Push-ups are one of the most traditional exercises used to get stronger and in better condition. Traditional push-ups are done with your hands flat on the ground and in a position about shoulder-width apart. Push-ups done on the knuckles — known as knuckle push-ups or "knucks" — build extra strength in the hands, wrists and knuckles and are frequently used by boxers, wrestlers and martial artists.
The skin on your knuckles will become tougher when you do knuckle push-ups. As you get used to doing knuckle push-ups, your skin in that area will become calloused. This will help your when you throw a series of power punches, by enabling you to throw those punches without fear of damaging the skin on your knuckles. Standard push-ups will not give you this advantage.
Because you are putting so much pressure on your knuckles when doing knuckle push-ups, your wrists have to do much of the work. This will help get your wrists significantly stronger and more powerful. This helps the martial artist when blocking kicks and punches and also helps when you deliver hard punches. It also helps you throw a punch and hit your opponent with a sharp blow. Instead of spraining your wrist, you will simply be able to throw another punch. Standard push-ups help build strength in the wrists but not as much as knuckle push-ups.
When you do push-ups on your knuckles, you are using the same muscles you use to make a fist. This means you are doing push-ups with your arm muscles contracted. This will help your forearms to get much stronger in that area in a shorter period of time. Push-ups will build arm strength, but more of it will be focused in the shoulders than in the lower arms.
A push-up done on the knuckles requires you to have your hands almost directly under your shoulders. This narrow set-up will help you build more strength in your chest. Standard push-ups will be done with your hands having a wider set. The wide set will build strength in your chest but not as effectively as push-ups done with the hands closer to the frame of the body.
When you do knuckle push-ups, your weight is centered on a small area of your hands. As you do those push-ups, you may become fatigued, and there is a chance you could lose your balance and slip out of position. The fatigue in your hands and arms may not allow you to protect yourself when you fall, and thus the possibility of injury is greater.