Fig. 8. The mass-specific external work done at each step (A) and the step frequency (B) are plotted as a function of running speed for the experiments made at 1 g (open circles) and at 1.3 g (filled circles). It can be seen that, on average, step frequency is increased by gravity more than the work per step: in other words, an increase in gravity increases the mechanical power output mainly through an increase in step frequency. Lines are least-squares linear regressions (1 g) or weighted mean (1.3 g) of all the data (Kaleidagraph 3.6.4).