Motor Unit Synchronization of the Vasti Muscles in Closed and Open Chain Tasks Rebecca Mellor, Mphty, Paul W. Hodges, PhD ABSTRACT. Mellor R, Hodges PW. Motor unit synchroni- zation of the vasti muscles in closed and open chain tasks. Arch Phys Med Rehabil 2005;86:716-21. Objectives: To investigate motor unit synchronization be- tween medial and lateral vasti and whether such synchroniza- tion 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, trig- gered from motor unit action potentials in the VMO, and the proportion of power in the power spectral density of the trig- gered 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. Key Words: Exercise therapy; Motor activity; Muscles; Rehabilitation. © 2005 by American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation O PTIMAL CONTROL OF the quadriceps is essential for normal function of the knee. In particular, coordination of the medial and lateral forces of the vasti is critical for control of alignment and stability of the patellofemoral joint. One strategy to simplify this control may be for the central nervous system (CNS) to provide a degree of common input to the vasti muscles. Recently, we showed that synchronous firing is com- mon between motor units in the vastus medialis obliquus (VMO) and vastus lateralis muscle components of the quadri- ceps group. 1 However, how this degree of synchronization differs between functional contexts is not known. Motor unit synchronization is defined as the increased ten- dency for motor units to discharge within a few milliseconds of each other more often than would be expected because of chance and can be quantified by techniques such as cross- correlation of the individual discharge times of motor unit pairs. 2,3 The degree of motor unit synchronization can provide some indication about the organization of synaptic input to motoneurons during voluntary motor tasks. 4 Differences in muscle function (eg, prehension and postural control) are reflected by different neural control mechanisms, and it appears from most studies of motor unit synchronization that it is greatest within muscles involved in postural control and stability. For example, Marsden et al 5 showed coherence between motor unit pairs located in the paraspinal muscles ipsilaterally and bilaterally at low frequencies but not bilater- ally between the interossei of the hand. The lack of common drive between the right and left interossei muscles could allow for skilled independent control of the 2 hands, whereas its presence in the paraspinals may allow the axial skeleton to act more as a functional unit during postural control. Several studies have found that the degree of synchroniza- tion between muscles may be task specific. Semmler et al, 6 using cross-correlation and coherence analysis of single motor unit pairs, found differences in the degree of common inputs received by the motoneurons between postural, shortening, and lengthening contractions of a hand muscle. Semmler and Nordstrom 7 also used cross-correlation of motor unit pairs in the first dorsal interosseous (FDI) muscle of weightlifters, musicians, and untrained subjects to examine the effect of habitual physical activity on motor unit synchronization. Dur- ing isometric abduction of the FDI, they found greatest strength of synchronization in the muscles of the weightlifters and least in the musicians, showing how different skilled activities re- quire different degrees of common input. Bremner et al 8 also found that the degree of synchronization between hand muscles depends on the task performed, reflecting the different levels of separate control necessary for prehension. By using cross- correlation analysis, Gibbs et al 9 found greatest synchroniza- tion between the discharge of motoneuron pools innervating muscle pairs in the lower limb that were anatomically and functionally related, especially during the postural tasks of balancing and standing. Halliday et al, 10 by using both time- and frequency-domain analysis of the coupling between the electromyographic activity from pairs of leg muscles, found that firing of motor units in different muscles is only weakly coupled during walking, which may provide a basis for the great adaptability of the human gait pattern. These data suggest that the CNS coordinates motor unit activity differently during postural and movement tasks. Because the quadriceps contract in a weight bearing or closed chain situation during stance and balance, one func- tional consideration is whether motor unit synchronization From the Division of Physiotherapy, University of Queensland, Brisbane, Australia. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the author(s) or on any organization with which the author(s) is/are associated. Reprint requests to Paul W. Hodges, PhD, Div of Physiotherapy, University of Queensland, Brisbane, Queensland 4072, Australia. e-mail: p.hodges@shrs.uq.edu.au. 0003-9993/05/8604-9221$30.00/0 doi:10.1016/j.apmr.2004.07.354 716 Arch Phys Med Rehabil Vol 86, April 2005