Weight support distribution during quadrupedal walking in Ateles and Cebus.

While it is well known that primate quadrupeds are distinctive in supporting more body weight on their hind limbs than their forelimbs, it has been unclear how this support asymmetry comes about. The simple explanation of a difference in body center of mass (COM) position is not supported by empirical data, leaving two less intuitive proposals. The first involves the position of the hands and feet relative to the COM, and has recently received support by Raichlen et al. (Am J Phys Anthropol 138 (2009) 395-402) who document that chimpanzees walk with a relatively protracted hind limb, bringing their feet closer to the COM for greater hind limb weight support. The second posits the use of hip retractor muscles to actively shift weight off the fore- limbs and onto the hind limbs, and has received recent support by Larson and Stern (Am J Phys Anthropol 138 (2009) 343-355), who report higher levels of hip extensor activity in primates that bear more weight on their hind limbs. Since the study by Raichlen et al. (Am J Phys Anthropol 138 (2009) 395-402) is based on a limited empirical dataset, we have undertaken a replicate study to document average limb posture and weight support distribution in two additional primate species: Ateles and Cebus. Using the equations presented by Raichlen et al. (Am J Phys Anthropol 138 (2009) 395-402), predicted forelimb weight support based on limb posture was com- pared to observed values based on vertical impulses. In contrast to their findings, the limb posture model dra- matically overestimated actual forelimb weight support in the monkeys, calling into question the general applic- ability of this model. Am J Phys Anthropol 144:633-642, 2011. V C 2011 Wiley-Liss, Inc.