Muscle force recruitment and biomechanical modeling: An analysis of masseter muscle function during mastication in Macaca fascicularis

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Abstract
The main purpose of this study is to test the hypothesis that as subjects chew with increasing levels of force, the ratio of the working- to balancing-side jaw-muscle force (W/B) decreases and begins to approach 1.0. We did this by analyzing relative masseter force in Macaca fascicularis using both strain gage and surface electromyographic (EMG) techniques. In addition, we also analyzed: 1) the relationship between jaw position using cineradiographic techniques and relative masseter force, 2) the timing differences between relative masseter force from the working and balancing sides, and 3) the loading and unloading characteristics of the masseter muscle. Our findings indicate that when macaques increase the amount of overall masticatory force during chewing, the W/B ratio for masseter force frequently (but not always) decreases and begins to approach 1.0. Therefore, our working hypothesis is not completely supported because the W/B ratio does not decrease with increasing levels of force in all subjects. The data also demonstrate timing differences in masseter force. During apple-skin mastication, the average peak masseter force on the working side occurs immediately at or slightly after the initial occurrence of maximum intercuspatiori, whereas the average peak masseter force on the balancing side occurs well before maximum intercuspation. On average, we found that peak force from the balancing-side masseter precedes the working-side masseter by about 26 msec. The greater the asynchrony between working- and balancing-side masseter force, the greater the difference in the relative magnitude of these forces. For example, in the subject with the greatest asynchrony, the balancing-side masseter had already fallen to about one-half of peak force when the working-side masseter reached peak force. Our data also indicate that the loading and unloading characteristics of the masseter differ between the working and balancing sides. Loading (from 50 to 100% of peak force) and unloading (from 100 to 50% of peak force) for the balancing-side masseter tends to be rather symmetrical. In contrast, the working-side masseter takes much longer to load from 50 to 100% of peak force than it does to unload from 100 to 50% of peak force. Finally, it takes on average about 35 msec for the working-side zygoma and 42 msec for the balancing-side zygoma to unload from 100 to 50% of peak force during apple-skin mastication, indicating that the unloading characteristics of the macaque masseter during mastication closely approximates its relaxation characteristics (as determined by muscle stimulation).