The mechanics of running in children

1 The effect of age and body size on the bouncing mechanism of running was studied in children aged 2‐16 years. 2 The natural frequency of the bouncing system (fs) and the external work required to move the centre of mass of the body were measured using a force platform. 3 At all ages, during running below ≈11 km h−1, the freely chosen step frequency (f) is about equal to fs (symmetric rebound), independent of speed, although it decreases with age from 4 Hz at 2 years to 2.5 Hz above 12 years. 4 The decrease of step frequency with age is associated with a decrease in the mass‐specific vertical stiffness of the bouncing system (k/m) due to an increase of the body mass (m) with a constant stiffness (k).Above 12 years, k/mand fremain approximately constant due to a parallel increase in both kand mwith age. 5 Above the critical speed of ≈11 km h−1, independent of age, the rebound becomes asymmetric, i.e. f< fs. 6 The maximum running speed (Vf,max) increases with age while the step frequency at remains constant (≈4 Hz), independent of age. 7 At a given speed, the higher step frequency in preteens results in a mass‐specific power against gravity less than that in adults. The external power required to move the centre of mass of the body is correspondingly reduced.