Nonlinear control of movement distance at the human elbow.

The kinematic, kinetic, and electromyographic (EMG) patterns observed during fast, single-joint flexion movement have been widely studied as a paradigm for understanding voluntary movement. Several patterns have been described that depend upon the movement task (e.g., distance, speed, and load). A previous model that interpreted differences in EMG patterns in terms of pulse-height or pulse-width modulation of rectangular pulses of motoneuron pool excitation cannot explain all the EMG patterns reported in the literature. We proposed a more general version of that model, consisting of a set of four equations, which specify the parameters of the excitation pulses for a wide variety of movement tasks. Here we report experiments in which subjects performed fast elbow flexions over a range of distances from 2.8 degrees to 45 degrees. The EMG patterns that we observe are consistent with this more general model. We conclude that this model is sufficient to specify muscle excitation patterns that will launch a movement toward and stop it in the neighborhood of a target. This model operates on the basis of prior knowledge about the task rather than feedback received during the task.