He most likely didn't get those knuckle injuries from just knuckle push-ups however. It's most likely because he punched a tree or a “makiwara board” for 2 hours.
Perfected by boxers and martial artists, fist or knuckle pushups are an excellent way to strengthen your wrists apart from reaping all the benefits.
The knuckle push-up is a bit of a gym legend, but how does it stack up to the standard variety? Time to weigh the benefits and the risks.