Surface electromyogram potentials of motor units; Relationship between potential size and unit location in a large human skeletal muscle

The muscle fibers of a motor unit are spread over only part of the total transverse cross section of a skeletal muscle. As a consequence, both the geometric and electrical centers of different units are distributed throughout the cross section. It follows that the estimate of a unit's size, if based on its surface potential measured by an electrode placed on the skin above the muscle, will depend on a number of variables; most specifically, the properties of the volume conductor and the “unit center-to-electrode” distance. An experimental approach to the problem of “depth normalization” of unit potentials is described. The approach is based on the use of a multielectrode and approximate geometric reconstruction of the unit's intramuscular course. The experimental findings are generalized to cross sections of various geometry. The approach is applied to an evaluation of both the size distribution and the size-threshold relationship of the human tibialis anterior muscle.