Masseter EMG as a measure of bite block use under various Gz levels

Biting down against a mandibular orthopedic repositioning appliance (MORA) that properly aligns the temporomandibular joint (TMJ) requires voluntary contraction of the masseter and temporalis muscles. The effects of this biting maneuver on G tolerance were evaluated. Subjects were fitted with mouthpieces by a dentist qualified in TMJ alignment. Noninvasive BP, ECG (electrocardiogram), and facial EMG (electromyogram) were recorded at 1 G during isometric straining in two sessions of two strains with biting and two strains without biting. Using no anti-G suit, subjects were then exposed to sessions of 0.5 Gz increasing 20 second G plateaus on the W-PAFB dynamic environment simulator (human centrifuge) until loss of peripheral vision. Seven physiological variables were recorded including facial EMG, ear pulse, and transcranial Doppler signals of cerebral artery flow. G tolerance was recorded for three exposures using the MORA and three without it. The resulting EMG and RMS EMG recordings indicated that subjects were not maximally contracting the involved muscles during biting and that muscle fatigue did not occur. Using the MORA resulted in 0.3 Gz mean increase in tolerance compared with not using the MORA, although the difference was not statistically significant at a 0.05 significance level