Task-dependence of jaw elevator and depressor co-activation.

Elevator muscle activity per unit bite-force has been shown to be higher in chewing than in isometric biting. We tested the hypothesis that surplus elevator activity is evoked in response to a possible co-activation of jaw-opener muscles during the masticatory power stroke. In 32 subjects, digastric and bilateral masseter and temporalis activities were recorded during unilateral chewing of test foods, isometric biting on a force transducer, and during balancing of the jaw against maximum effort of depressor muscles. During elevator peak effort in chewing, the digastric activity was 113% higher than during peak effort in isometric biting. Comparison of balancing and chewing trials revealed that a 6% increase of elevator activity would suffice to compensate for this increased depressor action. Elevator activity in chewing, however, was up to 130% higher than in clenching. We conclude that depressor counteraction could have only a minor influence on the generation of surplus muscle activity in chewing.

Preconditions for estimation of masticatory forces from dynamic EMG and isometric bite force-activity relations of elevator muscles.

PURPOSE: Previous studies proposed to estimate masticatory forces from electromyograms (EMG) by substituting chewing activities of a particular muscle into bite force-activity relations obtained from isometric clenching. For biomechanical reasons, this method requires that ratios between muscle forces in chewing be the same as in clenching. The purpose of this study was to investigate whether this assumption is valid. MATERIALS AND METHODS: In 32 asymptomatic subjects, EMGs of bilateral temporal and masseter muscles were registered in unilateral clenching on a bite fork and in chewing. Ratios of peak activities of different muscles were compared between the two biting tasks. RESULTS: In clenching, the working: balancing ratio of the masseters amounted to 0.92, while in chewing this ratio increased to 1.9. The working:balancing ratio of the temporalis was 1.8 in clenching and dropped to 1.3 in chewing. The temporalis:masseter ratio on the balancing side increased from 0.65 in clenching to 1.1 in chewing. CONCLUSION: The finding that ratios of muscle activities in chewing differed from corresponding ratios in clenching implies different combinations of muscle forces and/or different bite force-activity relations in the two biting tasks. In any of these cases, prediction of chewing force from the EMG of a particular muscle could lead to indefinite overestimations.