Methods: Twenty-four human subjects with no chronic condition participated. SMU electromyographic activity was recorded with fine-wire electrodes inserted into the SHLP. Electrode location was verified by computer tomography. An optoelectronic jaw tracking system (JAWS3D, Zurich) recorded contralateral, ipsilateral, and protrusive jaw movements (standardized tasks), and retrusive, clenching and/or open-close jaw movements (non-standardized tasks). For the standardized tasks, subjects tracked a target at fast (6.5 mm/s), intermediate (2.2 mm/s) and slow (1.3 mm/s) jaw velocity. Univariate ANOVA was done for each subject on each jaw movement across velocities.
Results: Subjects demonstrated activity during contralateral (n=24 subjects), ipsilateral (5), protrusive (24), retrusive (3), and/or open-close (24) jaw movements, and clenching (9). Of the 69 SMUs discriminated from 24 subjects, 54 were active during contralateral, 52 during protrusive and 8 during ipsilateral jaw movements. SMU threshold significantly increased (mean difference: contralateral 0.4 ± 0.7 mm, p=0.02, and protrusion 0.5 ± 0.4 mm, p=0,) with a decrease in jaw velocity.
Conclusion: SHLP SMUs showed velocity-dependent functional properties that suggests an important role for SHLP in the fine control of horizontal jaw movements.
Supported by Australian NHMRC, Dental Board of NSW, ADRF.