TOPOGRAPHICAL MAP OF INNERVATION ZONES IN THE BACK MUSCLES INVESTIGATED BY A MULTICHANNEL SURFACE EMG

Abstract

In skeletal muscles which have muscle fibers running parallel to each other, motor unit action potentials (MUAPs) arise from the innervation zones (motor end-plate regions) and propagate bidirectionally along the muscle fibers. Using this property, it is possible to estimate the location of the innervation zones by recording the EMG signals simultaneously with multiple-contact surface electrodes. This method for defining the innervation zones has been successfully applied to the muscles in the upper and lower extremities, because these muscles have long fibers running parallel to each other and consequently the propagation of MUAPs is easily detectable. However, if this technique were applicable only to the muscles in the extremities, it would have a limited application. The purpose of this study is to clarify the applicability of this technique to the back muscles: the trapezius, the latissimus dorsi, and the erectores spinae. These muscles are related to ergonomic problems such as a stiff neck or the low back pain. Five healthy male subjects were studied during an isometric contraction with a 15-channel surface electrode array. In the muscles which show the propagation of MUAPs, a topographical map of the innervation zones was obtained by moving the electrode array over the surface of the muscle. In the trapezius, which is separated into four portions, the MUAPs showed a clear propagation pattern, and the map of the innervation zones was clarified. The innervation zones lay in each portion at the middle of the muscle, forming a line perpendicular to the muscle fibers. In the latissimus dorsi the propagation was detected, but it was not so clear as in the trapezius. The innervation zones were scattered concentrically around the axillary portion, and the distribution of the innervation zones differed between the subjects. In the erectores spinae the bidirectional propagation of MUAPs was not observed except in one subject. This is perhaps because the muscle fibers have a complex configuration in this muscle.