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Arm swing in human locomotion


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.

Studies on the role of arm swing consist mainly of analysis of bipedal walking models 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.

Amplitude or frequency of arm movements is determined by the gait, meaning that the swing motion is adaptive to changing conditions and perturbations. 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.

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. 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.

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.


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