The goals of this study were to determine the degree to which subjects could voluntarily modulate static endpoint stiffness orientation and to quantify the effects of simultaneously generated voluntary endpoint forces on this ability. Static endpoint stiffness, which characterizes the relationship between externally imposed displacements of the hand and the elastic forces generated in response, was estimated in real time during the application of planar, stochastic perturbations of endpoint position. This estimation was accomplished using a real-time parametric identification algorithm on measured force and position data. Subjects were provided with real-time visual feedback of endpoint stiffness, and their ability to modulate the orientation of maximum static stiffness was measured for different endpoint force magnitudes and directions. We found that individuals can voluntarily change stiffness orientation but that the magnitude of these changes is small, the range of available stiffness orientations decreases as endpoint force exertion increases, and endpoint force direction significantly constrains direction and magnitude of the stiffness orientations that can be achieved. Given these findings it appears unlikely that static endpoint stiffness orientation is controlled independently of force by voluntary neural mechanisms during postural tasks.
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