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 unco nstrained arm. Series one and two required subjects to move as fast as possible with different weights attached to the wrist. The first requ ired flexion at both shoulder and elbow joints. The second required sh oulder flexion and elbow extension. The third series required flexion at both joints and subjects were intentionally instructed to vary move ment speed. These three pointing tasks were selected as the simplest p rogression from single to multiple degree of freedom movements in whic h different patterns of motoneuron excitation are required depending o n whether movements are made against different loads or at different i ntended speeds. 2. Changes in load and changes in intended speed both produced systematic but different changes in the patterns of muscle ac tivity and joint torque in both the elbow and shoulder muscles. These patterns are the same found during constrained, single-joint elbow fle xion movements. The changes are expressed in the rates of rise, durati ons, and latencies of the electromyographic (EMG) bursts and in the ra tes of rise of torque that have specific dependencies based on the for ce requirements of the task. 3. A consistent, almost linear relationsh ip is observed between muscle torque at the shoulder and at the elbow for all three tasks. Similar systematic changes were not seen in the k inematic description of joint angles. 4. The systematic relationship b etween torque and EMC as well as between muscle torque at the shoulder and at the elbow across the different tasks is consistent with our hy pothesis that movement planning and control can best be described in t erms 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.