Motor unit synchronization between medial and lateral vasti muscles

https://doi.org/10.1016/j.clinph.2005.04.004Get rights and content

Abstract

Objective

Accurate neuromuscular control of the patellofemoral joint is important in knee joint mechanics. Strategies to coordinate the vasti muscles, such as motor unit synchronization, may simplify control of patellar tracking. This study investigated motor unit synchronization between vastus medialis (VM) and lateralis (VL).

Methods

Electromyographic (EMG) recordings of single motor unit action potentials (MUAPs) were made from VM and single- and multi-unit recordings were made from VL. Synchronization was quantified from peaks in the cross-correlogram generated from single MUAP pairs in VL and VM. The proportion of motor units in VM with synchronized firing in VL was also quantified from peaks in averages of multiunit VL EMG triggered from the VM MUAP.

Results

A high degree of synchronization of motor unit firing between VM and VL was identified. Results were similar for cross-correlation (∼45% of cases) and triggered averages (∼41% of cases).

Conclusions

The data suggest that synchronization between VM and VL is higher than expected. Agreement between traditional cross-correlation and triggered averaging methods suggest that this new technique may provide a more clinically viable method to quantify synchronization.

Significance

High synchronization between VM and VL may provide a solution to simplify control of the mechanically unstable patellofemoral joint.

Introduction

Coordinated activity of the medial and lateral vasti muscles is essential to control the translation of the patella in the femoral groove. Biomechanical studies indicate that a delay in onset of activity of the medial vasti relative to the lateral muscles of only 5 ms can cause significant change in loading at the patellofemoral joint (Neptune et al., 2000) and poor coordination between the two muscles has been implicated in patellofemoral pain (PFP) (Cowan et al., 2002). One possible mechanism to facilitate coordination and simplify this control is to synchronize the neural drive to the muscles.

Motor unit synchronization is the near simultaneous discharge of action potentials in separate motor units (De Luca et al., 1993, Sears and Stagg, 1976) as a result of events that raise the joint probability of firing of the motoneurons. Evidence of synchronization of the firing of motor units may indicate the presence of shared input to the motoneurones, and can provide insight into the organization and activity of common, branched-stem presynaptic fibres within and between motoneuron pools (Datta et al., 1991). Although motor unit synchronization is not equivalent to the simultaneous onset of EMG activity of the vasti muscles, it could be interpreted to represent an attempt by the central nervous system (CNS) to coordinate the activity of the two muscles.

Studies of the biomechanical properties of the vasti muscles (Farahmand et al., 1998, Lieb and Perry, 1968, Nozic et al., 1997) indicate that the medially directed fibres of the distal oblique portion of vastus medialis (VMO) have an important role in counterbalancing the lateral pull of the other quadriceps muscles on the patella, due to its mechanical advantage, whereas the more longitudinal fibres contribute primarily to knee extension. We hypothesize that activity of motor units in the medial and lateral vasti muscles may be synchronized during isometric knee extension efforts as a possible CNS mechanism to simplify patellofemoral control, and to contribute to functional coordination between the muscles (Bremner et al., 1991a, Bremner et al., 1991b, Lemon and The, 1993).

Methods described for evaluation of motor unit synchronization generally involve cross-correlation and spike-triggered averaging (Hamm et al., 1985). Generation of cross-correlograms between individual motor units to assess synchronization requires recordings from a number of motor unit pairs in order to reliably estimate the extent of motor unit synchronization, as its strength varies both within and between muscles. It is also difficult to evaluate the same motor unit pair over repeated experiments. The shape of the motor unit action potential (MUAP) waveform can change over time, especially as long recordings are necessary to accurately detect synchronization (Sears and Stagg, 1976), and additional units are recruited at higher force levels, making reliable discrimination of the motor units difficult (Milner-Brown et al., 1975). The reliability of surface EMG techniques for evaluation of synchronization is also questionable due to problems such as cross-talk between muscles, variation in ongoing EMG, signal to noise ratio changes that reduce the sensitivity of the measure (Yue et al., 1995) and other methodological difficulties that prevent direct comparison between surface EMG and single motor unit cross-correlation techniques (Semmler and Nordstrom, 1999). In view of these limitations, we used two techniques to quantify motor unit synchronization; cross-correlation between single motor units in VM and VL, and a new technique that involves identification of peaks in averages of VL EMG triggered from a motor unit in VM. Because this latter technique involves recordings from separate muscles, it is not hampered by the problems identified for averages generated from multi- and single unit activity in a single muscle.

Thus, the primary aim of the present study was to determine the presence of motor unit synchronization between the functionally related medial and lateral vasti muscles during voluntary isometric contractions. A second aim was to evaluate a modified method of determining the degree of synchronization from triggered averages of VL EMG against conventional techniques of cross-correlation of motor unit firing.

Section snippets

Subjects

Fifteen healthy volunteers (aged 30±4.76, mean±SD) participated in the study. Subjects were excluded if they had a history of knee pain, history of lower limb pathology, previous knee surgery, osteoarthritis, or were involved in high intensity training. The study was approved by the Institutional Medical Research Ethics Committee and all procedures were conducted in accord with the Declaration of Helsinki.

Electromyographic activity

Motor unit synchronization was measured in two ways; cross-correlation of pairs of single

Cross-correlation of single motor unit pairs

In five subjects cross-correlograms were generated from MUAPs in needle EMG recordings of VM and VL. The mean (SD) discharge rate of the single motor units in VM and VL was 7.89±1.72 and the variability in the interpulse interval was 22%. Significant peaks were identified in 45% of cases (34 of 75 pairs). The prevalence of significant peaks ranged from 27 to 66% between subjects (Table 1), which concurs with the variability in motor unit synchronization described between different subjects in

Discussion

The findings of this study indicate that firing of motor units in VL and VM is synchronized during isometric knee extension efforts. This is consistent with the hypothesis, based on biomechanical data, that tight coordination between the medial and lateral muscles would be required to control the patellofemoral joint during contraction of the quadriceps muscle. Considering the degree of synchronization reported within single muscles, this degree of synchronization between two muscles is

Conclusion

The presence of motor unit synchronization provides a measure of the common drive to the motoneuron pools of the medial and lateral vasti muscles. The present study confirms that there is a surprisingly high degree of motor unit synchronization between the medial and lateral vasti muscles, and this may represent a mechanism by which the CNS simplifies and coordinates the activity of these two muscles in control of the patellofemoral joint.

Data from a recent study suggest that synchronization

Acknowledgements

We thank the National Health and Medical Research Council for financial support, and Johan Sandblad for assistance with data collection and analysis.

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