Kinematics of jaw movements during chewing at different frequencies.

The three-dimensional jaw movement trajectory and its velocity profile were investigated to provide information regarding the motor control of chewing. This was done on a cycle-by-cycle basis across three voluntarily guided chewing frequencies to test the lack of invariance at the velocity level and the evidence for a cost function. The data revealed that an increase in chewing frequencies resulted in a significant reduction of the three-dimensional path, being significantly higher in opening than closing. The ratio of peak speed/three-dimensional distance, a kinematic index of stiffness, increased with increase in chewing frequency; higher stiffness being associated with faster chewing, the stiffness being significantly higher in opening than in closing. The ratio of peak velocity to mean velocity decreased as a function of increase in chewing frequency only at the highest frequency and this decrease again was different in opening than in closing. The results demonstrated that the velocity profile of chewing movements, like that of speech movements, lacked invariance. Rather than minimizing one cost function, the system used an efficient trade-off between time, space and effort, based on the objective of movements.