Movements of the upper and lower lips during speech: interactions between lips with the jaw fixed at different positions.

Inferior-superior displacements of the upper lip, lower lip, and jaw were transduced with a strain-gauge system in 4 normal-speaking adults. Movements of the upper and lower lips were compared across conditions in which the jaw was free to move and when bite blocks were used to fix the jaw at four different vertical positions. As jaw-open position was increased with the bite blocks, it was found that: Positions of both lips changed for bilabial closure, but the closing movements did not usually maintain consistent proportions between lips across different bite-block sizes; although the lips maintained fairly consistent maximum interlabial opening across many conditions, this opening was reduced in the small bite-block conditions; and in a few cases there was an increase in the duration of lip-closing movements, but these were small and inconsistent. The findings are discussed relative to possible organizational systems that would produce the observed interactions among speech articulators.