Why isn't motility most efficient at pendulum frequency?

3 reasons:

1. The power dissipated by a damped resonant system is not minimum, but maximum, at resonance:
       

   
   

2. Animal locomotion is certainly not resonant! —more like an inverted pendulum:
   

   
   

3. Empirically the frequencies don't agree with the calculated (inverted) pendulum ones:

   Predicted and observed stepping frequencies for l = 1 m Predicted | Observed Walk:   0.6 Hz   0.85–0.9 Hz Run:   1.13 Hz   1.65–2.0 Hz