Discharge rate of sternohyoid motor units activated with surface EMG feedback

We analyzed individual motor units of the sternohyoid muscle with the aim of characterizing their minimum and maximum discharge rates and their variability in discharge during voluntary contractions. Surface EMG signals were recorded with an array of eight electrodes from the sternohyoid muscle of seven healthy men (age: 30.2+/-3.5 yr). The multichannel surface EMG signals were displayed as feedback for the subjects who identified and modulated the activity of one target motor unit in 30-s contractions during which the discharge rate was increased from minimum to maximum (ramp contraction), sustained at maximum level (sustained), or increased in brief bursts (burst). During the ramp contractions, the minimum average discharge rate over epochs of 1 s was 11.6+/-1.5 pulses per second (pps) and the maximum 57.0+/-5.7 pps (P<0.001). During the sustained contractions, the motor unit discharge rate decreased from 65.5+/-8.4 pps at the beginning to 52.9+/-7.6 pps at the end of the contraction (P<0.05). The coefficient of variation for the interspike interval during the sustained contractions was 40.2+/-9.8% and a large percentage of discharges had instantaneous rates>50 pps (52.2+/-12.5%) and>100 pps (8.0+/-1.2%), with peak values>150 pps. During the burst contractions, the instantaneous discharge rate reached average maximum values of 97.6+/-36.8 pps. The observed discharge rates and their variability are higher than those reported for limb muscles, which may be due to large synaptic input and noise received by these motor neurons.