Mechanical power and efficiency in running children

Abstract

The effect of age and body size on the total mechanical power output (Ẇ_tot) during running was studied in children of 3–12 years of age and in adults. Ẇ_tot was measured as the sum of the power required to move the body's centre-of-mass relative to the surroundings (the "external power", Ẇ_ext) plus the power required to move the limbs relative to the body's centre-of-mass (the "internal power", Ẇ_int). At low and intermediate speeds (less than about 13 km h^–1) the higher step frequency used by young children resulted in a decrease of up to 40–50% in the mass-specific external power and an equal increase in the mass-specific internal power relative to adults. Due to this crossed effect, the mass-specific Ẇ_tot is nearly independent of age. At high speeds the mass-specific Ẇ_tot is 20–30% larger in young children than in adults, due to a greater forward deceleration of the centre-of-mass at each step. The efficiency of positive work production, calculated as the positive mechanical power divided by the net energy consumption rate, appears to be similar in children and adults (i.e. 0.40–0.55).