Recruitment threshold and muscle fiber conduction velocity of single motor units

The average muscle fiber conduction velocity (CV) measured with multicontact surface electrodes has been reported to increase with the contraction force. To understand this behavior better, we studied the relationship between the recruitment threshold and the muscle fiber CV of single motor units (MUs). Myoelectric signals were recorded simultaneously with a linear surface electrode array and a selective needle electrode. From the signals detected by the meedle electrode, the discharges of single MUs were isolated by the decomposition technique. By using the firing of single MUs as the trigger point, we averaged the surface myoelectric signals and extracted the single MU action potentials from the interference surface signals. The CV of single MUs, calculated by a cross-correlation analysis, was higher for MUs recruited at higher contraction force. This result indicates that the larger MUs with higher muscle fiber CV contribute to increase the average CV during varying force contractions.