Localised task‐dependent motor‐unit recruitment in the masseter

Abstract

Localised motor-unit (MU) recruitment in the masseter was analysed in this study. We investigated whether differential activation behaviour, which has already been reported for distant masseter regions, can also be detected in small muscle subvolumes at the level of single MUs. Two bipolar fine-wire electrodes and an intra-oral 3D bite-force transmitter were used to record intra-muscular electromyograms (EMG) resulting from controlled bite-forces of 10 healthy human subjects (mean age 24.1 ± 1.2 years). Two-hundred and seventeen decomposed MUs were organised into localised MU task groups with different (P < 0.001) force-direction-specific behaviour. Proportions of MUs involved in one, two, three or four examined tasks were 46%, 31%, 18% and 5%, respectively. This study provides evidence of the ability of the neuromuscular system to modify the mechanical output of small masseter subvolumes by differential control of adjacent MUs belonging to distinct task groups. Localised differential activation behaviour of the masseter may be the crucial factor enabling highly flexible and efficient adjustment of the muscle activity in response to complex local biomechanical needs, for example, continually varying bite-forces during the demanding masticatory process.

Small vertical changes in jaw relation affect motor unit recruitment in the masseter

“…The results of the quantitative assessment of the MU recruitment characteristics is in fairly good agreement with the findings of a recent study report which reported a dominance of mono‐modal MUs (46%) followed by 31% bi‐modal, 18% tri‐modal and 5% quadri‐modal activation. The occurrence of task‐specific and poly‐modal MUs in the masseter also confirms results from a previous study .…”

“…The EMG activity of the right masseter was measured at three different sites by means of intra‐muscular bipolar fine‐wire electrodes described in detail in a previous paper . To enable recording from approximately the same regions of the muscles of different subjects, the point of intersection of two diagonals was marked as described elsewhere .…”

“…The EMG activity of the right masseter was measured at three different sites by means of intra‐muscular bipolar fine‐wire electrodes described in detail in a previous paper . To enable recording from approximately the same regions of the muscles of different subjects, the point of intersection of two diagonals was marked as described elsewhere . On a line parallel to the lower edge of the zygomatic arch and drawn through the point of intersection, the positions of posterior and anterior wire insertion were marked at a distance of 10 mm from the point of intersection.…”

“… A ‘coincidence index’ (CI) was chosen to analyse which task combinations were managed by the same MUs. This CI enabled quantitative comparison of the different recruitment patterns for two different motor tasks, for example i and j , with a specific number of active MUs (nActMU) as follows: For example, when five MUs were active in task i (e.g.…”

Static balancing behaviour of the mandible

“…Obviously, most of the subjects used the right condyle as a virtual fulcrum whereas the left condyle used all the biomechanical degrees of freedom available to equalize the vertical distance between the upper and lower dental arches. This asymmetric behaviour during BAL might have been affected by the structural asymmetry of the TMJ , differences between bilateral muscle volumes , and/or neuromuscular compartmentalization . It is also possible that, similarly to other neuromuscular preferences, for example handedness, a tendency of neuromuscular laterality might have been involved ; however, it seems that chewing‐side preferences did not have a significant effect on the results of our study.…”