Circle-drawing movements at different speeds: role of inertial anisotropy

This study investigated the role of inertial anisotropy at the hand in causing distortions in movement. Subjects drew circles in the horizontal plane at four locations in the workspace at three instructed paces using elbow and shoulder movements. Specifically, we tested two hypotheses, which we would expect if the anisotropy of inertia were not completely accounted for by the CNS when generating circle-drawing movements: 1) speed will affect the circularity of figures, with faster movements associated with greater elongation into an oval shape, irrespective of workspace location for configurations with a similar angle between the forearm and upper arm. 2) The elongation of the circle at fast speeds will be in the direction of least inertia. The results showed that despite individual differences in the speed dependence of the relative motions at the elbow and the shoulder, the circularity decreased (distortion increased) with increased speed, and workspace location had no effect on circularity. We also found that the elongation of the circles at fast speeds was in a direction close to but significantly different from the direction of least inertia for three workspace locations and was in the direction of least inertia for the fourth location. We suggest that the elongation results from lack of full accounting by the CNS of the anisotropy of viscosity and inertia.