In a manga, a spectator is describing the way a boxer throws his punches:
What is the meaning of 拳の山の骨? Does it refer to the knuckles? Here you can see the page it is taken from. Thank you for your help!
拳 is "knuckle", so 拳の山 should refer to a part of a knuckle. Judging from the picture, 拳の山 seems to refer to the protrusion made by the metacarpophalangeal joints. 拳の山 is not a well-known term at least among laypeople. It doesn't look like boxer jargon, either.
so perhaps it's a made-up word. Some orthopedists seem to use this phrase. Anyway, this is not to be memorized but to be understood ad-hoc.