Work of the motor units in human antagonists during rhythmic movements

Firing of single motor units (MUs) from m. biceps brachii and m. triceps brachii has been studied with rhythmic successive flexion and extension in the elbow. The time of appearance of the first of the series of impulses from this MU is compared with the time of the beginning of flexion and extension. This recruitment time (the recruitment threshold in time) is very variable. It depends on the average velocity of the isotonic movement, whereby at higher velocities the number of cases when it can be negative increases, i. e. the impulses of the MU to start before the change in the direction of the movement.