The aim of this experiment was to see if his mother was right and that cracking the knuckles causes arthritis4.

这个实验的目的就是看看他母亲是否正确，因为他母亲认为扳指关节会导致关节炎。

youdao

But because of the Academies Bill's timetable, "we have the spectacle of ministers who have already told us they will accept no amendment, period, and the sight of whips new and old cracking their knuckles off-stage and perfecting basilisk-like stares in the mirror" to prevent MPs voting for any changes, he said.

BBC: MP accuses government of intimidating backbenchers