Lip and jaw motor control during speech: responses to resistive loading of the jaw.

Resistive loads were applied to the jaw during speech production. Loads were initiated during the jaw closing movement associated with the production of bilabial stops, creating a situation in which bilabial closure would be disrupted if motor control were independent of peripheral feedback. Three subjects were observed during control and experimental conditions. In all utterances in which a load was appropriately introduced, closure of the lips was achieved and the bilabial stop was adequately produced. To assess the nature of this control, displacement of the upper lip, lower lip, and jaw, in the inferior-superior dimension, were recorded along with EMG from medial pterygoid, anterior temporalis, masseter, and orbicularis oris superior muscles. Based on observation of these variables, it appears that the muscles of the lips and jaw are capable of on-line compensatory motor reorganization.