Fig. 4. (A) Percent recovery of mechanical energy via the inverted-pendulum-like mechanism in walking as a function of the dimensionless Froude number. Data from the literature are presented for children and adults walking at a range of speeds in normal gravity, and for adults walking at a range of speeds in simulated reduced gravity. Percent recovery is similar at equal Froude numbers, suggesting that at equal Froude numbers the inverted-pendulum dynamics are mechanically equivalent. Child data from Cavagna et al. (1983) for 3–4, 7–8 and 11–12-year olds. Adult and reduced gravity data from Griffin et al. (1999). (B) Percent recovery for a biped (humans) and quadruped (dogs) versus Froude number for a range of walking speeds. Percent recovery decreases more precipitously at faster speeds for the quadruped compared with the biped at similar Froude numbers. Quadruped data are from Griffin et al. (2004), and biped data are for adults walking in normal gravity from Griffin et al. (1999).