Task dependent patterns of muscle activation at the shoulder and elbow for unconstrained arm movements

1. Six subjects performed three series of pointing tasks with the unconstrained arm. Series one and two required subjects to move as fast as possible with different weights attached to the wrist. The first required flexion at both shoulder and elbow joints. The second required shoulder flexion and elbow extension. The third series required flexion at both joints and subjects were intentionally instructed to vary movement speed. These three pointing tasks were selected as the simplest progression from single to multiple degree of freedom movements in which different patterns of motoneuron excitation are required depending on whether movements are made against different loads or at different intended speeds. 2. Changes in load and changes in intended speed both produced systematic but different changes in the patterns of muscle activity and joint torque in both the elbow and shoulder muscles. These patterns are the same found during constrained, single-joint elbow flexion movements. The changes are expressed in the rates of rise, durations, and latencies of the electromyographic (EMG) bursts and in the rates of rise of torque that have specific dependencies based on the force requirements of the task. 3. A consistent, almost linear relationship is observed between muscle torque at the shoulder and at the elbow for all three tasks. Similar systematic changes were not seen in the kinematic description of joint angles. 4. The systematic relationship between torque and EMG as well as between muscle torque at the shoulder and at the elbow across the different tasks is consistent with our hypothesis that movement planning and control can best be described in terms of the force requirements of the task. This hypothesis, developed for single joint movements, may be extended to the individual joints during movements of the entire limb.