The development of infant upright posture: sway less or sway diVerently?

Postural control is an important factor for early motor development; however, compared with adults, little is known about how infants control their unperturbed upright posture. This lack of knowledge, particularly with respect to spatial and temporal characteristics of infants’ unperturbed independent standing, represents a signiWcant gap in the understanding of human postural control and its development. Therefore, our Wrst analysis oVers a thorough longitudinal characterization of infants’ quiet stance through the 9 months following the onset of independent walking. Second, we examined the inXuence of sensorymechanical context, light touch contact, on infants’ postural control. Nine typically developing infants were tested monthly as they stood on a small pedestal either independently or with the right hand lightly touching a stationary contact surface. In addition to the longitudinal study design, an age-constant sample was analyzed to verify the inXuence of walking experience in infant postural development without the confounding eVect of chronological age. Center of pressure excursions were recorded and characterized by distance-related, velocity, and frequency domain measures. The results indicated that, with increasing experience in the upright, as indexed by walk age, infants’ postural sway exhibited shifts in rate-related characteristics toward lower frequency and slower, less variable velocity oscillations without changing the spatial characteristics of sway. Additional touch contact stabilized infants’ postural sway as revealed by decrease in sway position variance, amplitude, and area as well as lower frequency and velocity. These results were conWrmed by the age-constant analysis. Taken together, our Wndings suggest that instead of progressively reducing the sway magnitude, infants sway diVerently with increasing upright experience or with additional somatosensory information. These diVerences suggest that early development of upright stance, particularly as it relates to increasing postural and locomotor experience, involves a reWnement of sensorimotor dynamics that enhances estimation of self-motion for controlling upright stance.