Comparative Study of Shoulder Joint Stiffness during Dynamic Movement and Static Posture

Characterizing human shoulder joint stiffness provides insight into how the shoulder contributes to arm stability under varying task demands. This study characterizes shoulder joint stiffness under two different task conditions-dynamic movement and static posture-in 3D space, and examines differences in stiffness between male and female participants. During dynamic movement trials, participants performed flexion-extension movements while wearing a parallel-actuated shoulder exoskeleton robot, whereas during static posture trials, they maintained fixed arm postures. In both conditions, the robot applied controlled position perturbations at the shoulder joint in the horizontal flexion-extension direction, and the resulting output torques were measured. Data collected from 20 participants (10 males and 10 females) showed that task condition significantly affected shoulder joint stiffness. Specifically, across three different horizontal flexion- extension arm postures, shoulder joint stiffness during dynamic movement was, on average, 23.1% lower than stiffness during static posture. Furthermore, a significant sex-based difference in shoulder joint stiffness was observed during static postures, whereas no significant difference was found during dynamic movement. These findings enhance the understanding of task-dependent modulation of shoulder joint mechanics and have important implications for quantifying altered shoulder stiffness following neurological impairments and for developing task-specific, individualized rehabilitation and assistive strategies.