Now, what is it that causes that, mechanically,

从物理角度讲是什么原因
that transition to take place?

造成了这样的转变
What's different about running than walking?

跑步和走路有什么区别呢
Well, one of the things that's different is

其中一个区别是
that you don't always have one foot on the ground when you're running.

在跑步时不会保持一只脚始终着地
In fact, if you watch somebody run or you run yourself,

其实如果你观察某人跑步或自己跑步
you'll know that for a lot of the time you're only--

你会发现大部分时间里你只是
you're not--no foot's on the ground.

你并非...你的双脚都不着地
There's not time that both feet are on the ground,

没有两只脚都着地的时候
one foot hits, you sort of bounce off of it,

当脚触地的时候你的脚好似弹了起来
you bounce off the other one,

然后另一只脚弹起来
you bounce off the other one,

然后另一只脚再弹起来