CONTROL OF HUMAN ARM MOVEMENTS IN 2 DIMENSIONS - USE OF THE WRIST IN SHORT POINTING MOVEMENTS

In order to examine how humans utilize redundant degrees of freedom in the arm, movements of the shoulder, elbow and wrist were recorded as subjects made short pointing movements in a horizontal plane. The results show that the wrist is used considerably less than the elbow, even when geometrical relationships would allow either joint to perform the movement. The wrist is used relatively more in short movements (5 to 10 cm), particularly those at right angles to the line of the hand and lower arm, and in fast movements. These changes are not predicted by cost functions which describe static postures. Repetition of movements does not greatly change the amount of wrist movement, but when use of the wrist is explicitly requested, large wrist movements can be incorporated into fast movements with no penalties in duration or peak velocity. For directions where wrist movement is larger and shoulder movement is small, peak velocity is higher and duration is shorter, indicating that subjects do not fully compensate for the anisotropy in the effective inertia of the arm to keep durations and peak velocities constant but instead take advantage of the low inertia at the wrist.