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So why is it that we move the particular ways we do?
那我们为什么用这种特定的模式移动呢？
Well let's think about what really happens. Maybe we don't all quite move the same way.
来想想真实情况可能是什么。假如我们并不都用同一种模式移动。
Maybe there's variation in the population.
假如人群中有差异。
And maybe those who move better than others have got more chance of getting their children into the next generation.
而且假如那些比其他人移动得更好的个体有更多机会让他们的子代进入下一代的群体里面。
So in evolutionary scales, movements get better.
那么在进化级别上，动作越来越优化。
Perhaps in life, movements get better through learning.
另外，也许在一生当中，学习会让动作更优化。
So what is it about a movement which is good or bad?
那么，是什么决定了动作的好与坏呢？
Imagine I want to intercept this ball.
来想象我要截下这个球。
Here are two possible paths to that ball.
这里有两种路径来做到。
Well if I choose the left-hand path, I can work out the forces required in one of my muscles as a function of time.
假如我选了左手边这条路径，我可以算出某一块肌肉需要施多大力，用关于时间的函数表示。
But there's noise added to this.
但是还有杂音附加在上面。
So what I actually get, based on this lovely, smooth, desired force, is a very noisy version.
所以根据这条美好、光滑、期望中的曲线，我事实上得到的是个杂音很大的版本。
So if I pick the same command through many times, I will get a different noisy version each time, because noise changes each time.
那如果我多次发布同样的动作指令，每次会得到不同的含杂音的版本，因为每次杂音不一样。
So what I can show you here is how the variability of the movement will evolve if I choose that way.
所以我现在在这儿展示的只是这个动作的可变性有多大，前提是如果我选这种方式的话。
If I choose a different way of moving -- on the right for example -- then I'll have a different command, different noise, playing through a noisy system, very complicated.
如果我选另一种方式--比如右边这个--那我就会有不同的指令，不同的杂音，还要透过杂音系统，总之非常复杂。
All we can be sure of is the variability will be different.
我们唯一能确定的是，两个方式的可变性会不同。
If I move in this particular way, I end up with a smaller variability across many movements.
要是我用这种方式运动的话，最后得到的可变性在很多种动作里面是最小的。
So if I have to choose between those two, I would choose the right one because it's less variable.
所以要是我在这两者里面选其一，我就会选右边那个，因为可变性更小。
