The typical fighter athlete looks strong. He has cut, well-defined muscles and competes with reasonable success against other trained and conditioned athletes. These fighters participate in a holistic program of strength training, cardio training and diet. Although working a punching bag is primarily intended to build fighting skills, it also contributes to physical strength.
How it Works
Your muscles grow stronger when they meet resistance. If you force your body to work harder than it does over the course of your general day, they suffer microscopic tears. Those tears heal thicker and stronger, like scars from a cut on your skin. This is how resistance training builds muscle. When you punch a punching bag, it resists the force of your blow, building the muscles that powered the punch.
Kind of Bag
The variety of punching bags on the market includes speed bags, heavy bags, double-ended bags, slip bags and "headache" bags. For strength training, a heavy punching bag is your best choice. Although the other, smaller bags work well for building other attributes important to a fighter, they lack the weight to contribute significantly to your strength training.
The heaviest standard punching bags weigh in at 100 to 120 pounds. For most people, that's enough weight to pose a legitimate challenge and build strength. However, eventually you'll grow strong enough that it only maintains your body strength, not increases it. Like other exercises of high repetitions and low to moderate weight, you won't see the increased bulk or extreme strength typical of power lifting.
When punching a bag for strength, you don't want to fire off combinations or dance around the bag. Plant your feet firmly and strike the bag as hard as you can. Punch when the bag is hanging still, or swinging toward you, for maximum resistance and best strength-building results. Because of the forces involved, wear boxing gloves and hand wraps when strength training on a punching bag. If you don't, you risk injury to your knuckles and wrists. Start off with short bouts of punching. Progress from 15 seconds to up to three minutes over time; and decrease the amount of rest time in between, from three minutes, down to two and so forth.
Your actual strength is one part physical power and two parts how you apply that power. Boxers use punching bags to learn how to best apply their power in the context of throwing a punch. If you get instruction from a qualified coach, you can learn how best to use what strength you have while using the bag. That translates to more applied strength as you both improve your muscle capacity and better learn how to use those muscles.