Do arm postures vary with the speed of reaching?

Do arm postures vary with the speed of reaching? J. Neuuophysiol. 81: 2582- 2586, 1999. For reaching movements in one plane, the hand has been observed to follow a similar path regardless of speed. Recent work on the control o f more complex reaching movements raises the question of whether a similar "speed invariance" also holds for the additional degrees of freedom. Theref ore we examined human arm movements involving initial and final hand locati ons distributed throughout the three-dimensional (3D) workspace of the arm. Despite this added complexity, arm kinematics (summarized by the spatial o rientation of the "plane of the arm" and the 3D curvature of the hand path) changed very little for movements performed over a wide range of speeds. I f the total force (dynamic + quasistatic) had been optimized by the control system (e.g., as in a minimization of the change in joint torques or the c hange in muscular forces), the optimal solution would change with speed; sl ow movements would reflect the minimal antigravity torques, whereas fast mo vements would be more strongly influenced by dynamic factors. The speed-inv ariant postures observed in this study are instead consistent with a hypoth esized optimization of only the dynamic forces.