Changes in activity of motor units preceding a rapid voluntary movement.

Motor unit activity was studied during slightly sustained contraction of the erector spinal muscle before a rapid trunk extension from a moderately bent position, with special reference to a change of the surface EMG to a rhythmic slower wave with a periodicity of 20-30 msec preceding the movement. Synchronous discharge of a few motor units appeared at regular intervals of 20-30 msec during the slower wave. EMG peaks in the slower wave corresponded to synchronous discharges at the regular intervals.