Refers to the patella, the small bone at the front of the knee. Synonymous with 膝頭 (ひざがしら).
See also: 膝頭
I fell and hit my kneecap hard.
knee wheel (judo)
A judo technique where the attacker uses their knee as a fulcrum to throw the opponent. Domain-specific martial arts term.
He executed a knee wheel in the judo match.
Compound of 膝 (ひざ, 'knee') and 車 (くるま, 'wheel'). The anatomical sense likely comes from the round, wheel-like shape of the kneecap. The judo technique name is a direct metaphorical extension of this imagery.