Energy transformation during erect and 'bent-hip, bent-knee' walking by humans with implications for the evolution of bipedalism.

We have previously reported that predictive dynamic modeling suggests that the 'bent-hip, bent-knee' gait, which some attribute to Australopithecus afarensis AL-288-1, would have been much more expensive in mechanical terms for this hominid than an upright gait. Normal walking by modern adult humans owes much of its efficiency to conservation of energy by transformation between its potential and kinetic states. These findings suggest the question if, and to what extent, energy transformation exists in 'bent-hip, bent-knee' gait. This study calculates energy transformation in humans walking upright, at three different speeds, and walking 'bent-hip, bent-knee'. Kinematic data were gathered from video sequences and kinetic (ground reaction force) data from synchronous forceplate measurement. Applying Newtonian mechanics to our experimental data, the fluctuations of kinetic and potential energy in the body centre of mass were obtained and the effects of energy transformation evaluated and compared. In erect walking the fluctuations of two forms of energy are indeed largely out-of-phase, so that energy transformation occurs and total energy is conserved. In 'bent-hip, bent-knee' walking, however, the fluctuations of the kinetic and potential energy are much more in-phase, so that energy transformation occurs to a much lesser extent. Among all modes of walking the highest energy recovery is obtained in subjectively 'comfortable' walking, the next highest in subjectively 'fast' or 'slow' walking, and the least lowest in 'bent-hip, bent-knee' walking. The results imply that if 'bent-hip, bent-knee' gait was indeed habitually practiced by early bipedal hominids, a very substantial (and in our view as yet unidentified) selective advantage would have had to accrue, to offset the selective disadvantages of 'bent-hip, bent-knee' gait in terms of energy transformation.