noun
The joint itself. Used in medical, everyday, and figurative contexts.
転んで膝をすりむいた。
I fell and scraped my knee.
膝が痛くて正座できない。
My knees hurt so I can't sit in seiza.
noun
Refers to the area from the knees to the thighs when sitting, especially as a surface for holding something or someone.
猫が膝の上で寝ている。
The cat is sleeping on my lap.
子どもを膝に乗せて本を読んだ。
I read a book with the child on my lap.
Native Japanese word. The kanji 膝 is a phono-semantic compound with 肉 (flesh) and 桼 (phonetic, also suggesting bending), but the reading ひざ is not derived from Chinese.