Postural adjustments during sitting at preschool age: presence of a transient toddling phase.

The present study investigated developmental changes in postural adjustments during preschool age. Postural responses during sitting on a moveable platform were assessed in 21 healthy children aged 1 1/2 to 4 1/2 years. Multiple surface EMGs of neck, trunk, and leg muscles were recorded during forward and backward translations. Comparable data were available for 11 infants seen three times between the ages of 5 and 10 months. The data revealed the existence of a transient period between the ages of 9 to 10 months and 2 1/2 to 3 years, during which perturbations in a sitting position are accompanied by high activity in the direction-specific agonist muscles and in the antagonist muscles. After this period, agonist activity became more variable, particularly so during backward translations, and antagonist activity disappeared. These changes could be attributed to biomechanical factors and to maturation of the nervous system.