Distribution of innervation zones in the human biceps brachii

Two types of multicontact surface electrodes were used to define the distribution of innervation zones (motor end-plate regions) in human skeletal muscles: a linear array electrode and a two-dimensional grid electrode. These electrodes detected motor unit action potentials (MUAPs) propagating along the muscle fibers during a voluntary contraction and marked the source of the propagation, which was assumed to indicate the position of the innervation zones. The linear array electrode defined the absolute position of the innervation zones in the muscle, but it could not clarify the detailed distribution, within the innervation zones. On the other hand, the grid electrode clarified the detailed configuration of innervation zones under the pickup area of the electrode, but the absolute position of the sources was inaccurate. The records obtained with these two electrodes were combined to draw a detailed map of the innervation zones over the entire surface of the muscle. In seven healthy normal adults, the biceps brachii was studied. The innervation zones were positioned at the middle length of the muscle in a band 30-60 mm wide. The detailed configuration of the innervation zones varied between the subjects.