Heavy bag training is great for strength and conditioning training. It can a be a fun and beneficial alternative to traditional cardio training. Some days I just prefer to hit something in lieu of other conditioning options. Primary running. A nice warm up with the jump rope, followed by shadow boxing and then 20 mins on the bag is a truly intense workout and will leave you spent...if you're doing it right.
Many people are aware of this and get heavy bags for their garage or basement and assume it's as easy as setting one up and getting some gloves. They bang on the bag wildly with no real concept of form. This can have several negative affects.
1. You risk injury: Many people don't even know to wrap their hands, let alone how to do it properly. This leaves their wrists exposed to injury. Even with expensive gloves with superior wrist support, the lack of knowledge on how to throw a proper punch puts you at risk. Let's not even get into the risk surrounding throwing sloppy kicks by slapping your ankle into the bag, or your foot, or toes. Another risk is the bag burn many novices gain by dragging their kneecaps, or elbows across the bag surface because they don't know how to properly throw these strikes or gauge their distance. There is also risk of back injury when a person isn't properly educated and hunches forward when trying to throw body strikes.
2. You develop bad form: This may not be a concern to some who just hit bags to let out aggression and burn calories. But I say if you're going to engage in combat sports training, you should at least learn a little about combat. You don't go bike riding for training without knowing how to at least ride a bike. Many beginners never keep their hands up. They cock back their arms and swing really wide leaving their entire face and torso exposed. They throw kicks with their hands at their waist and so on. This all develops horrible habits that not only impede your ability to fully benefit from the training method, but you are learning bad fighting habits that will be hard to break later. Another issue with bad form is not learning to use your entire body and put weight and momentum behind your strikes. This is ultimately what fighters use heavy bags for. To develop their power.
3. You don't fully benefit from the workout: You've heard the saying, "You can do 100 reps wrong and do 10 right, and those 10 will benefit you more." This is true of the heavy bag. Without the proper guidance most people use just their arms. They slam into their bag without using the proper compound movements that will engage their entire body. Proper form will give you an full strength and conditioning session by engaging not must your arms, but your core, and legs.
If you think about it logically. You learn how to use proper form in all the forms of exercise you engage in (I hope). Why should heavy bag training be any different. It's much more intricate that people give credit. Get into a proper gym and learn how to attack your bag properly and you will gain the full benefits of this outstanding form or exercise.