The rise and fall of the plantar response in infancy.

To determine when the plantar response becomes reliably flexor in infants, 169 infants underwent serial evaluation from 2 weeks to 12 months of age during routine well-child visits. The plantar response, elicited in a standardized fashion, was extensor in 95.5%, 64.8%, 10.9%, 0.7%, and 0% of the infants at 2, 4, 6, 9, and 12 months, respectively. The plantar response becomes predominately flexor by 6 months of age in normal infants. This maturation of response correlates closely with that of other infantile reflexes.