Dynamics of postural control in the child with Down syndrome.

We examined the development of neural control processes underlying stance balance in both developmentally normal children and children with Down syndrome to test the hypothesis that motor deficiencies in children with Down syndrome are associated with deficits within the automatic postural control system. We compared children with Down syndrome and developmentally normal children in two age groups (1-3 and 4-6 years) by using displacements of a platform and measuring electromyograms from leg muscles. The automatic muscle response pattern in both normal children and children with Down syndrome were directionally specific, although the pattern were more variable than in adults. Responses in children with Down syndrome showed no adaptive attenuation to changing task conditions. Onset latencies of responses in children with Down syndrome were significantly slower than in normal children. Presence of the monosynaptic reflex during platform perturbations at normal latencies suggests that balance problems in children with Down syndrome do not result from hypotonia, which researchers have defined as decreased segmental motoneuron pool excitability and pathology of stretch reflex mechanisms, but rather result from defects within higher level postural mechanisms.