Effect of Speed, Step Frequency and Age on the Bouncing Step

Abstract

This chapter shows how the on-off-ground asymmetry and the landing-takeoff asymmetry of the rebound of the body change with running speed, step frequency and age. An increase in running speed causes an increase of the on-off-ground asymmetry and a decrease of the landing-takeoff asymmetry, suggesting that the length change of tendon versus that of muscle in the stretch-shorten cycle of muscle-tendon units increases with speed. At low and intermediate running speeds the freely chosen step frequency equals the resonant frequency of the bouncing system, coincides with the frequency minimizing the metabolic energy expenditure, but is lower than the frequency minimizing the mechanical power output, i.e. metabolic energy is saved by tuning step frequency to the resonant frequency even if this requires a greater mechanical power. At high running speeds a compromise is attained to minimize the aerobic power using longer leaps at a low step frequency within the limit set by the anaerobic-limited push-average power allowing these leaps. In children, as in adults, the freely chosen step frequency equals the natural frequency of the bouncing system up to ~11 km/h, although it decreases with age from 4 Hz at 2 years to 2.5 Hz above 12 years. Above ~11 km/h, the rebound becomes on-off-ground asymmetric in children as in adults. In the old subjects, on the contrary, the bounce is on-off-ground symmetric at all running speeds. The landing-takeoff asymmetry is greater in the oldest than in the youngest, qualitatively consistent with the more asymmetric force-velocity relation described in aged muscle.