Early development of postural adjustments in standing with and without support.

This study investigates the early development of postural adjustments during external perturbations in two different standing positions: standing with support and standing without support. The aim of the study was to assess a group of 13 infants four times during the period in life when independent standing is achieved; at 8, 10, 12 and 14 months. However, longitudinal data could be achieved only in four infants. Muscle activations of the neck, hip and ankle were recorded using surface electromyography. Based on earlier studies and controversies, three main issues were addressed: (1) Is direction specificity present before independent standing is established? (2) How do postural adjustments change with increasing age (8-14 months)? (3) Are postural adjustments task-specific in the young child? The results showed that our small sample of infants aged 8 and 10 months, who were not yet able to stand independently, exhibited direction-specific postural adjustments both during standing with and without support, though not consistently during all trials and at all body levels. Therefore, we argue that direction specificity might constitute a prerequisite for the development of independent standing. We also found that the development of postural adjustments in standing with support resembles that of sitting, i.e. great variation in the postural adjustments at early age, and fine-tuning to the situation with increasing age and experience. This, we find that this is in agreement with the proposal that postural control develops through a selection process of the most suitable postural adjustments for the situation from a repertoire of direction-specific postural adjustments. The development of postural adjustments during standing without support is discussed. Additionally, differences in response rates were noted between the two standing positions, indicating that even before independent standing is established, sophisticated sensorimotor integration enables task-specific postural adjustments.