All pushup variations build strength and endurance in your chest, shoulders and triceps. But the standard palms-flat pushup position forces you to hyperextend your wrists, which can be painful. If you don't have pushup handles available, doing pushups on your knuckles is an appropriate modification to eliminate the wrist pain. Knuckle pushups also closely mimic the movement of throwing a punch, so mixed martial arts fighters and other martial artists sometimes use these as an exercise to build punching power and range of motion.
Assume the standard pushup position, with your hands directly beneath your chest, slightly wider than shoulder-width apart. Balance on the balls of your feet with your body straight from head to heels.
Make a fist with each hand. Rest the flat of each fist on the floor, directly beneath your elbows. Your body position has not changed but your wrists and hands are now straight.
Lower your chest toward the floor. If you're doing this exercise for everyday conditioning, stop when your chest is level with your elbows. If you're exercising for martial arts or another sport that requires you to exert power through an increased range of motion, dip lower. The standard rule is that you should strength train in the same range of motion you use for your everyday or sporting activities.
Exhale and push off, straightening your arms. For general strength and conditioning, count slowly to two as you lower yourself, then press up to a slow count of three or four. If you're training to build power, allow yourself to move more quickly but still under control as you explode up out of the low position, with no pause at the bottom.