Cobo-Lewis, A. B., Oller, D. K., Lynch, M. P., & Levine, S. L. (1996). Relations of motor and vocal milestones in typically developing infants and infants with Down Syndrome. American Journal on Mental Retardation, 100, 456–467.

We measured the ages at which typically developing infants and infants with Down syndrome achieved an important vocal milestone (canonical babbling), a rhythmic motor milestone (hand-banging), and six other motor milestones. The interrelations of the milestone onsets, and their relations with Down syndrome, were assessed quantitatively. Hand-banging and canonical babbling were associated and were somewhat delayed by Down syndrome. Stepping, standing, sitting, and creeping/crawling were associated and were severely delayed by Down syndrome. Rolling and reaching were also delayed by Down syndrome, though they were not strongly associated with other milestones or with one another. These results suggest that the rhythmic behaviors (canonical babbling and hand-banging) may be internally linked by common neuromuscular underpinnings and that the postural behaviors may be similarly linked.