In order to contribute to a more comprehensive model of speech motor development, we examined the movement trajectories of the upper lip, lower lip, and jaw to determine (a) if there are changes in articulatory motor control in late adolescence; (b) if there are sex differences during this developmental period, perhaps related to differences in craniofacial growth rates; (c) if control of jaw motion is adultlike earlier than control of the upper and lower lip; and (d) if control of spatial and temporal aspects of articulatory movement co-develop in adolescence. Participants were 12-, 14-, and 16-year-olds, and young adults (mean age 21.2 years), with 15 males and 15 females per group. A measure reflecting spatiotemporal consistency in trajectory formation for repeated productions of a phrase was calculated for the upper lip, lower lip, and jaw movements. Overall trajectory variability was higher for adolescents compared to young adults. Jaw trajectories were less variable than upper lip or lower lip trajectories, but all effectors showed parallel decreases in variability as age increased, suggesting that control of jaw movement does not reach adult performance before control of the lips. Separate temporal and spatial measures revealed that adolescents had significantly longer movement durations, lower velocities, smaller displacements, and greater variability on these measures than young adults. There were no sex differences on any measure examined, suggesting that peripheral growth factors do not account for this protracted developmental time course. These results provide initial evidence of significant changes in speech motor control processes during adolescence.
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