New method of three‐dimensional analysis of bipedal locomotion for the study of displacements of the body and body‐parts centers of mass in man and non‐human primates: Evolutionary framework

Abstract

The current biomechanical interpretation of the chimpanzee's bipedal walking argues that larger lateral and vertical displacements of the body center of mass occur in the chimpanzee's "side-to-side" gait than in the human striding gait. The evolutionary hypothesis underlying this study is the following: during the evolution of human bipedalism one of the necessary changes could have been the progressive reduction of these displacements of the body center of mass. In order to quantitatively test this hypothesis, it is necessary to obtain simultaneously the trajectories of the centers of mass of the whole body and of the different body parts. To solve this problem, a new method of three-dimensional analysis of walking, associated with a volumetric modelling of the body, has been developed based on finite-element modeling. An orthogonal synchrophotographic device yielding four synchronous pictures of the walking subject allows a qualitative analysis of the photographic sequences together with the results of their quantitative analysis. This method was applied to an adult man, a 3-year-old girl and a 9-year-old male chimpanzee. Our results suggest that the trajectory of the body center of mass of the human is distinguished from that of the chimpanzee not by a lower movement amplitude but by the synchronization of the transverse and vertical displacements into two periodic curves in phase with one another. The non-human primate uses its repertoire of arboreal movements in its bipedal terrestrial gait, provisionally referred to as a "rope-walker" gait. We show that the interpretation of a "side-to-side" gait is not applicable to the chimpanzee. We argue that similarly this interpretation and the initial hypothesis presuppose a basic symmetric structure of the gait, in relation to the sagittal plane of progression, similar to the human one. This lateral symmetry of the right and left displacements of the center of gravity, in phase with the right and left single supports of walking, is probably a very derived feature of the human gait. We suggest that low lateral and vertical displacements of the body center of mass are not indicative of a progressive bipedal gait and we discuss the new evolutionary implications of our results.

The dynamics of hylobatid bipedalism: evidence for an energy-saving mechanism?

“…Taking size differences into account, gibbons (with a hind limb length of 0.37·m) seem to have relatively larger vertical oscillations of the COM than during human walking (with an average human hind limb length of 0.85·m). Thus there is no evidence for a flattened path of the COM, or the so-called 'rope-walker' pattern (Tardieu et al, 1993), during overground bipedalism of gibbons. It might, however, be possible that gibbons adopt a modified compliant gait during arboreal bipedalism, although there are no indications to suspect that gibbons alter their gait dynamics to substrate type (see also Alexander, 1991b;Bonser, 1999).…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…During bipedal walking when the body is supported by a single leg, the pelvis has a tendency to tip towards the unsupported side. When apes walk bipedally, they compensate by leaning their trunk towards the supported side or stretching out their arms [30,31], but such side-to-side weight shifts are energetically costly [30]. In humans, the gluteal muscles on the support side are able to balance the trunk more efficiently by pulling up the unsupported side of the pelvis [22,23,32].…”

Legs evolved only at the end!

“…Those trunk motions may be dominated by the need of guiding the CoM in a potential field formed by gravity and elasticity of hip muscles (this hypothesis may be derived from the observations of Tardieu 1990a,b;Tardieu et al 1993), or it may serve the placement of the CoM relative to the point of ground contact; in either case, legs and arms have to be looked at as servants of the trunk. Extremities have to mediate between the task to carry the trunk through potential fields and the boundary conditions offered by the environment.…”