Walking speed as a function of gravity (expressed as a fraction of the terrestrial one). Thick parabola arcs represent the prediction of dynamic similarity according to the Froude number equation:

where l = 0.92 m (average leg length for adult males) and Fr has been set to 0.25 (maximum pendulum-like energy exchange, optimal walking speed on Earth) and to 0.5 (walk-run transition speed on Earth). The thin curve shows the physical limit to walking speed (Fr = 1.0) as imposed by the balance, in an inverted pendulum, between the centrifugal (m.v2.l-1) and the gravitational (m.g) force, necessary to maintain the contact with the ground. Square and open circle symbols refer to the measurements in reduced simulated gravity of the optimal walking speeds and walk-run transition speed, respectively. A previous estimate of speed reduction on the Moon is shown by the open triangle. Filled circles represent recent results of optimal speed obtained by using parabolic flight simulating hypo/hypergravity conditions.