Variability of lip and jaw movements in children and adults: implications for the development of speech motor control.

The development of speech motor skill was studied by measuring the variability of lower lip and jaw movements. Groups of five adults, and children at ages 4, 7, and 10 years produced [mae] and [bae] 20 times each. The duration of lip-opening movements, jaw-opening movements, lip-open postures, jaw-open postures, and the timing between the onset of lower lip opening and jaw opening decreased in variability between the child and adult groups. No significant differences were observed in the variability of these measures across the child groups. The variability of lower lip displacement decreased significantly between the 4-year-old and 7-year-old groups, but not between any other age groups. Jaw displacement variability did not change significantly between any groups. No significant differences in variability were found between [bae] and [mae]. It is hypothesized that different developmental motor processes affect the variability of speech movements at early, intermediate, and older ages.