The organization of leg movements in preterm and full-term infants after term age.

In a sample of 13 full-term and 10 preterm infants, the development of kicking movements was studied at 6, 12, and 18 weeks (corrected) age. In healthy full-term infants some characteristics are strikingly stable, such as the duration of the flexion and extension phase and the within-joint organization. These parameters did not differ in preterm compared to full-term infants. For other features, however, developmental changes and differences were observed. Full-term infants tended to decrease their kick frequencies slightly with age. In preterm infants much higher initial kick rates were found, followed by a steep decrease, which resulted in kick frequencies comparable to the full-term levels after the (corrected) age of 12 weeks. There is a tight coupling between the movements in the different joints of the leg in full-term newborns. Preterm infants, in contrast, initially show much lower cross-correlations between hip and ankle and between knee and ankle. This is particularly the case for those preterm infants who were born before 32 weeks gestation. Again, the differences resolved after the age of 12 weeks, which might be related to a transformation in neural functions reported previously around this age. The initial differences in the characteristics of kicking appeared to be more readily explainable by differences in neurological condition than by contrasts in leg volume or postural control.