Frequency control of motor unit action potentials.

Abstract The onset interval of the single motor unit action potential (MUAP), defined as the longest regular interval of firing at minimal effort, was determined for various skeletal muscles. Intervals found were 132±32 msec, mean for all limb and girdle muscles, and 86±29 msec, mean for all facial muscles studied. Distal muscles in the upper extremities had significantly longer onset intervals than did proximal shoulder girdle and lower extremity muscles. In studying single units, awareness of the level of effort seemed more critical to the control of firing rate than did movement. The recruitment interval was less variable than onset interval, with a mean of 90±19 msec for limb and girdle muscles and 40±16 msec for facial muscles. The range of control, expressed as the difference in frequency of a MUAP between onset and recruitment relative to the recruitment frequency, decreased with cephalo-caudal location of the muscle from 0.55 for the frontalis muscle to 0.29 for the biceps femoris muscle.