An ostrich, the fastest living biped^[1] at 70 km/h^[2][a]

A Man Running - Eadweard Muybridge

Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal ormachine that usually moves in a bipedal manner is known as a biped /ˈbaɪpɛd/, meaning "two feet" (from the Latin bi for "two" and ped for "foot"). Types of bipedal movement include walking, running, or hopping.

Few modern species are habitual bipeds whose normal method of locomotion is two-legged. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare,^[4] hopping mice, pangolinsand homininan apes, as well as various other extinct groups evolving the trait independently. In the Triassic period some groups of archosaurs (a group that includes the ancestors of crocodiles) developed bipedalism; among their descendants thedinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds descended from one group of exclusively bipedal dinosaurs.

A larger number of modern species utilize bipedal movement for a short time. Several non-archosaurian lizard species move bipedally when running, usually to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment. Several arboreal primate species, such as gibbons and indriids, exclusively utilize bipedal locomotion during the brief periods they spend on the ground. Many animals rear up on their hind legs whilst fighting or copulating. A few animals commonly stand on their hind legs, in order to reach food, to keep watch, to threaten a competitor or predator, or to pose in courtship, but do not move bipedally.

Arm swing in human bipedal walking is a natural motion that each arm swings with the motion of the opposing leg. Swinging arms in an opposing direction with respect to the lower-limb reduces the angular momentum of the body, balancing the rotational motion produced during walking. Although such pendulum-like motion of arms is not essential for walking, recent studies point that arm swing improves the stability and energy efficiency in human locomotion. Those positive effects of arm swing have been utilized in sports, especially in racewalking and sprinting.

Kinematics[edit]

Studies on the role of arm swing consist mainly of analysis of bipedal walking models^[1] and treadmill experiments on human subjects. Bipedal walking models of various complexity levels provided an explanation for the effects of arm swing on human locomotion. On the course of bipedal walking, the leg swing results in an angular momentum that is balanced by the ground reaction moments on the stance foot. Swinging arms create an angular momentum in the opposing direction of lower limb rotation, reducing the total angular momentum of the body. Lower angular momentum of the body results in a decline on the ground reaction moment on the stance foot.^[2]

Amplitude or frequency of arm movements is determined by the gait, meaning that the swing motion is adaptive to changing conditions and perturbations.^[3] As the walking speed increases, the amplitude of the arm swing increases accordingly. The frequency of the arm movements changes with the speed as well. Studies showed that at speeds lower than approximately 0.8 m/s, the frequency ratio between arm and leg movements is 2:1 whereas above that speed the ratio becomes 1:1.^[4]

Theories[edit]

Stability[edit]

[Stability is the utility, not the reason behind] Both simulations on skeletal models and experiments on force plate agree on that since the total angular momentum is lowered with the counterbalancing swing of arms with respect to the lower-limb the free arm swing limits the ground reaction moments effective on the stance foot during walking.^[5] In other words a subject needs less reaction moment applied by the ground surface when there is arm swing. Having a smaller need for a reaction moment implies that the friction force between the stance foot and the ground surface does not have to be as high as it would when there is no arm swing. Less dependence on the friction force, hence environment, is a possible outcome of arm swing.^{[citation needed]}

Energy efficiency[edit]

Whether arm swing is a passive, natural motion caused by the rotation of torso or is an active motion that requires active muscle work has been a critical discussion on arm swing that could illuminate its benefit and function. A recent study concentrated on the energy consumption during walking showed that at low speeds arm swing is a passive motion dictated by the kinematics of torso, no different from a pair of pendula hung from the shoulders. Active upper extremity muscle work, controlled by the brain, only takes part when there is a perturbation and restores that natural motion. However, at higher speeds, the passive motion is insufficient to explain the amplitude of the swing observed in the experiments. The contribution of active muscle work increases with the walking speed. Despite the fact that a certain amount of energy is consumed for the arm movements, the total energy consumption drops meaning that arm swing still reduces the cost of walking. That reduction in the energy is up to 12 per cent at certain walking speeds, a significant saving.^[6] 

^{[if you do not swing your arm, you have to actively repress the mothion that you will not naturally.} 

^{That means, if you do not do the arm swing, you are actually doing 2 jobs: 1. unstoppable natural} 

^{command + 2. Active participation of to make that thing . it is doing and then undoing, both of which is active.]} 

Evolution[edit]

The inter-limb coordination in human locomotion, questioning whether the human gait is based on quadruped locomotion, is another major topic of interest. A recent research indicates that inter-limb coordination during human locomotion is organized in a similar way to that in the cat, promoting the view that the arm swing may be a residual function from quadruped gait.^[7] Another work on the control mechanisms of arm movements during walking corroborated the former findings, showing that central pattern generator (CPG) might be involved in cyclic arm swing. However, these findings do not imply vestigiality of arm swing, which appears to be debateful after the 2003 evidences on the function of arm swing in bipedal locomotion.^[8]