Effects of fatigue on intermuscular common drive to the quadriceps femoris.

The purpose of this investigation was to examine intermuscular common drive to motor units in the vastus lateralis and vastus medialis during isometric contractions of the leg extensors. A secondary purpose was to investigate the effects of fatigue on the common drive. Fourteen healthy subjects (mean ± SD, age = 21.7 ± 1.5 years) volunteered to perform two separate isometric contractions of the leg extensors at 50% of the maximum voluntary contraction (MVC). These contractions were performed before (PRE) and immediately after (POST) a fatigue protocol of 10, 10 s isometric MVCs with 10 s of rest in between (i.e., 10 s on, 10 s off). During the PRE and POST isometric contractions, surface electromyographic (EMG) signals were detected from the vastus lateralis and vastus medialis and decomposed into individual motor unit action potential trains. A mean firing rate curve was then calculated for each motor unit, and the curves for the vastus lateralis were cross-correlated with those from the vastus medialis. The results showed that the average peak cross-correlation coefficients were always between 0.39 and 0.55, with coefficients for individual motor unit comparisons occasionally as high as 0.93. However, these coefficients were not affected by fatigue. These findings demonstrated a significant level of intermuscular common drive that was generally unaffected by fatigue.