The development of postural adjustments during reaching in 6-to 18-month-old infants - Evidence for two transitions

The present study focused on the developmental changes of postural adjustme nts accompanying reaching movements in healthy infants. We made a longitudi nal study of ten infants between 6 and 18 months of age. During each sessio n multiple surface electromyograms of arm, neck, trunk and leg muscles at t he right side of the body were recorded during right-handed reaching moveme nts in two positions ("upright sitting" in an infant chair and "long-leg" s itting without support). Simultaneously the whole session was recorded on v ideo, Comparable data were present from the same infants at 3-5 months. Add itionally, 18 infants (8-15 months) were assessed once during similar reach ing tasks, but in these infants electromyographic activity of the trunk and neck muscles at both sides of the body were recorded. Our data revealed tw o transitions in the development of postural adjustments. The first transit ion was present around 6 months of age. At this age the postural muscles we re infrequently activated during reaching movements. At 8 months ample post ural activity reappeared and the infants developed the ability to adapt the postural adjustments to task-specific constraints such as arm movement vel ocity or the sitting position at the onset of the reaching movement. The se cond transition occurred between 12 and 15 months. Before 15 months the inf ants did not show consistent anticipatory postural activity, but from 15 mo nths onwards they did, particularly in the neck muscles.