Infants Adapt Sitting to a Decreasing Base of Support

As infants acquire postural control, they learn to keep balance over a progressively smaller base of support. We systematically tested the development of postural control from a floor sit, where infants' thighs and lower legs are supported by the ground, to a bench sit, where infants' lower legs are unsupported. Six- to 9-month-old infants (N = 25) began in a floor sit with their thighs on a stationary platform and lower legs on a platform that was lowered in 2-cm increments until infants were in a bench sit with feet dangling in the air or they lost balance and fell. Each decrease in the height of the bottom platform decreased infants' base of support and made postural control more challenging. Although bench sitting was presumably novel, all infants maintained balance in a bench sit if their feet rested on the ground. However, infants' trunk control predicted their ability to maintain balance with their feet dangling in the air. Infants with more trunk control compensated for decreased support by progressively increasing their trunk-thigh angle (leaning backward rather than forward) and hence keeping their center of mass within their decreasing base of support. Thus, improvements in trunk control provide infants with more efficient postural-control strategies to cope with novel threats to balance. We suggest that flexibility and adaptability are integral to learning to sit and prepare infants for the next challenges in motor development.