The electromyographic (EMG) activity of the human genioglossus (GG) muscle during chewing efforts is not fully understood. In this study, the EMG activity of the human GG muscle during unilateral gum chewing was illustrated and correlated with the activities in the anterior temporalis (AT), the anterior digastric (DG), and the inferior orbicularis oris (OI) muscles. GG muscle activity was measured with customized surface electrodes, while other muscles were recorded with conventional surface electrodes. EMG activities during tongue displacement and the articulation of long vowels, recorded by the customized electrodes, were consistent with the recordings obtained by fine wire electrodes placed in the GG muscle. Jaw displacement was monitored by means of a kinesiograph with a transducer attached to the mandibular central incisors. Mean normalized GG muscle activity showed an onset in the last one-fifth of the intercuspal phase, gradually increasing during jaw-opening, and at its greatest immediately before the maximum jaw-opening position. It then decreased during jaw-closing and ceased in intercuspation but showed a small rebound in the third fifth of the intercuspal phase. The GG muscle burst showed phase lags with the DG and OI muscles and an opposite phase with the AT muscle (all P < 0.0001). All correlations were statistically significant (all P < 0.0001, r values between 0.88 and 0.97). The results suggest central coordination of the timing of the activities of the jaw, lip, and tongue muscles in chewing.

Human limb movements are successfully modeled based on the assumption that the central nervous system controls the movements by maximizing movement smoothness. Movement smoothness is quantified by means of a time integral of squared jerk (jerk-cost), where jerk is defined as the rate of change in acceleration. This study was performed to investigate whether the control of human masticatory vertical jaw movements can also be explained by a minimum-jerk (maximum-smoothness) model. Based on the assumption that minimum-jerk models account for vertical jaw-opening and -closing movements during chewing, the actual time profile of the movement trajectory was simulated by the model. The simulated jerk-costs and peak velocities were compared with those obtained by actual measurements of jaw movements during chewing. Jerk-costs and peak velocities of the jaw movements during chewing were significantly correlated with those predicted by minimum-jerk models (P < 0.0001, r between 0.596 and 0.799). The minimum-jerk models predicted closing movement trajectories more accurately than opening movement trajectories (jaw opening, root-mean-square error = 1.19 mm; jaw closing, 0.52 mm, t = 4.375, P < 0.0001). The results indicated that the vertical jaw movement control during chewing was represented by the minimum-jerk control model and that the vertical jaw-closing movement is smoother than the opening movement during gum-chewing.

Rate of change in movement acceleration (jerk) is a universally accepted quantity to evaluate irregularities of human limb and eye movements. This study was aimed to attest reliability of the jerk measurement of masticatory jaw movements and to identify its sensitivity to discriminate between movements performed with the presence of varied occlusal interference. Jaw movements during gum chewing were recorded with a 3D tracking device. Twelve adult subjects participated in the experiments. For five subjects, normalized jerk-costs (NJC) during jaw closing were compared between those measured on two separate occasions. For seven subjects, the NJCs during closing were compared with those measured with/without introduction of four different types of occlusal interference. The NJCs did not differ significantly between the two recording occasions. The interference at the canine tooth induced greater increase in the NJCs than that at the molar tooth. The comparison between repeated measures revealed reproducibility of the NJCs. In addition, the NJC was shown to be capable of discriminating between irregularities of the movements induced by occlusal interference at the canine and molar tooth sites. These findings suggest that the NJC is a valid indicator of masticatory jaw movement irregularity induced by disturbances of full intercuspation between upper and lower teeth.

It has been demonstrated that the vertical jaw movement trajectories during gum-chewing can be explained by jerk-cost minimization. However, it is uncertain whether the masticatory jaw movement in space can be predicted by the minimum-jerk model. The aims of the present study were to develop minimum-jerk models that would explain 3D masticatory jaw movements with different hardnesses of foods, and to evaluate if the models can predict the movements accurately. The 3D masticatory jaw movement during food breakage was formulated for two types of test foods. The coefficients of determination (R2) between the measured and model-based values ranged from 0.846 to 0.882. Differences were found in the kinematic parameters between the test foods. The results suggest that the models predict the 3D jaw movements during food breakage and are effective in differentiating among the kinematic features of masticatory jaw movements that are peculiar to the mechanical properties of foodstuffs.

Objective: To investigate whether individuals with malocclusion show less skillfulness, as represented by kinematic parameters that characterize masticatory jaw movement, compared with those having normal occlusion and, if so, to examine whether more skilled movements are achieved after completion of orthodontic treatment. Materials and Methods: Lower incisor point movement in space during gum chewing was recorded, and the kinematic traits of such movement were compared among four subject groups: a Control Group (36 females with good occlusion), a Malocclusion Group (24 females with dental malocclusions), an Extraction Group (31 females who had received orthodontic treatment with premolar extraction) and a Nonextraction Group (27 females who had been treated orthodontically without tooth extraction). Before treatment, all subjects in the three experimental groups exhibited dental malocclusions and skeletal class I jaw-base relationship. Results: Compared with the Malocclusion Group, the lower normalized jerk-cost, the shorter phase durations, the more symmetric property of the velocity profile, and the smaller variance of lateral jaw-closing trajectories near the tooth intercuspation position were determined in the Extraction Group and the Nonextraction Group as well as in the Control Group. Conclusions: As measured by kinematic parameters such as normalized jerk-costs, velocity profile, and variance of movement trajectories near the endpoint of movement, dental malocclusions were associated with significantly lower skillfulness of masticatory jaw motion, whereas good occlusion and orthodontically improved occlusion (either with or without premolar extraction) were both associated with more skillful motion.