Infant Behavior & Development 30 (2007) 16–25 Two steps forward and one back: Learning to walk affects infants’ sitting posture Li-Chiou Chen a , Jason S. Metcalfe a , John J. Jeka a,b , Jane E. Clark a,b,∗ a Department of Kinesiology, University of Maryland, College Park, MD 20742-2611, USA b Program in Neuroscience and Cognitive Science, University of Maryland, College Park, MD 20742-2611, USA Received 3 November 2005; received in revised form 18 April 2006; accepted 12 July 2006 Abstract The transition from sitting to walking is a major motor milestone for the developing postural system. This study examined whether this transition to walking impacts the previously established posture (i.e., sitting). Nine infants were examined monthly from sitting onset until 9 months post-walking. Infants sat on a saddle-shape chair either independently or with their right hand touching a stationary contact surface. Postural sway was measured by sway amplitude, variability, area, and velocity of the center of pressure trajectory. The results showed that for all the postural measures in the no-touch condition, a peak before or at walk onset was observed in all the infants. At the transition age, when peak sway occurred, infants’ postural sway measures were significantly greater than at any other age. Further, infants’ postural sway was attenuated by touch only at this transition. We suggest that this transient disruption in sitting posture results from a process involving re-calibration of an internal model for the sensorimotor control of posture so as to accommodate the newly emerging bipedal behavior of independent walking. © 2006 Elsevier Inc. All rights reserved. Keywords: Posture; Infant; Walking; Transition; Sensorimotor; Re-calibration 1. Introduction Infants first demonstrate sitting independently at 6 months of age and walking independently at 1 year (Bayley, 1993; Piper & Darrah, 1994). For the developing postural system, these two behaviors pose very different postural demands. In sitting, the infant’s head, trunk and arms are balanced over a very broad base of support with the center of mass close to the base. Once the infant rises to stand, the base narrows (over the two small feet), the number of body segments to be controlled increases (now including the thigh and shank), and the center of mass is considerably higher; all of which increase the postural challenge. The postural dynamics become even more complex for walking as the multi-segmented body moves over its changing base of support. In adults, upright postural control has recently been characterized as a combination of estimation and control (Kiemel, Oie, & Jeka, 2002; van der Kooij, Jacobs, Koopman, & van der Helm, 2001). Estimation, in this context, is the process in which sensory information from multiple sources is combined to give continuously updated estimates of body position and velocity (i.e., dynamics). Complementary to ∗ Corresponding author at: Department of Kinesiology, University of Maryland, College Park, MD 20742-2611, USA. Tel.: +1 301 405 2452; fax: +1 301 405 5578. E-mail address: jeclark@umd.edu (J.E. Clark). 0163-6383/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.infbeh.2006.07.005