AN EXAMINATION OF THE DEGREES OF FREEDOM OF HUMAN JAW MOTION IN SPEECH AND MASTICATION

The kinematics of human jaw movements were assessed in terms of the th ree orientation angles and three positions that characterize the motio n of the jaw as a rigid body. The analysis focused on the identificati on of the law's independent movement dimensions, and was based on an e xamination of jaw motion paths that were plotted in various combinatio ns of linear and angular coordinate frames. Overall, both behaviors we re characterized by independent motion in four degrees of freedom. In general, when law movements were plotted to show orientation in the sa gittal plane as a function of horizontal position, relatively straight paths were observed. In speech, the slopes and intercepts of these pa ths varied depending on the phonetic material. The vertical position o f the jaw was observed to shift up or down so as to displace the overa ll form of the sagittal plane motion path of the jaw Yaw movements wer e small but independent of pitch, and vertical and horizontal position . In mastication, the slope and intercept of the relationship between pitch and horizontal position were affected by the type of Food and it s size. However, the range of variation was less than that observed in speech. When vertical jaw position was plotted as a function of horiz ontal position, the basic form of the path of the jaw was maintained b ut could be shifted vertically. In general, larger bolus diameters wer e associated with lower jaw positions throughout the movement. The tim ing of pitch and yaw motion differed. The most common pattern involved changes in pitch angle during jaw opening followed by a phase predomi nated by lateral motion (yaw). Thus, in both behaviors there was evide nce of independent motion in pitch, yaw, horizontal position, and vert ical position. This is consistent with the idea that motions in these degrees of freedom are independently controlled.