背要直，只弯两膝。

Keep back straight and bend knees.

youdao

双膝弯曲，两脚着地，躺在这个滚筒上，让它沿着你的脊柱，一直从尾骨到头顶。

Lie on the roller with your knees bent and your feet on the floor, so the roller extends along your spine from your sitting bones to the top of your head.

youdao

因为双膝一直僵直，那么整个重量势必都将由下背部的肌肉负担。

Because both knees are always tetanic, lower back's muscle will burden all weight.

youdao

I think even if you're in a chair, and you're sitting up straight with good posture, maybe hands on knees.

哪怕只是坐在椅子上，坐直并保持良好姿势，也许双手放在两膝上。

普林斯顿公开课 - 人性课程节选