These experiments were undertaken to evaluate whether the kinematics and end-point control of learned movements were affected by changes in dynamic loads or were determined largely by centrally specified motor programs. Human subjects performed flexion movements about the wrist in a discrete visual tracking task for a range of movement sizes. For some movements, viscosity was increased at movement onset. When the viscous load opposed movement unexpectedly, subjects initially overshot the intended target for all movement sizes, but only for the smaller movements did the overshoot persist. Unexpected introduction of heavier loads was more effective in inducing these behavioral changes; the lightest loads did not alter end-point positioning. When subjects had visual guidance about performance when load changes occurred, the effect of the unexpected occurrences of viscous loads was diminished, suggesting that subjects rapidly adjusted their movement strategy, depending on task demands and performance. The movement responses were mediated by short-latency and long-duration muscle responses triggered by the change in viscous loading. Although the triggered muscle responses were larger when the loads were encountered during performance of large, in comparison to small, movements, smaller muscle responses affected small movements more than large triggered responses did large movements. This suggests that triggered muscle responses are compensatory in certain movement situations but disruptive in others. In addition, these findings demonstrated that dynamic loads especially affect the kinematics and end-point control of smaller movements, suggesting that kinesthetic inputs and central motor commands interact so subjects may achieve accurate positioning for certain classes of movements.
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