Human movements and mandibular stability

Abstract

Intensive studies of the positioning and movements of the human jaw in both healthy and diseased subjects have been undertaken because of the importance of the mouth in daily human activities, such as verbal and nonverbal communication, breathing, chewing and swallowing. The observation that our teeth do not crash together in a potentially harmful way during locomotion and other whole body activities is common, although the reason for this is not clear. The aim of the present study was to investigate the mechanisms controlling the rest position of the human jaw and the influence of whole body movements such as running and hopping on these mechanisms. While there have been numerous studies relating jaw elevator muscle activity with jaw movement and position in seated subjects, there are no reports of this in fully ambulatory humans. I developed specific equipment, built around miniature accelerometers and magnetic sensors, to enable a rapid and accurate measurement of small excursions of the mandible in response to take-off and landing during multi-dimensional movements. Accurate detection and recording of short-latency reflex activation in muscles attached to the jaw was required during unconstrained movements of the subject. I developed a recording system that was robust yet non-invasive for this purpose. In the present study short-latency excitation was found in the masseter after an abrupt landing (heel land) from a hop. No reflex activation was seen in either a soft landing (toe land) or a hard landing with the teeth fully occluded in which there was minimal displacement of the mandible with respect to the maxilla. This provides strong evidence that the excitation is a stretch reflex and not of vestibular origin. This is the first report of short-latency stretch reflex in the human masseter resulting from a natural movement. Landing on the heel or on the toe in different forms of locomotion on a treadmill was followed by rapid deceleration of the downward movement of the head and slightly less rapid deceleration of the downward movement of the mandible, i.e., the mandible moved downwards relative to the maxilla, then upwards again to near its normal posture, within 200 ms. No tooth contact occurred in any forms of gait or at any inclination of the treadmill. The movement of the mandible relative to the maxilla was found to depend on the nature and velocity of the locomotion and the effects on head deceleration. The least deceleration, and hence the minimal mandibular displacement, occurred during toe-landing, such as occurs during "uphill" running. The maximum displacement of the mandible relative to the head was less than 1 mm, even at the fastest running speed. The mechanisms that limit the vertical movements of the jaw within such a narrow range are not known, but are likely to include passive soft-tissue visco-elasticity and stretch reflexes in the jaw-closing muscles. The techniques developed in the present study will allow the investigation of the role of stretch reflexes under these conditions in a future study.