By detecting the propagation of motor unit action potentials (MUAPs) with a linear surface electrode array we can locate the innervation zones (motor endplate regions) and estimate the muscle fibre conduction velocity. This technique has mostly been applied to muscles in the upper and lower extremities. The purpose of the present study was to clarify whether the same technique is applicable to three back muscles: the trapezius, the latissimus dorsi and the erector spinae, which are related to application fields including ergonomic problems and low back pain studies. The results indicated that the trapezius showed clear propagation of MUAPs and the innervation zones defined by the source of the propagation were distributed in a narrow band running along the midpoint of muscle fibres. Propagation of MUAPs in the latissimus dorsi was not so clear as that in the trapezius, but it was still possible to locate the innervation zones, which were scattered in a large area and distributed concentrically with respect to the fossa axillaris. The erector spinae, which consisted of many distinct muscles, showed no clear propagation of MUAPs and its innervation zones could not be defined by the surface electrode array technique.