Electromyographic responses to an unexpected load in fast voluntary movements: descending regulation of segmental reflexes.

This study examined the effects of unexpected loading on muscle activation during fast goal-oriented movements. We tested the hypothesis that the electromyographic (EMG) response to an unexpected load occurs at a short latency after the difference between the expected and the unexpected movement velocity exceeds a fixed threshold. Subjects performed two movement tasks as follows: 1) 30 degrees fast elbow flexion movement with an inertial load added by a torque motor; and 2) 50 degrees fast elbow flexion movement with no added load. These movement tasks were chosen to have similar timing parameters, such as movement time, time-to-peak velocity, and duration of the first agonist burst, while the magnitudes of the angular displacement, velocity, and acceleration were different. In task 1, in random trials a viscous load was substituted for the inertial load at movement onset. In task 2, the same viscous load was added in random trials. The earliest consistent response to the unexpected load was detected in the agonist (biceps) EMG at the same time, about 200 ms from the EMG onset, in both tasks. However, the velocity errors were different in the two tasks and no velocity error threshold dependency could be found. Therefore we reject the hypothesis that the timing of the EMG response to an unexpected load is related to a velocity error threshold. Instead, we suggest that the timing of the EMG response is primarily determined by descending regulation of segmental reflex gain.