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A biped is an animal that travels across surfaces supported by two legs.


Bipedalism in general

Bipedal locomotion is walking, running, and standing on two legs (as opposed to four). It is a process requiring complex interaction of mechanical and control-system characteristics. Humans are capable of performing such motion because their spines are s-curved and their heels are round. Whether such a system has evolved in an animal species or been engineered in a robot, many of the same issues are inevitably involved.

Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid overcorrection.

Efficient walking complicates these issues, as it entails tipping slightly off-balance forward and to the side, and correcting balance with the right timing.

Running is an inherently continuous process, in contrast to walking; a bipedal creature or device, when efficiently running, is in a constant state of falling forward, that is maintained as relatively smooth motion only by repeatedly "catching oneself" with, again, the right timing, but in the case of running only delaying the nearly inevitable fall for the duration of another step.

The phenomenon of "tripping" is also informative in this regard. One popular way to think of it is as having one's leg pulled out from under them. In fact, however, merely stopping the movement of one leg of a walker, and merely slowing one leg of a runner, is sufficient to amount to tripping them. They were already falling, and preventing the tripped leg from aborting that fall is sufficient to literally "drop them like a sack of dirt".

Engineers who study bipedal walking describe it as a repeatedly interrupted fall.

Animals and humans

Many animals, including humans, have evolved bipedalism, with anatomical adaptations constituting the required mechanical systems and neurological adaptations the control-system ones.

As to anatomy, contrast in domesticated poultry the meaty drumstick and thigh, against measures like the assertive flavors that Buffalo-wing recipes use to "dress up" the small and bony wing. The technique of power-lifters highlights the similar difference in dimensions, even in untrained humans, between the muscles of the thigh and the upper arm. This difference is extreme: the large muscle in the human upper arm is the biceps, which bends the arm at the elbow; few people know the name of, or pay any attention to, the muscle that is used to straighten the arm; the quadriceps and hamstring muscles of the thigh are both so crucial to bipedal activities, that each alone is much larger than even a well-developed biceps.

The famous knee jerk (or patellar reflex) emphasizes the necessary bipedal control system: the only function served nerves involved being connected as they are is to ensure quick response to imminent disturbance of erect posture; it not only occurs without conscious mental activity, but also involves none of the nerves which lead from the leg to the brain.

A less well-known aspect of bipedal neuroanatomy can be demonstrated in human infants who have not yet developed toward the ability to stand up. They can nevertheless run with great dexterity, provided they are supported in a vertical position and offered the stimulus of a moving treadmill beneath their feet.

Human walking is composed of several separate processes:

  • rocking back and forth between feet
  • pushing with the toe to maintain speed
  • combined intruption in rocking and ankle twist to turn
  • shortening and extending the knees to prolong the "forward fall"

Evolution of bipedalism

Bipedalism and associated traits can offer a species several advantages:

  • Some evolutionary biologists have suggested that a crucial stage in the evolution of some or all bipeds was the ability to stand, which generally improves the ability to see (and perhaps otherwise detect) distant dangers or resources.
  • In vertebrate species, for whom evolution of additional limbs would be an enormous genetic change, it can serve to free the front limbs for such other functions as manipulation (in primates) and flight (in birds).
  • In some species with predominantly prone locomotion and often inability to stand erect while stationary, bipedal behavior appears only for rapid motion, "rearing up" on their hind legs.
  • Humans are generally thought to have evolved bipedalism either through living on plains (the Savanna Theory), or wading like their semi-bipedal wading cousins the bonobo chimps and proboscis monkeys, the Aquatic Ape Theory.

Many animals that do not use bipedal locomotion in nature can be trained to walk on hind legs. This includes dogs, cats, elephants, horses and pretty much every animal that has legs. Some animals (and humans) can also be trained to walk on front limbs, although this method lacks any practical benefits. Primates usually use both forms of locomotion - bipedal and walking on all fours, though there was one recorded case of a macaque switching to bipedal walking completely after recovering from a serious illness, and at least one example of a captive chimp who only walked upright.

Bipedal biological taxa

Biological examples of bipedality beyond humans and other primates are mostly vertebrates. Birds are bipeds, whether flying or ratite, and the ostrich and kin demonstrate that not even large bipeds have to be mammals. Another mammalian group of bipeds are the kangaroos.

The pattern of bipedality only in the form of "reared-up" running can be seen in some of the cockroaches, and in at least one genus of lizard (the basilisk lizards) that can run across the surface of water.

Bipeds are almost exclusively terrestrial animals, perhaps because the advantages of erect motion are offset, for aquatic animals, by the greater resistance to motion, in dense and somewhat viscous water in contrast to air, incurred by presenting a large cross-sectional area perpendicular to the direction of motion. Obvious exceptions to this rule include several animals which are partially bipedal, semi-aquatic mammals, including the bonobo and proboscis, and also the raccoon, which does not walk on its hind feet but often stands erect, or squats in water to use its hands to manipulate food and rocks/sticks.


For nearly the whole of the 20th century, bipedal robots were very difficult to construct. Increasingly cheap and compact computing power, however, has made two-legged robots more feasible. In recent years, Honda and Sony have developed these machines.

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