Bipedal walking was studied in chimpanzees between 1 and 19 years of age by measuring the external energy generated by the movement of the centre of gravity of the body. The ‘recovery’ of external energy in infant chimpanzees less than five years of age is smaller than in juveniles from 5 to 19 years of age. During the single stance phase, the short, flexed hindlimb of chimpanzees younger than 5 years is not able to lift the body centre of gravity high enough, so that these infants have a considerable energy output during bipedal walking. Extension of the hindlimb is one of the bases for energy economy in human bipedalism. The development of this low-energy-cost mechanism is an important component of the evolution of human bipedal