Heterogeneous activation of the medial pterygoid muscle during simulated clenching

Summary

The aim of this study was to investigate whether the medial pterygoid muscle shows differential activation under experimental conditions simulating force generation during jaw clenching.

To answer this question, the electromyographic activity of the right medial pterygoid was recorded with two intramuscular electrodes placed in an anterior and posterior muscle region, respectively. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with strain gauges. The activity distribution in the muscle was recorded in a central mandibular position during generation of eight different force vectors at a constant amount of force (F = 150 N).

The investigated muscle regions showed different amounts of EMG activity. The relative intensity of the activation in the two regions changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for various motor tasks.

The results indicate a heterogeneous activation of the medial pterygoid muscle under test conditions simulating force generation during clenching. This muscle behaviour might offer an explanatory model for the therapeutic effects of oral splints.

Introduction

Jaw muscles are traditionally considered as a homogeneous unit. This corresponds to the idea that all motoneurons of a specific muscle receive the same synaptic input¹ and that they are activated in a predetermined order corresponding to their cell size.² Recent findings show, however, that – similar to some muscles in the extremities3, 4, 5 – the jaw musculature is capable of differential, i.e., heterogeneous, activation.6, 7, 8, 9, 10 As a result, regions of an individual muscle demonstrate distinct differences in their activation relative to one another¹¹ when various motor tasks are performed. In contrast to the ‘mosaic pattern’ in the extremities with fibre distribution throughout large areas of the muscle cross-section,¹² the more focal distribution of the motor units in the jaw muscles¹³ as well as the regionally heterogeneous histochemical fibre profile¹⁴ point to local functional differences. Additionally, the heterogeneous distribution of muscle spindles as found in the masseter muscle15, 16 suggests a special role for certain muscle regions in the control of a particular motor task. Apart from the implications for biomechanical modelling, such data are of special clinical interest, because the regional differences in the jaw muscle activation might give a deeper insight into pathophysiological mechanisms which are supposed to be responsible for regional muscle pain caused by overload.17, 18, 19, 20 Thus far, the phenomenon of differential activation has been substantiated for the masseter,8, 21 the lateral pterygoid,²² and the digastric muscles.²³ Evidence for the temporalis is conflicting, because the heterogeneous activation has been demonstrated by some groups,⁷ but not by others.²⁴ The goal of this study was to investigate if a differential activation can also be detected for the medial pterygoid.