The resonant step frequency in human running

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.