A new method for the localization of the innervation zone based on monopolar surface-detected potentials

In monopolar electrode configuration, the shape of the motor unit potential (MUP) undergoes essential morphologic changes with increasing distance from the innervation zone. In particular, the rising phase of the MUP slows down and becomes longer as the electrode is moved further from the innervation zone. Based on this, it is hypothesized that the maximum slope of the rising phase of a monopolar MUP (Slope_MAX) would reach its highest value at the innervation zone. Herein, we sought to determine whether the location of the innervation zone coincides with the position in the muscle of the monopolar MUP with highest Slope_MAX and whether new criteria for the localization of the innervation region can be proposed based on the Slope_MAX of monopolar potentials. Multichannel surface (13×5 electrode array) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25mm) EMG signals were concurrently recorded in monopolar configuration from the biceps brachii muscle of 10 healthy subjects. The spatial distribution of monopolar and bipolar MUPs along the fibers' direction were obtained by spike-triggered averaging of the surface EMG. We found that the monopolar MUP with steepest rising phase (i.e., highest Slope_MAX) was located either just above or at half of the inter-electrode distance from the innervation zone. High levels of agreement (94-98%) were found between the position of the innervation zone assessed with the Slope_MAX criteria of monopolar potentials and the position identified using the phase reversal criteria of bipolar potentials. The present results legitimize the use of compound muscle action potentials (M waves) to localize the innervation zone: this location would correspond to the electrode position which yielded the monopolar M wave with highest Slope_MAX.