Symmetric force response of human masticatory muscles to stretch and unloading

During chewing, the force exerted by the jaw-closing muscles must constantly adapt to changing resistances between the teeth, as the food is broken down. In the present study, the changes in biting force resulting from small, controlled displacements imposed on isometrically contracting jaw-closing muscles were measured. We found that the force changes resulting from small loading and unloading movements were normally highly symmetrical. The initial force change was linear, and preceded the onset of reflex changes in muscle activity. Later changes in force were the result of both short- and long-latency reflexes in the jaw-closing muscles, the long-latency component being quantitatively greater. The long-latency unloading reflex in the jaw-closing muscles has not been described hitherto. The symmetry of force increase with loading and decrease with unloading was absent in one subject with atypical stretch and H-reflexes.