EMG responses to unexpected perturbations are delayed in slower movements.

It has previously been found that in fast point-to-point arm movements, proprioceptive feedback is centrally suppressed at the beginning of movement and is facilitated at a time that is correlated with temporal parameters of the planned movement. Here, we show that this correlation holds when subjects are explicitly instructed to move at less than maximal speed. We studied elbow flexion movements made at maximal speed and at 70% of maximal speed over a short distance against a light inertial load and over a long distance against a heavy inertial load. A small number of trials were unexpectedly perturbed by using a servo-controlled motor to decrease the movement velocity. The servo control was turned on early in the movement. The main novel finding is that responses in the surface EMG in the elbow muscles to the perturbation occurred later in the slow-speed conditions than fast-speed conditions. When viewed across all conditions, the onset of the EMG responses to the perturbation increased with the time to peak acceleration in unperturbed movements. In the inertial loaded movements, the time of peak acceleration coincides with the time of peak inertial torque, and so the observed correlation can be interpreted as reflecting the relation between either the planned movement kinematics or the planned movement dynamics. These results are compatible with a hypothesis that a descending command suppresses the proprioceptive feedback control at the movement onset and facilitates it at a time that depends on the time parameters of the planned movement.