Electromyographic activity from the jaw-adductor muscles was recorded during mastication and bone crushing in domestic dogs (Canis familiaris). During mastication, balancing-side temporalis electromyographic activity was much less than that of the working side while masseter muscle electromyographic activities were of similar amplitude. Despite the large bite forces that were produced during bone crushing, balancing-side masseter and temporalis electromyographic activities were always smaller than the working-side electromyographic amplitudes. Based upon geometric modelling, it is proposed that balancing-side muscle activity is reduced because of its tendency to produce mechanically disadvantageous jaw-joint forces. This hypothesis was tested by correlating bone strain adjacent to the jaw joint measured during manipulations of the mandibular condyle with bone strain recorded during the simulation and stimulation of jaw-adductor muscle activity. Working-side muscle activity produced bone strain that correlated with a compressive joint loading, while balancing-side muscle activity, with an occlusal fulcrum at the carnassial teeth, produced bone strain indicative of an anteroventral movement of the working-side mandibular condyle which eventually ruptured the joint capsule. When the temporalis and masseter muscles were stimulated bilaterally with a carnassial bite point, bone strain indicative of a ventrally directed and potentially damaging condylar movement was produced. It is proposed that working-side muscle activity exceeds balancing-side muscle activity during carnassial biting to maintain jaw-joint stability.
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