Boxing is an effective cardiovascular activity. But because of its explosive nature, it builds not only the cardiovascular system, but also many muscle groups. In fact, an editor at "Fitness" magazine found that six months of boxing training made more changes to her physique than the previous 10 years of gym workouts. Knowing which muscles are active will help you box as well as train for boxing.
Boxing works the shoulders intensely. When you begin boxing, your shoulders are where you'll feel the most burn. Whether you're sparring or working the bag, you'll be wearing gloves that weigh up to a pound, and throwing dozens of punches will wear out your deltoids quickly. While the front and middle deltoid heads are active when a punch is thrown, the rear deltoid head also gets a workout from retracting punches.
When you think of punching, it's natural to look at it as a pushing exercise. The same muscles are used to throw a punch as to perform a shoulder press. But during sparring or an actual match, as many punches miss as land, and even punches that land must be retracted quickly in order to avoid getting counter-punched. This is why, besides the deltoids, some of the most developed muscles on a boxer's body are the teres major of the back. The book "Strength Training Anatomy" explains that the teres major sits above the latissimus dorsi, making up the back of the armpit. It attaches to the upper humerus and the scapula, and along with the lats, are responsible for retracting the arms.
All the muscles of the upper arms are used during boxing. The triceps aid the deltoids in throwing punches such as the jab and cross, while the biceps stabilize the arms during hooks and help retract the arms quickly after all punches.
As your coach will tell you, the strength of a punch doesn't come from the arms and the shoulders alone, but from the core. A true power punch requires the body to move in unison, from the back toes to the forward fist, and that movement is coordinated by the core. "Strength Training Anatomy" explains that the external obliques are the primary muscles for twisting the body back and forth, but the rectus abdominis and transverse abdominis are also very active during a punch.
The shoulders are versatile and flexible, and because of this, they are easily injured. This is especially true during an explosive sport such as boxing. To prevent injury, perform rotator-cuff specific exercises such as external shoulder rotations. Also, don't try to go 10 rounds right away. Give your body time to adjust to the new load by building up slowly. If you find that you are having lower back pain, it could point to a deficiency in core strength. Perform exercises that develop the lower back and the external obliques. Improper technique can be a problem. Finding a knowledgeable trainer who works at your pace can be the key to avoiding injury.