Multijoint dynamics and postural stability of the human arm.

The goal of this study was to examine how the mechanical properties of the human arm are modulated during isometric force regulation tasks. Specifically, we examined whether the dynamic stability of the limb remained nearly invariant across a range of voluntarily generated endpoint forces and limb postures. Previous single joint studies have demonstrated that dynamic joint stability, as quantified via estimates of the joint damping ratio, is nearly invariant during isometric torque regulation tasks. However, the relevance of these findings to the control of multijoint posture has not been investigated previously. A similar degree of invariance at the multijoint level could suggest a fundamental property of the motor system that could be incorporated into the planning and execution of multijoint tasks. In this work, limb mechanics were quantified using estimates of dynamic endpoint stiffness, which characterizes the relationship between imposed displacements of limb posture and the forces opposing those displacements. Endpoint stiffness was estimated using a two-link robot operating in the horizontal plane at the height of each subject's glenohumeral joint. The robot was used to apply stochastic position perturbations to the arm and to measure the resulting forces. Endpoint stiffness dynamics were estimated nonparametrically and subsequently summarized using inertial, viscous and elastic parameters. We found that in the tasks studied, there was a differential modulation of endpoint elasticity and endpoint viscosity. Elasticity increased nearly linearly with increases in voluntary force generation while viscosity increased nonlinearly. This differential regulation resulted in limb dynamics that had a remarkably consistent damping ratio across all subjects and all tested conditions. These results emphasize the importance of considering the full dynamic response of a limb when investigating multijoint stability, and suggest that a minimal degree of limb stability is maintained over a wide range of force regulation tasks.