RECRUITMENT PATTERNS OF SINGLE MOTOR UNITS IN THE HUMAN MASSETER MUSCLE DURING BRISK JAW CLENCHING

Activity of single motor units was recorded with highly-selective electrodes from the masseter muscle in intact man with normal dentition. The inter-occlusal isometric force was simultaneously recorded. The fractional contribution of the masseter to jaw clenching estimated from integrated EMG and isometric tetanic tension was about 0.33 both in slow or brisk voluntary clenches. The recruitment order of motor units raises problems in brisk contractions because the spikes of motor units cannot be directly related in time to the steeplyincreasing force record. A method was used which relates threshold activation of any unit to the peak force of brisk contractions. The rank ordering of masseter motor units was identical in the tonic reflex to chin vibration, in slow and brisk voluntary clenches. No evidence was found for any reversal of the recruitment order over a wide range of contraction speeds. © 1979.