我们在走路的时候手臂会不自觉的随着步伐前后摆动，而且还很有规律地在摆动，比如左脚出右手之类的。那么，这样自然的动作搭配是为什么呢？

It's all part of a fine-tuned dance of physics and engineering. The skeleton is made of long bones and the many-jointed spine which alone cannot support the body against gravity. Coordinated muscle activity enables it to stand erect. Man is a very tall structure balanced on a small base, with a rather high center of gravity just below the small of the back. For stability, the center of gravity must be kept within the small area determined by the vertical projection of that base. Reflexes and postural mechanisms allow us to maintain equilibrium under unstable conditions produced by movement. There are no "center-of-gravity" receptors to provide afferent input, but rather afferent signals from muscles, joints, skin, vestibular system and eyes provide integrated information about the location of the center of gravity, sort of an internal map or positioning system.

In walking, the body is balanced on the base provided by one foot. The weight is supported alternately by one leg and then the other, so the body moves from side to side to poise the center of gravity over each leg in turn. As this happens, the foot is raised and advanced and the trunk sways to counterbalance the weight of the leg -- as the hips sway, the arms must move out of the way, and there is also some natural propelling of their weight from the shoulders. All of this is so delicately coordinated, rhythmically and gracefully within the brain. You may notice that when one becomes self-conscious of this process, one may try to inhibit the natural movement as one becomes ultra-aware of it, producing a stiff-armed effect. By focusing only on, say, the arms swinging, one loses the fine coordination of all of the movements.

（来源：cornell.edu 英语点津 Helen 编辑）