Design and Preliminary Evaluation of a Wearable Passive Cam-Based Shoulder Exoskeleton

Abstract

\n BackgroundMechanically passive (i.e. spring-powered) exoskeletons may be a practical and affordable solution to meet a growing clinical need for continuous, home-based movement assistance. We designed, fabricated, and preliminarily evaluated the performance of a wearable, passive, cam-driven shoulder exoskeleton (WPCSE) prototype. MethodsThe novel feature of the WPCSE is a modular spring-cam-wheel module, which generates an assistive force that can be customized to compensate for any proportion of the shoulder elevation moment due to gravity. We performed a benchtop experiment to validate the mechanical output of the WPCSE against our theoretical model. We also conducted a pilot biomechanics study (four able-bodied subjects) to quantify the effect of a WPCSE prototype on muscle activity and shoulder kinematics during three one-degree-of-freedom shoulder movements. ResultsThe shoulder elevation moment produced by the spring-cam-wheel module alone closely matched the desired, theoretical moment. However, when measured from the full WPCSE prototype, the moment was lower (up to 30%) during positive shoulder elevation and higher (up to 120%) during negative shoulder elevation compared to the theoretical moment, due primarily to friction. Even so, a WPCSE prototype, compensating for about 25% of the shoulder elevation moment due to gravity, showed a trend of reducing root mean square (up to 50%) and peak (up to 53%) electromyogram magnitudes of several muscles crossing the shoulder during shoulder elevation and horizontal adduction/abduction movements. Subjects verbally reported that the WPCSE did not physically constrain them during the tested movements. ConclusionThe results provide proof-of-concept evidence that our WPCSE can potentially assist shoulder movements. The proposed WPCSE, once refined, could provide clinical and home-based rehabilitation for patients with shoulder disability.