His palms are sweaty, knees weak, arms are heavy.

他的手掌出汗，双膝发软，两臂沉重。

youdao

His sudden smile made her go weak at the knees.

他突然笑了笑，使得她两膝发软。

《牛津词典》

You literally feel flushed and your knees go weak.

你觉得满脸通红，膝盖变得虚弱无力。

youdao

Paul's knees became weak.

VOA: special.2009.04.11

"I fought to keep my nerve despite my knees feeling weak, " she told the news agency.

CNN: Reporter first on pope story, thanks to her Latin skills

And this does not cover the legacy of injuries (bad knees, weak shoulders, arthritic hips, etc.) that football players will carry for the rest of their lives.

FORBES: What the Manti Te'o Hoax Says About Big-Time College Sports

What if a swab of saliva could tell if a player has genetically weak knees, or ankles?

FORBES: Is The NFL Going To Test Players For Genetic Defects?