Initial agonist burst is modified by perturbations preceding movement.

Experiments were conducted to investigate the role of peripheral feedback in modulating the initial agonist burst. Subjects performed visually guided step-tracking movements about the elbow. Brief (50 ms) torque pulse perturbations were randomly applied immediately prior to movement and either opposed (load) or assisted (unload) the forthcoming movement. Load perturbations caused both components of the initial agonist burst to increase in magnitude. In contrast, unloading increased the magnitude of the first component but decreased the second component. These effects were graded with the size of the perturbation, with large perturbations causing a greater change in component magnitude than small perturbations. These results support recent evidence that the initial agonist burst is comprised of individual subunits of activity which can be independently modulated by the central nervous system prior to movement.