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° to 45°. 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.