Human Arm Stiffness Changes in a Ball-Bouncing Task and its Effects on Stability and Performance

This research focuses on a specific facet of human motor control: the modification of the arm stiffness during ball-bouncing. This study was prompted after observing variations in arm stiffness during visual-motor rhythmic tasks. In previous studies, stiffness varies during the task and seems to increase before the impact between the ball and the racket. This paper questions the effect of heightened stiffness at impact from the point of view of closed-loop control. To this end, a closed-loop system that replicates human motor control during ball-bouncing from [1] is used. The simulation is modified with an enhanced impact model that permits adjustments of the arm stiffness. This paper presents a theoretical analysis of the difference created by the stiffness modification in a simplified version of the closed-loop system using a Poincaré map and the eigenvalues of Jacobian linked to it. This analysis is then compared to the results obtained on the non-simplified simulation for further verification. The main conclusion of this study is that stiffness plays a major role in reducing variability in limit cases but has a lesser impact on the overall stability of the system.