Spontaneous kicking behavior in infants: age-related effects of unilateral weighting.

The age-related effects of unilateral weighting on spontaneously generated kicks in 18 healthy, full-term infants were investigated. The main question was whether infants during the first half-year after birth reveal changes in how they adjust to unilateral weighting. At 6 weeks, infants reduced the frequency of the weighted leg and increased that of the unweighted leg whereas at 12 weeks the frequency of kicking increased in both legs. At both ages, unilateral weighting also resulted in differences on a number of kinematic parameters. By 18 and 26 weeks, such frequency and kinematic effects were no longer present. With regard to interlimb couplings, a clear pattern of bilateral coordination was only present at 26 weeks; these results suggest that the effects of unilateral weighting are not directly related to the tightness of interlimb couplings. The implications of these age-related differences for understanding developmental changes in the control of leg movements are discussed. It is suggested that the infants' improved ability to act in a task-specific manner as well as nonlinear changes in the musculo-skeletal system and fine-tuning processes at a neural level might be factors of importance.