Neuromuscular control of balancing side contacts in unilateral biting and chewing

When jaw gape in unilateral biting or chewing narrows, the working/balancing side activity ratio (W/B ratio) of masseter muscles increases due to decrease of balancing side (BS) activity. This was interpreted as a neuromuscular strategy to delimit the impact of BS contacts during chewing. To test this hypothesis, we studied whether W/B ratios are associated with incidence of BS tooth contacts. In 40 healthy subjects, bilateral masseter activity was recorded during unilateral biting with different jaw gapes and during various chewing tasks. Biting was performed with absence and with deliberate avoidance or generation of BS tooth contacts. Subjects were divided into three groups according to jaw gapes of 2, 1, and 0.5 mm for which BS contact was first noticed in strong biting. The smaller this gape was, the higher were the mean W/B ratios. In biting with contact avoidance, the W/B ratios in each group increased with decreasing gape. In biting with generation of BS contacts, W/B ratios were smaller than with contact avoidance. W/B ratios in chewing with minimum interocclusal distances below 0.5 mm were bigger than in biting with contact generation and were mostly bigger than in biting with contact avoidance. The findings confirm that increasing the masseter W/B ratio is a neuromuscular measure suitable to avoid BS contacts and support the idea that motor control uses jaw gape-related activation to limit the impact of BS contacts. Clarification of this protection mechanism might contribute to uncover the etiology of functional disorders and occlusal malfunctions.