Interactions in the labial musculature during speech.

Interactions in electromyographic activity of the upper and lower lips during speech were studied by manipulating the magnitude of bursts of activity related to bilabial closure. Four pairs of electrodes were placed in the labial musculature in each of four normal-speaking young adults. Manipulation of muscle activity usually resulted in positively correlated changes in activity recorded from the other three electrode pairs. Similar effects were found when lower lip muscle activity was manipulated and when upper lip muscle activity was manipulated, suggesting there is no asymmetry in the interactions between lips. Measurements of lower lip closing movement often correlated with the modulated muscle activity, suggesting that the size of lip opening was varied to accommodate different closing forces. The flexibility of a system producing positive correlations in the activity of different labial regions is discussed and contrasted with the suggestions that there are complementary variations, producing negative correlations, in the activity of different labial muscles during speech.