Heel contact as a function of substrate type and speed in primates.

In this report we provide detailed data on the patterns and frequency of heel contact with terrestrial and arboreal supports in primates. These data can help resolve the question of whether African apes and humans are uniquely "plantigrade" (Gebo [1992] Am. J. Phys. Anthropol. 89:29-58; Gebo [1993a] Am. J. Phys. Anthropol. 91:382-385; Gebo [1993b] Postcranial Adaptation in Nonhuman Primates), or if plantigrady is common in other primates (Meldrum [1993] Am. J. Phys. Anthropol. 91:379-381). Using biplanar and uniplanar videotapes, we recorded the frequency and timing of heel contact for a variety of primates (32 species) walking on the ground and on simulated arboreal supports at a range of natural speeds. Our results indicate that Pongo as well as the African apes exhibit a "heel-strike" at the end of swing phase. Ateles and Hylobates make heel contact on all supports shortly after mid-foot contact, although spider monkeys do so only at slow or moderate speeds. Data available from uniplanar videotapes suggest that this pattern occurs in Alouatta and Lagothrix as well. No other New or Old World monkey or prosimian in this study made heel contact during quadrupedalism on any substrate. Thus, heel contact occurs in all apes and atelines, but only the great apes exhibit a heel-strike. We suggest that heel contact with the substrate is a by-product of an active posterior weight-shift mechanism involving highly protracted hindlimbs at touchdown. Force plate studies indicate that this mechanism is most extreme in arboreally adapted primate quadrupeds walking on arboreal supports. Although heel contact and heel-strike may have no evolutionary link, it is possible that both patterns are the result of a similar weight shift mechanism. Therefore, the regular occurrence of heel contact in a variety of arboreal primates, and the absence of a true biomechanical link between limb elongation, heel contact, and terrestriality, calls into question the claim that hominid foot posture was necessarily derived from a quadrupedal terrestrial ancestor.