“stretch one's legs”别理解成“把腿伸展”，真实含义是(久坐后)散步，进行必要的(散步)锻炼。

【例句】
Let's stop off at this rest area for a while. After three hours of driving, I'm sure everyone would like to stretch their legs a bit.
让我们在这个休息区域停一会儿吧，在三小时的开车后，我确定大家都想下来散散步。

You’d better stretch your leg after working the whole day.
在工作一整天后，你最好能锻炼一下。