Stability and change in children's skill.

This paper explores age-related performance variability, both within trials and between sessions, in repetitive hopping. The mean, the standard deviation (SD), and the coefficient of variation (CV) of several timing and ground-reaction-force variables of hopping were analysed by repeated-measures ANOVA for age-related effects across test sessions and foot used. Forty-five subjects in five age groups (3-4 years, 4-5 years, 6-7 years, 8-9 years, or Adult) performed self-paced, one-footed hopping on three occasions within one week. As was expected, the results showed main effects for Age in all force and time variables, with the exception of CV of medio-lateral force. No significant main effects for Feet were revealed. However, significant Feet x Session interactions were found in flight-time measures, with higher flight-time SD and lower CV for the non-preferred foot in Session 1, a reversal in Session 2, and a negligible difference in Session 3. Across sessions, decreased SD and CV for both vertical and medio-lateral force and shorter flight time indicated more efficient hopping. Overall, it was concluded that SD and CV measures were more sensitive measures of children's performance across repeated sessions than were mean scores and that the order of testing the limbs is an important consideration in experimental protocols when lateralized tasks are measured.