Strength of infants' bimanual reaching patterns is related to the onset of upright locomotion

Infants typically exhibit a shift from unimanual to bimanual reaching toward the end of their first year, which has been linked to walking onset. Until now, however, it has been unclear whether it was the onset of walking per se that influenced reaching patterns or whether a more general shift to an upright posture might have prompted the reorganization of the motor system. To address this question, the current study longitudinally chronicled the uni- and bimanual reaching preferences of 25 infants every 3 weeks starting at 7 months, prior to the onset of pulling-to-stand and through the onset of cruising. Experimenters recorded infants' reaching behavior via a semi-structured reaching procedure and documented their motor development. There was no relationship between the shift from uni- to bimanual reaching and the onset of pulling-to-stand. However, the onset of cruising was related to a shift reaching pattern preference, suggesting that the increase infants' bimanual reaching was prompted by a reorganization of the motor system which the arms are recruited for use new ways to support locomotion. We also discuss individual differences the trajectory of reaching activity terms of the pitfalls of using age as an explanatory variable.