cs.ROFeb 20, 2026

Design and Characterization of a Dual-DOF Soft Shoulder Exosuit with Volume-Optimized Pneumatic Actuator

Authors: Rui Chen, Domenico Chiaradia, Daniele Leonardis, Antonio Frisoli

Organizations: The authors are with the Institute of Mechanical Intelligence and Department of Excellence in Robotics & AI, Scuola Superiore Sant’Anna (SSSA), 56127 Pisa, Italy

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Abstract

Portable pneumatic systems for 2 degree-of-freedom (DOF) soft shoulder exosuits remain underexplored, and face fundamental trade-offs between torque output and dynamic response that are further compounded by the need for multiple actuators to support complex shoulder movement. This work addresses these constraints through a volume-optimized spindle-shaped angled actuator (SSAA) geometry: by reducing actuator volume by 35.7% (357mL vs. 555mL), the SSAA maintains 94.2% of output torque while achieving 35.2% faster dynamic response compared to uniform cylindrical designs. Building on the SSAA, we develop a curved abduction actuator (CAA) based on the SSAA geometry and a horizontal adduction actuator (HAA) based on the pouch motor principle, integrating both into a dual-DOF textile-based shoulder exosuit (390 g). The exosuit delivers multi-modal assistance spanning shoulder abduction, flexion, and horizontal adduction, depending on the actuation. User studies with 10 healthy participants reveal that the exosuit substantially reduces electromyographic (EMG) activity across both shoulder abduction and flexion tasks. For abduction with HAA only, the exosuit achieved up to 59% muscle activity reduction across seven muscles. For flexion, both the single-actuator configuration (HAA only) and the dual-actuator configuration (HAA,+,CAA) reduced EMG activity by up to 63.7% compared to no assistance. However, the incremental benefit of adding the CAA to existing HAA support was limited in healthy users during flexion, with statistically significant additional reductions observed only in pectoralis major. These experimental findings characterize actuator contributions in healthy users and provide design guidance for multi-DOF exosuit systems.

Figures & tables

Fig. 1: Conceptual overview of the shoulder exosuit. (a) Shoulder abduction assistance. (b) Exosuit in deflated state. (c) Actuators of the exosuit. (d) Shoulder flexion assistance.
Fig. 2: Comparison of uniform cylindrical angled actuator (UCAA) and spindle-shaped angled actuator (SSAA). (a) Illustration and dimensions of the two actuators. (b) Moment-angle relationship of the two actuators under different pressures. (c) Step response of the two actuators at 50 kPa and 90 kPa. (d) Rise time and (e) fall time comparison of the two actuators.
NomenclatureDescriptionUnitUCAASSAA
lul_{u}Upper segment lengthmm162162
lpl_{p}Lower segment lengthmm192192
w1w_{1}Upper widthmm9052
w2w_{2}Middle widthmm9090
w3w_{3}Bottom widthmm9056.5
wseal1w_{\text{seal1}}Sealing widthmm7.57.5
TABLE I: Nomenclature and Dimensions of UCAA and SSAA Actuators
Fig. 3: curved abduction actuator characterization. (a) Fabrication process of the curved abduction actuator. (b) Experimental setup for moment measurement. (c) Moment-angle relationship of the abduction actuator at various pressure levels. (d) Moment-pressure relationship of the abduction actuator. (e) Bode plot of the actuator at different pressure levels. (f) Force output stability over 2000 inflation cycles.
Fig. 4: Fabric horizontal adduction actuator characterization. (a) Fabrication and dimensions of a pouch motor. (b) Sewing two pouch motors together to create the horizontal adduction actuator. (c) Parameters of the pouch motor. (d) Experimental setup for pouch motor characterization. (e) Parameters for pouch motor modeling. (f) Validation of the force–height model against experimental measurements at various pressure levels. (g) Geometric parameters for torque–angle modeling of the horizontal adduction actuator. (h) Comparison of modeled and experimentally derived torque as a function of horizontal adduction angle.
Fig. 5: Subject testing protocol. (a) Setup for flexion experiments. (b) Communication between system and subjects. (c) Movement and actuator pressure conditions. (d) Procedure of the subject test.
IDGenderAge (years)Height (cm)Weight (kg)
1M3417874.0
2M2617682.5
3F3015950.0
4F3016050.0
5F2715658.0
6M2817071.0
TABLE II: Participant Demographics
Fig. 6: Muscle activity comparison during shoulder flexion movements under different exosuit and weight conditions. (a) Flexion movement with weight. (b-c) Polar plot of muscle activation patterns during shoulder flexion without load and with load. (d-j) Statistical results for each muscle during shoulder flexion movements under different exosuit and weight conditions. Statistical analysis: Wilcoxon signed-rank tests with Bonferroni correction (**: p<0.0167p<0.0167 ; *: 0.0167≤p<0.050.0167\leq p<0.05 ).
Fig. 7: Muscle activity comparison during shoulder abduction movements under different exosuit and weight conditions. (a) Abduction movement with weight. (b-c) Polar plot of muscle activation patterns during shoulder abduction without load and with load. (d-j) Statistical results for each muscle during shoulder abduction movements under different exosuit and weight conditions. Statistical analysis: Wilcoxon signed-rank tests (**: p<0.01p<0.01 ; *: p<0.05p<0.05 ).
Fig. 8: ROM comparision and user evaluation results. (a) Comparison of ROM with and without the exosuit. (b) Fatigue scores for flexion and abduction movements. (c) User ratings for ease of donning/doffing, freedom of motion, comfort, and assistance effectiveness.
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