Methods for estimating the number of motor units in human muscles

The number of motor units in the thenar muscle group was calculated by dividing the surface electromyogram and twitch force, in maximal stimulation of the median nerve, by estimates of the average electromyogram and twitch force from single units. The following three techniques were used to estimate the average electromyogram from single units: spike-triggered averaging from units recorded with a needle electrode, intramuscular microstimulation of motor nerve branches, and graded whole nerve stimulation at the wrist. The first two techniques also provided independent estimates of motor unit numbers based on the average force generated by single units. The five estimates (three based on the electromyogram and two on force) ranged from 116 to 170 motor units in the thenar group. Correcting for cancellation when unit responses sum to form the compound action potential or twitch increased the estimated number of units, which ranged from 130 to 179. The estimates were not statistically different from one another but were substantially lower than some previous electrophysiological estimates based on graded whole nerve stimulation. The recruitment pattern of single units during whole nerve stimulation was recorded and simulated mathematically. The most likely reason for the higher estimates in previous studies using graded whole nerve stimulation is shown to be alternation of motor units. Potential errors in all the techniques are discussed and compared.