The resonant step frequency in human running

Abstract

 At running speeds less than about 13 km h^–1 the freely chosen step frequency (f _free) is lower than the frequency at which the mechanical power is minimized (f _min). This dissociation between f _free and f _min was investigated by measuring mechanical power, metabolic energy expenditure and apparent natural frequency of the body’s bouncing system (f _sist) during running at three given speeds with different step frequencies. The f _free requires a mechanical power greater than that at f _min mainly due to a larger vertical oscillation of the body at each step. Energy expenditure is minimal and the mechanical efficiency is maximal at f _free. At a given speed, an increase in step frequency above f _free results in an increase in energy expenditure despite a decrease in mechanical power. On the other hand, a decrease in step frequency below f _free results in a larger increase in energy expenditure associated with an increase in mechanical power. When the step frequency is forced to values above or below f _free, f _sist is forced to change similarly by adjusting the stiffness of the bouncing system. However the best match between f _sist and step frequency takes place only in proximity of f _free (2.6–2.8 Hz). It is concluded that during running at speeds less than 13 km h^–1 energy is saved by tuning step frequency to f _sist, even if this requires a mechanical power larger than necessary.