Inertial properties of hominoid limb segments

Quantitative, accurate data regarding the inertial properties of body segments are of paramount importance when developing musculo-skeletal locomotor models of living animals and, by inference, their ancestors. The limited number of available primate cadavers, and the destructive nature of the post-mortem, result in such data being very rare for primates. This study builds on the work of Crompton et al. (Am. J. Phys. Anthropol. 1996, 99, 547-570) and reports inertial properties of the body segments of gorillas, chimpanzees, orangutans and gibbons. Segment mass, centre of mass and the radius of gyration of five ape cadavers were measured using a complex-pendulum technique and compared with the results derived from external measurements of segment lengths and diameters on the same animals. With additional data from external measurements of eight more hominoid cadavers, and published data, intergeneric differences between the inertial properties and the distribution of mass between limb segments are analysed and related to the locomotor habits of the species. We found that segment inertial properties show extensive overlap between ape genera as a result of large interindividual variation. Segment mass distribution also overlaps between apes and humans, with the exception of the shank segment. However, owing to a different distribution of mass between the limb segments, the centre of mass of both the arms and the legs is located more distally in apes than in humans, and the natural pendular period of ape forelimbs is larger than that of the hindlimbs. This suggests that, in contrast to the limbs of cursorial mammals and cercopithecoid primates, hominoid limbs are not optimized for efficiency in quadrupedal walking, but rather reflect a compromise between various locomotor modes. Common chimpanzees may have secondarily evolved a more efficient quadrupedal gait.