The generation of the efferent command and the importance of joint compliance in fast elbow movements.

We describe the effects of changing an elastic or inertial load on the trajectory and EMG patterns during fast, voluntary elbow flexions. These effects depend upon whether the load is known to the subject in advance of the movement or is only determined as the movement is performed. The EMG patterns change in a well-defined manner when the load is known in advance. When the load change is unexpected, the change in the EMG patterns is smaller and later and the effects on the trajectory are greater. In neither case did the loads used here have large effects. We show that joint compliant properties are responsible for minimizing the effects of external load changes on the trajectory. We conclude that there is no evidence for a large contribution by length-sensitive stretch reflexes to this process.