Similarly, although t c for quadrupedal chimpanzees were slightly longer than for humans, they activated so much more muscle per unit of ground force that V musc/ t c was 72.8% (±4.6%) lower for humans than for quadrupedal chimpanzees, matching the 68.5% (±4.3%) difference in COL ( Figs. These differences caused a 79.4% (☑.6%) lower ratio of V musc/ t c, which corresponded closely to the observed 76.8% (☒.6%) decrease in locomotor cost ( Fig. When human walking was compared with chimpanzee bipedal walking, humans activated smaller muscle volumes per unit of ground force and used longer t c than bipedal chimpanzees ( Fig. This difference corresponds closely to the observed 32.2% (☓.2%) increase in COL during bipedal walking for these subjects ( Figs. In the three chimpanzees for which V musc were estimated (subjects C1-C3 see Methods), an increase in V musc and shorter t c increased V musc/ t c by 35.2% (±5.2%) during bipedal walking compared with quadrupedal walking. As predicted, differences in kinematics and estimated muscle activation explained observed differences in cost between bipedal and quadrupedal walking as well as between humans and chimpanzees. Given these results, we investigated potential biomechanical sources of the observed differences in cost among chimpanzees and between chimpanzees and humans. For three chimpanzees (C1-C3) bipedalism was 32.2% more expensive ( P < 0.001, Student's paired t test), but for two other chimpanzees, bipedal costs were similar ( P = 0.39 C5) or even less than quadrupedal costs ( P < 0.05 C4). 1), and in contrast to the study by Taylor and Rowntree ( 6), most subjects exhibited significant differences between gaits. However, differences in bipedal and quadrupedal cost varied among individuals ( Fig. This finding is consistent with previous work on juvenile chimpanzees ( 6), which indicated that bipedal and quadrupedal locomotion were equally costly for chimpanzees. Within the entire chimpanzee sample, bipedal walking was modestly, but not significantly, more costly (≈10%) than quadrupedal walking ( Fig. In contrast, human walking was less expensive than expected for their body size and substantially (≈75%) less expensive than chimpanzee locomotion ( Fig. The mass-specific cost of transport (ml of O 2 kg −1 m −1) for chimpanzees was greater than expected for their body size ( 15) ( Fig.
Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb.
Furthermore, human walking is ≈75% less costly than both quadrupedal and bipedal walking in chimpanzees. However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Consistent with previous work on juvenile chimpanzees, we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors. Bipedal walking is evident in the earliest hominins, but why our unique two-legged gait evolved remains unknown.
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