Fig. 4. Metabolic rate increased over fourfold with frequency of isolated leg swinging, for motion faster than the leg's natural frequency. (A) vs frequency of leg swinging f, showing metabolic rate increasing approximately with f ⁴ as predicted by force/time hypothesis (Equation 12). Data shown are for all frequencies applied, but the curve fit was only performed on data for fast leg swinging. (B) vs , showing metabolic rate increasing approximately linearly with the hypothesized force/time cost (Equation 13). Rate of mechanical work is also likely to contribute, but cannot accurately be distinguished from force/time in overall metabolic cost.