Yep. Those bendy things between your thighs and shins. Those parts that make their presence felt around about age fifty, even if all you did was walk with them.
More famous verses will come to mind (Isa 45:23, for example), but for now, let me give you Job 4:4: “Your words have upheld him who was stumbling, and you have made firm the feeble knees.”
Send your haikus and images about knees to dawn[at]tenwaystopray[dot]com.
See Prayer Light if you want to read more about this thirty-day prayer challenge.