the biceps muscle of the thigh
to sit knee to knee with one―sit opposite each other
ひざの関節炎
(膝を屈めるだけの意味なら)to bend one's knees―(礼拝、服従するの意味なら)―bow the knee (to or before god or man)―(屈服するの意味なら)―come to one's knees
a cordlike tendon at the back of the knee, called hamstring