Walking and Sloshing 边走边溅

You're at a party. Somebody hands you a glass. Hey, come on over here! Somebody else shouts. You take a step and the next thing you know you are wearing that tasty beverage.
你在一聚会上。有人递给你一杯饮料。“来，到这来”，有人喊到。你迈出步子，你知道的，接下来你将洒掉那杯可口的饮料。

But why? When we walk, we keep our torsos straight, don't we? Walking feels entirely steady and constant. So why is it so hard to carry a liquid?
为什么？我们走路时，我们的躯干是挺直的，不是吗？感觉我们的步态是完全平稳的啊。那为什么端一杯液体饮料（不洒出来）就那么困难？

If you doubt the difficulty of this exercise, we suggest you try filling up a bucket with water and carrying it at chest level without spilling any. But try it outside first!
如果你怀疑这并不困难，我们建议你试试装满一桶水，提到与胸部齐高，并保证水不洒出来。但首先请在室外试（因为你一定会洒出来的，别把家里弄湿了）。

The answer is that although we feel ourselves to be quite steady when we walk, in truth that is anything but the case. When you walk your torso bobs in all directions.
答案是：即使我们自我感觉步态平稳，然而事实绝不如此。当你走路时，你的躯干向四面八方摆动。

Walking is a complex muscular phenomenon that amounts to allowing yourself to fall slightly forward and then preventing yourself from falling completely by extending your foot. The extended foot halts the motion which was begun partly by gravity and partly by the pushing of the other foot. Now the torso moves over the base of the resting foot which, once the torso has passed it, begins pushing it into a forward fall again until the other foot arrests the motion.
走路是一种很复杂的肌肉活动现象，就等于说允许你的身体轻微的向前倾一点，而又通过脚步的迈开让你不至于完全向前倾倒。迈开的脚停止了这个动作， 这个一部分是由重力，另一部分是由伸出的另一只脚引起的。现在躯干在静止的那只脚上方移动，一旦倾过了它，静止的脚又开始推动其向前倾，直到另一只脚停止了此动作。

On top of this, we must also constantly prevent ourselves from falling sideways by flexing and releasing the gluteus muscles around our buttocks and hips. That's why people sway as well when they walk.
另外，我们也必须不断地通过收缩和放松臀部和髋部周围的臀肌，来防止我们倒向两边。那就是为什么人们走路时也在摇摆的原因。

This is what walking is: not the smooth, undifferentiated motion that it seems to be, but an ongoing series of up-down, start-stop, and side-to-side maneuvers. No wonder drinks fly!
步行就是：不是表面看起来的那样平稳，无差异的运动，而是一系列的自上而下，起起停停，从一边到另一边的不间断的动作。难怪液体会溅出来呢！