Boxers have a lean and toned physique when they step into the ring for a match. This comes from hours of hard training and mental sacrifices. Using a punching bag is a sport-specific component of a boxer's training regimen. If you are trying to build muscle, the punching bag might not be your best option.
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Cardiovascular activity involves a number of large muscles, but it varies from strength-training. With cardio, you move in a repetitive fashion for an extended period of time. This is done in the absence of heavy resistance. With strength-training, you lift weights through a series of repetitions for a short amount of time, which leads to muscle gain. Using a punching bag is a form of cardio so it does not cause a large amount of muscle growth.
Punching bags come in two basic types -- a heavy bag and speed bag. A speed bag is small, light and at eye level. As the name implies, it is used to develop speed and quickness. A heavy bag is large and heavy, and used in a slower fashion. Its main function is to develop punching power. When you punch this bag, you forcefully contract a high amount of muscle fibers. This will not make you bigger, but it will help improve your definition, especially because you burn calories while you work out. A 180 lb. person, for example, burns nearly 500 calories in 60 minutes working out on the punching bag, according to the website CaloriesPerHour.com.
Multiple muscles in the upper and lower body have to contract when you punch a bag, especially a heavy bag. The pectorals, deltoids, triceps, latissimus dorsi, glutes, quadriceps and hamstrings are examples. The pecs are the chest muscles, the delts are on the sides of the shoulders, the triceps are on the back of the arms and the lats are in the back. The quads, hamstrings and glutes are in the legs and butt. The abs also contract to generate force and to keep your body balanced. If you use a bag regularly, these are the primary muscles that will get toned.
When you punch a punching bag, the only resistance you have is the weight of your arms. Performing multiple punches over time builds a type of strength called muscular endurance. This has practical application if you are an actual boxer. It can also enable you to do upper body repetitive movements in every day life with more efficiency.
Even if you do not build big muscles with a punching bag, you still experience health benefits. Whenever you do aerobic activity, you improve the health of your respiratory and circulatory system. According to the American Heart Association, frequent physical activity improves blood pressure, reduces triglycerides, boosts healthy cholesterol, reduces diabetes risk and improves mood. The key is to work out for at least 30 minutes at least five days a week.