refinements as a viable path toward comfortable, prolonged, soft-exosuit use in daily activities and clinical contexts.

Abstract

Reduced shoulder mobility limits upper-limb function and the performance of activities of daily living across a wide range of conditions. Wearable exosuits have shown promise in assisting arm elevation, reducing muscle effort, and supporting functional movements; however, comfort is rarely prioritized as an explicit design objective, despite it strongly affects real-life, long-term usage. This study presents a redesigned soft shoulder exosuit (Soft Shoulder v2) developed to address comfort-related limitations identified in our previous version, while preserving assistive performance. In parallel, assistance was also improved, shifting from the coronal plane to the sagittal plane to better support functionally relevant hand positioning. A controlled comparison between the previous (v1) and redesigned (v2) modules was conducted in eight healthy participants, who performed static holding, dynamic lifting, and a functional pick and place task. Muscle activity, kinematics, and user-reported outcomes were assessed. Both versions increased endurance time, reduced deltoid activation, and preserved transparency during unpowered shoulder elevation. However, the difference between them emerged most clearly during functional tasks and comfort evaluation. The redesigned module facilitated forward arm positioning and increased transverse plane mobility by up to 30 deg, without increasing muscular demand. User-reported outcomes further indicated a substantial improvement in wearability, with markedly lower perceived pressure and higher ratings in effectiveness, ease of use, and comfort compared to the previous design. Taken together, these findings show that targeted, user-centered design refinements can improve comfort and functional interaction without compromising assistive performance, advancing the development of soft exosuits suitable for prolonged and daily use.

Figures (7)

• Figure 1: System overview. a) Comparative view of the Soft Shoulder version v1 (orange hue) and the Soft Shoulder version v2 (green hue). Each actuator consists of a fabric-based inflatable air chamber and is shown both alone and worn on a user, either powered off (pact = 0 kPa) or active to its upper pressure limit (pact = 70 kPa). Force diagrams provide a qualitative visualization of the working principle: as the actuator pressure increases, the two portions spanning the armpit (anchored respectively to the arm and the torso) begin to press against each other (white arrows), generating the initial driving force for the movement (blue arrows). As inflation progresses, the actuator chamber stiffens and approaches its straight configuration, resulting in an upward motion of the arm through the attachments that secure the actuator’s mobile portion to the upper arm. The anchoring forces at the attachment locations (purple arrows) are crucial for establishing a stable interaction with the body and ensuring that the intended primary direction of motion is consistently elicited (i.e., shoulder abduction for version 1 and shoulder flexion for version 2). b) Isolated view of each component of the renovated Soft Shoulder version 2: the soft actuator, the textile harness, and the 3D-printed upper arm cuffs. C) Step by step donning of the device.
• Figure 2: Static holding performance during target movements of shoulder exosuit v1 and v2. a, Representative time-series from a single participant (S6) during sustained static holding tasks. From left to right, shoulder elevation angle, normalized sEMG activity, and MDF are shown for the primary assisted movement of each exosuit version. The first three traces refer to shoulder abduction with exosuit version 1, and the last three traces to shoulder flexion with version 2, each shown in unassisted (robot OFF) and assisted (robot ON) conditions. Horizontal reference lines in the elevation traces indicate the target angle (90°) and the stopping threshold (80°). Across both movements, assisted conditions are associated with longer endurance, reduced muscle activation, and a slower decline in MDF. b, Group level summary of endurance time, normalized sEMG activity, and MDF$\Delta$ for each exosuit version in its target movement. Both versions improved task duration and reduced activation of the primary agonist muscle, while fatigue-related metrics exhibited consistent but non-significant trends toward slower decline under assistance.
• Figure 3: Comparison of exosuit versions in static task performance. a, Boxplots show endurance time and normalized sEMG activity (%MVC) of the AD, MD and PD muscles for the three conditions (robot OFF, v1 ON, v2 ON). Both exosuit versions increased endurance time relative to baseline, with version 1 primarily reducing MD activation and version 2 producing broader reductions across deltoid heads. b, Endurance time and deltoid sEMG activity are shown for the same tested conditions. Both versions enhanced task duration, while exhibiting muscle-specific support patterns, with version 1 reducing AD activity and version 2 reducing MD activity.
• Figure 4: Dynamic lifting task performance. a, Normalized sEMG activity of the AD, MD and PD heads during dynamic lifting in three isolated movement directions (abduction, flexion, and oblique lift). Boxplots report group level data across unassisted (robot OFF) and assisted (v1 ON, v2 ON) conditions. Both exosuit versions show a general tendency toward reduced muscle activation during assisted movements, with version 2 yielding the most consistent reductions, particularly during forward flexion lifting.
• Figure 5: Mechanical transparency during isolated shoulder movements. a, Shoulder elevation ROM during abduction, flexion, and horizontal adduction movements. Grey lines show normative functional ROMs 31 for reference. During shoulder elevation (abduction and flexion), both exosuit versions worn unpowered preserved full, physiologically relevant ROM without restricting movement. During horizontal adduction, version 2 allowed a wider shoulder azimuth ROM compared to version 1. b, Boxplots show normalized sEMG activity (%MVC) of AD, MD, and PD heads for each movement direction. Muscle activation did not exceed the no robot baseline in any version, indicating preserved mechanical transparency.