The electrophysiology of neuromuscular function can be analyzed at the level of the individual motor unit which is the functional element of the motor system by decomposition of the EMG interference pattern. Configurational and firing properties of 3,342 motor unit action potentials(MUAP) in the biceps brachii and 3,455 in the first dorsal interosseous were measured through automatic decomposition electromyography(ADEMG) using standard concentric needle electrode in 25 healthy adults(19 men, 6 women; mean age 24.3±4.2 years) at four levels of stable isometric contraction; threshold, 10% maximal voluntary contraction(MVC), 20% MVC and 30% MVC.

The mean number of simultaneously active MUAPs per recording site was increased as force increment in biceps brachii but no difference in first dorsal interosseous. In both muscles the increment of contractile force from threshold to 30% MVC was associated with a significant increase in amplitude, number of turns, firing rate and rise rate but decrease in duration of MUAP. The identification rate was decreased and coefficient of interspike interval variability(CIV) was increased as force increment

So the contractile force is a major determinant of MUAP shape and firing properties and so must be precisely measured or controlled in clinical EMG studies.