Control of human arm movements in two dimensions: influence of pointer length on obstacle avoidance.

Human subjects (N = 13) made arm movements in a horizontal plane by using the shoulder, elbow, and wrist to move a pointer to a target while avoiding simple obstacles. Movements with no pointer and with pointers of three different lengths were tested so that the lever arm of the wrist could be changed and its effective redundancy altered. With increasing pointer length, movement durations decreased, wrist movement used in avoiding obstacles increased, and variability in proximal joint angles increased. Most characteristics of paths and joint movements previously described for movements with the long pointer were found to generalize to movements with shorter pointers or with no pointer-the condition used in several previous comparisons of 2- and 3-joint movements. Both joint angles and path parameters changed in accommodating to the constraints added by obstacles and by increasing pointer length, implying that criteria defined in workspace and in joint space together influence paths in workspace and joint movement, as postulated by bidirectional models. Increased wrist extension and flexion during movement, compared with static pointing, indicate that dynamic factors are also important.