Developing harder knuckles won't improve the strength of your punches. Conditioning knuckles "may keep you from getting a scratch on your knuckles during a fight, but that's all," writes Tom Mizula. The power behind a punch derives from a person's body alignment, hip movement, mental state and other factors. The safest exercise for developing protective callouses on your knuckles are knuckle push-ups, which also tone and strengthen muscles that contribute to punching strength.
Assume a traditional push-up position, with your body at roughly a 45-degree angle in relation to the floor. Curl your hands into fists and rest your body weight on the larger knuckles of your index and middle fingers.
Lower your body until your chest is parallel to the ground.
Push down on your knuckles until your body returns to its original position.
Perform as many knuckle push-ups as your strength allows.
Striking a punching bag also develops callouses on the knuckles. When punching a bag, wear gloves made specifically for this activity to protect bones in the hand.