Control of jaw orientation and position in mastication and speech.

1. The kinematics of sagittal-plane jaw motion were assessed in mastication and speech. The movement paths were described in joint coordinates, in terms of the component rotations and translations. The analysis focused on the relationship between rotation and horizontal translation. Evidence was presented that these can be separately controlled. 2. In speech, jaw movements were studied during consonant-vowel utterances produced at different rates and volumes. In mastication, bolus placement, compliance, and size as well as chewing rate were manipulated. Jaw movements were recorded using the University of Wisconsin X-ray microbeam system. Jaw rotation and translation were calculated on the basis of the motion of X-ray tracking pellets on the jaw. 3. The average magnitudes of jaw rotation and translation were greater in mastication than in speech. In addition, in speech, it was shown that the average rotation magnitude may vary independent of the horizontal translation magnitude. In mastication, the average magnitude of vertical jaw translation was not dependent on the magnitudes of jaw rotation or horizontal jaw translation. 4. The magnitude of rotation and horizontal jaw translation tended to be correlated when examined on a trial by trial basis. Some subjects also showed a correlation between jaw rotation and vertical jaw translation. However, the proportion of variance accounted for was greater for all subjects in the case of rotation and horizontal translation. 5. Joint space paths in both mastication and speech were found to be straight. The pattern was observed at normal and fast rates of speech and mastication and for loud speech as well. Straight line paths were also observed when subjects produced utterances that had both the syllabic structure and the intonation pattern of speech. The findings suggest that control may be organized in terms of an equilibrium jaw orientation and an equilibrium jaw position. 6. Departures from linearity were also observed. These were typically associated with differences during jaw closing in the end time of rotation and translation. Start time differences were not observed in jaw closing and the movement paths were typically linear within this region.