This study investigated the effects of the changes in lower limbs length and body function with age on running ability. The subjects were 133 young boys (2 to 12yrs), 18 male students (18 to 19yrs), and 34 male sprinters (19 to 30yrs). Running abilities were measured by running velocity (RV), step length (SL), and step frequency (SF), respectively. To exclude the effects of lower limb length on running ability, dimensionless numbers; representing index of running velocity (IRV), index of step length (ISL) and index of step frequency (ISF) were calculated. RV and SL of young boys increased linearly with age (2 to 12yrs), but SF showed almost no change. Sprinters demonstrated higher RV and SL than students of similar age. However there were no differences in SF between the two subject groups. Although IRV and ISF increased linearly (r=0.778, p<0.001; r=0.719, p<0.001, respectively) with age (2 to 12yrs), ISL showed no such change (except below 6yrs). The above results indicated that increases in SL with age were caused by both the increasing length of the lower limbs and the improvement in body function (such as neuro-muscular function), and the constant SF with age was maintained by the improvement in body function which counteracts the increase of lower limb mass. Therefore, it was suggested that the development of running velocity with age was resulted from both the increasing length of lower limbs and the improvement in body function.
