Motor unit synchronization of the vasti muscles in closed and open chain tasks

Abstract

Mellor R, Hodges PW. Motor unit synchronization of the vasti muscles in closed and open chain tasks. Arch Phys Med Rehabil 2005;86:716–21. Objectives To investigate motor unit synchronization between medial and lateral vasti and whether such synchronization differs in closed and open chain tasks. Design Electromyographic recordings of single motor unit action potentials were made from the vastus medialis obliquus (VMO) and multiunit recordings from vastus lateralis during isometric contractions at 30° of knee flexion in closed and open chain conditions. Setting Laboratory. Participants Five volunteers with no history of knee pain (age, 30±3.32y). Interventions Not applicable. Main outcome measure The degree of synchronization between motor unit firing was evaluated by identifying peaks in the electromyographic averages of the vastus lateralis, triggered from motor unit action potentials in the VMO, and the proportion of power in the power spectral density of the triggered average at the firing frequency of the reference motor unit. The proportion of cases in which there was significant power and peaks in the triggered averages was calculated. Results The proportion of trials with peaks in the triggered averages of the vastus lateralis electromyographic activity was greater than 61.5% in all tasks, and there was a significantly greater proportion of cases where power in the spectrum was greater than 7.5% ( P=.01) for the closed chain condition. Conclusions There was a high proportion of synchronized motor units between the 2 muscles during isometric contractions, with evidence for greater common drive between the VMO and vastus lateralis in closed chain tasks. This has implications for rehabilitation because it suggests that closed chain tasks may generate better coordination between the vasti muscles.