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Review
, 204 (5), 321-30

Bipedal Animals, and Their Differences From Humans

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Review

Bipedal Animals, and Their Differences From Humans

R McN Alexander. J Anat.

Abstract

Humans, birds and (occasionally) apes walk bipedally. Humans, birds, many lizards and (at their highest speeds) cockroaches run bipedally. Kangaroos, some rodents and many birds hop bipedally, and jerboas and crows use a skipping gait. This paper deals only with walking and running bipeds. Chimpanzees walk with their knees bent and their backs sloping forward. Most birds walk and run with their backs and femurs sloping at small angles to the horizontal, and with their knees bent. These differences from humans make meaningful comparisons of stride length, duty factor, etc., difficult, even with the aid of dimensionless parameters that would take account of size differences, if dynamic similarity were preserved. Lizards and cockroaches use wide trackways. Humans exert a two-peaked pattern of force on the ground when walking, and an essentially single-peaked pattern when running. The patterns of force exerted by apes and birds are never as markedly two-peaked as in fast human walking. Comparisons with quadrupedal mammals of the same body mass show that human walking is relatively economical of metabolic energy, and human running is expensive. Bipedal locomotion is remarkably economical for wading birds, and expensive for geese and penguins.

Figures

Fig. 1
Fig. 1
A graph of relative stride length against dimensionless speed for humans, chimpanzees (Pan troglodytes) and bonobos (P. paniscus) moving bipedally and quadrupedally. From Aerts et al. (2000) Spatio-temporal gait characteristics of the hind-limb cycles during voluntary bipedal and quadrupedal walking in bonobos (Pan paniscus) Am. J. Phys. Anthropol. III, 503–517. Reprinted by permission of Wiley-Liss Inc., a subsidiary of John Wiley & Sons Inc.
Fig. 2
Fig. 2
A graph of relative stride length against dimensionless speed for humans and various birds. Dotted lines mark the range of speeds in which the transition from walking to running is made. From Gatesy SM, Biewener AA (1991) Bipedal locomotion: effects of speed, size and limb posture in birds and humans. J. Zool. Lond. 224, 127–147. Cambridge University Press.
Fig. 3
Fig. 3
Limb positions during the stance phase of slow walking and fast running of bobwhite quail (Colinus), ostrich and human. From Gatesy SM, Biewener AA (1991) Bipedal locomotion: effects of speed, size and limb posture in birds and humans. J. Zool. Lond. 224, 127–147. Cambridge University Press.
Fig. 4
Fig. 4
Schematic graphs of the vertical force F exerted on the ground, against time t, for examples of four types of walking. Each graph shows the forces exerted individually by the left and right feet in several successive steps and, by a broken line, the total force when both feet are on the ground. Shape factors are 0.4 in (i) and (ii), 0 in (iii) and (iv). Duty factors are 0.75 in (i) and (iii), 0.55 in (ii) and (iv). From Alexander & Jayes (1978), by permission.
Fig. 5
Fig. 5
(a) Schematic graphs of the height y of the centre of mass against time t for the four patterns of force illustrated in fig. 4. (b) A graph of shape factor q against duty factor β divided into the areas that give rise to each of the four styles of locomotion. From Alexander & Jayes (1978), by permission.

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