1 2 3 4 5 Unique characteristics of motor adaptation during walking in young children 6 By 7

30 Children show precocious ability in the learning of languages; is this the case with motor 31 learning? We used split-belt walking to probe motor adaptation (a form of motor 32 learning) in children. Data from 27 children (aged 8-36 months) were compared with 33 that from 10 adults. Children walked with the treadmill belts at the same speed (tied34 belt), followed by walking with the belts moving at different speeds (split-belt) for 8-10 35 min, followed again by tied-belt walking (i.e., post-split). Initial asymmetries in temporal 36 coordination (i.e., double support time) induced by split-belt walking were slowly 37 reduced, with most children showing an aftereffect (i.e., asymmetry in the opposite 38 direction to the initial) in the early post-split period, indicative of learning. In contrast, 39 asymmetries in spatial coordination (i.e., center of oscillation) persisted during split-belt 40 walking and no aftereffect was seen. Step length, a measure of both spatial and temporal 41 coordination, showed intermediate effects. The time course of learning in double support 42 and step length was slower in children than in adults. Moreover, there was a significant 43 negative correlation between the size of the initial asymmetry during early split-belt 44 walking (called error) and the aftereffect for step length. Hence, children may have more 45 difficulty learning when the errors are large. The findings further suggest that the 46 mechanisms controlling temporal and spatial adaptation are different, and mature at 47 different times. 48 49