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. Th ese data can help resolve the question of whether African apes and hum ans 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 c ommon in other primates (Meldrum [1993] Am. J. Phys. Anthropol. 91:379 -381). Using biplanar and uniplanar videotapes, we recorded the freque ncy 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 Africa n apes exhibit a ''heel-strike'' at the end of swing phase. Ateles and Hylobates make heel contact on all supports shortly after mid-foot co ntact, although spider monkeys do so only at slow or moderate speeds. Data available from uniplanar videotapes suggest that this pattern occ urs in Alouatta and Lagothrix as well. No other New or Old World monke y or prosimian in this study made heel contact during quadrupedalism o n any substrate. Thus, heel contact occurs in all apes and atelines, b ut only the great apes exhibit a heel-strike. We suggest that heel con tact with the substrate is a by-product of an active posterior weight- shift mechanism involving highly protracted hindlimbs at touchdown. Fo rce plate studies indicate that this mechanism is most extreme in arbo really adapted primate quadrupeds walking on arboreal supports. Althou gh heel contact and heel-strike may have no evolutionary link, it is p ossible that both patterns are the result of a similar weight shift me chanism. Therefore, the regular occurrence of heel contact in a variet y of arboreal primates, and the absence of a true biomechanical link b etween limb elongation, heel contact, and terrestriality, calls into q uestion the claim that hominid foot posture was necessarily derived fr om a quadrupedal terrestrial ancestor. (C) 1995 Wiley-Liss, Inc.