Identifying an optimal anti-gravity assistance level for select functional shoulder movements: A simulation study

The level of assistance torque is one key design parameter for passive shoulder exoskeletons. High assistance levels may perturb arm movements, while low assistance may not provide functional benefits. This study aimed to use computational tools to identify an optimal anti-gravity assistance level for passive shoulder exoskeletons. We used the task space framework to perform biomechanical simulations of arm movements in OpenSim (Stanford, CA, USA). The simulated movements included shoulder elevation and lowering movements in frontal and scapular planes, as well as forward and lateral reaching movements. These movements were simulated across a range of assistance torque levels from 0% (no-assistance) to 100% of the maximum shoulder gravity torque, in increments of 10%. The optimal assistance level was identified based on analysis of hand kinematics, muscular response efficiency, and glenohumeral joint stability. As the assistance level increased from 10% to 40%, the variability of hand movements nearly doubled, and this trend continued for higher assistance levels. The total muscle effort rate was minimized at an assistance level ranging from 20% to 30%. While the stability of the glenohumeral joint was mostly maintained across assistance levels, it decreased slightly at higher assistance levels. The results of this study indicated that, for the simulated movements, an optimal assistance level lies within the range of 20-30% of the maximum gravity torque at the shoulder joint. Assistance levels above 40% could cause undesired effects such as greater variability of end-limb kinematics, reduced muscular efficiency, and compromised glenohumeral joint stability.