Differential activity patterns in the masseter muscle under simulated clenching and grinding forces

Abstract

The aim of this study was to investigate (i) whether the masseter muscle shows differential activation under experimental conditions which simulate force generation during clenching and grinding activities; and (ii) whether there are (a) preferentially active muscle regions or (b) force directions which show enhanced muscle activation. To answer these questions, the electromyographic (EMG) activity of the right masseter muscle was recorded with five intramuscular electrodes placed in two deep muscle areas and in three surface regions. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with three strain gauges (SG). The activity distribution in the muscle was recorded in four different mandibular positions (central, left, right, anterior). In each position, maximum voluntary contraction (MVC) was exerted in vertical, posterior, anterior, medial and lateral directions. The investigated muscle regions showed different amount of EMG activity. The relative intensity of the activation, with respect to other regions, changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for the various motor tasks. The resultant force vectors demonstrated similar amounts in all horizontal bite directions. Protrusive force directions revealed the highest relative activation of the masseter muscle. The posterior deep muscle region seemed to be the most active compartment during the different motor tasks. The results indicate a heterogeneous activation of the masseter muscle under test conditions simulating force generation during clenching and grinding. Protrusively directed bite forces were accompanied by the highest activation in the muscle, with the posterior deep region as the most active area.

Heterogeneous activation of the medial pterygoid muscle during simulated clenching

“…These specific factors limit the direct comparison with activation states in maximum intercuspation. However, the good match of the mean EMG data during maximum contraction with and without the bearing pin device as shown for the masseter 10 and all other masticatory muscles 30 speaks for a similar stiffening behaviour of the system in the two positions, i.e., the neuromuscular system balances the muscle forces and joint reaction forces in a similar way to reach static equilibrium. This leads us to assume that our results represent a good approximation of clenching activities that occur in maximum intercuspation.…”

“…10,26 This might have led to the registration of different motor unit subpopulations in the two muscle regions of the Muscle activity during clenching 501 subjects. This possible source of error cannot be entirely excluded.…”

“…10 Briefly, the force was centrally transmitted by an intraoral bearing pin device, which was equipped with strain gauges and fixed on custom made metal splints. For individual adjustment of the measuring device, the casts of the subjects were mounted in maximum intercuspation in an articulator.…”

“…2 Recent findings show, however, that -similar to some muscles in the extremities [3][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 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.…”

Evaluation ofT2values and apparent diffusion coefficient of the masseter muscle by clenching

“…However, the actual electrical source detected by surface EMG is vague and the signal is faint when derived from deeply located muscles. [13][14][15][16][17] MRI has advantages over surface EMG in that it provides anatomical and physiological information.…”