EMG Decomposition of Vastus Medialis and Vastus Lateralis in Normal Subjects and Patellofemoral Patients: A New Way of Assessing the Balance of Muscle Function?

Abstract

INTRODUCTION: Much has been published on the electromyographic (EMG) differences between vastus medialis (VM) and vastus lateralis (VL); this work has mainly focused on the timing differences in the on¬set of muscle activation using surface EMG.1 However, little has been re¬ported on the frequency of the EMG signals, which can indicate differ¬ences in motor unit firing patterns.
METHODS: This study used surface EMG signal decomposition technology to explore the properties of numerous simultaneously active motor units. EMG decomposition was conducted on VM and VL during a weight-bearing closed-kinetic-chain isometric “squat” task in pathology-free in-dividuals and case studies of patients with patellofemoral pain. EMG sig¬nals were collected with 2 five-pin surface array sensors, each providing 4 channels of data, which were placed over the belly of VM and VL. Each channel was then sampled at 20 KHz using a modified 16-channel Ba¬gnoli EMG system (Delsys Inc, Boston, MA). The signals were then de¬composed into the constituent action potentials. The mean firing rate for each motor unit action potential train, the number of peaks per second (PPS), was then calculated.
RESULTS: The results from the normal subjects support previous findings of VM and VL EMG decomposition with approximately equal firing rates of VM and VL (FIGURE 1). However, the results in the patellofemoral pa¬tients show clear differences between the firing rates in VM and VL, with VM having significantly greater firing rates than any previously pub¬lished data from normal subjects2 (FIGURE 2).
DISCUSSION: The ability to conduct surface EMG signal decomposition is a recent technological development. The elevated firing rate measured in the VM in this study could be explained in a number of ways. For ex¬ample, it could be an indicator of localized muscle fatigue in the VM or it could indicate a change in recruitment pattern of the motoneuron pool; either of these explanations could contribute to patellar maltracking. Al¬though exploratory at this time, these differences in motor unit recruit¬ment patterns between healthy subjects and patients with patellofemoral pain syndrome could represent an important future outcome measure of knee control when treating patellofemoral pain. Certainly this is an area worthy of further study.
ACKNOWLEDGEMENTS: Delsys Inc for the loan of the EMG decomposition equipment.