Provided your hands are wrapped properly and you are using adequate boxing gloves,
your hands, wrists and knuckles should not hurt when punching a heavy bag, even when hitting that bag with a lot of force over and over again. It is NOT just a part of boxing training.
That being said, people have certainly reported feeling some discomfort at times. The good news is that if someone does experience any pain or hand issues at all from the heavy bag, there is always a solution (so long as there aren't any underlying problems that would lead to making heavy bag boxing unsafe). The solution of course, depends on the problem, and I am going to go through a list of the most common issues I hear accompanied by the cause and specific solution(s) for that issue.
First and foremost, for just about all of these potential issues, there are 2 important things you can do to prevent them from ever happening. The first is warming up your wrists and hands before hitting the bag. The idea of the warm-up is to get some some flexion, extension and blood flow happening. Here are 3 exercises I demonstrated in a video last year -
https://www.youtube.com/watch?v=x141CXJdI6U&list=PLN0PPf0o8X6X1Te2zEgHrqy-BtyI-RAg3&index=38 - and even when you do start punching, take that first round or so a little lighter to get your hands used to that contact.
The other thing you can do is strengthen your hands and wrist. The stronger they are, the less likely injury could potentially happen. Things like push-ups on your knuckles and bottoms-up kettlebell presses will really help you keep your wrists straight and stable and any exercise you can do to overall improve your grip strength will help for delivering tighter, more solid punches. There are a lot of little muscles in the hands and forearms and all of them are forced to stabilize when your punches land. You don't need to spend a ton of time on this training and it is very possible to do other lifting work at the same time. Even something like pull-ups, where the hands and wrists aren't targeted at all, can do wonders for your grip strength, especially with a fatter/thicker bar.
Before I finally get to the issues, I also wanted to say one thing about hand-wrapping. It is critical to wrap your hands before putting on the gloves and hitting the bag and it is also critical that they are wrapped correctly. There is no one, unified way to wrap and many different variations but as long as it...
1) Is tight enough that it doesn't become unraveled
2) But not so tight that you cannot feel your hands and get good circulation to them
3) Allows you to make a tight fist and fully close your hand
4) Doesn't bind any of your fingers together
5) Provides some padding for the knuckle
6) Provides some wrist support
...then your hands are wrapped properly. Once again,
here is a video I made showing a couple different ways to wrap -
https://www.youtube.com/watch?v=HZM6OXoCSfg - If all of these things are true and/or we wrap your hands for you at our club, any hand or wrist issues are likely NOT due to the wraps. I say this because it is often the first thing people wonder if they encounter pain. Maybe I'm just not wrapping correctly? Or not wrapping tight enough? Usually though, this is not the case and in fact, wrapping too tight can probably cause worse problems because if you do lose circulation and can't feel your hands or wrist, you won't be fully aware of what they are doing and where they are when they hit the bag. And that is the most important thing of all! Speaking of which, here are the more likely culprits and how to fix them:
Abrasion
Problem 1 - You have blisters on your front 2 knuckles (index and middle finger knuckles)
Solution a - This could be simply due to newness. You just started hitting the bag and/or have brand new gloves and/or a brand new heavy bag and your hands simply aren't used to it. As you get more used to it, these scrapes and blisters will go away. But punching a bag with blistered hands hurts, even if just in the interim while you heal. You can go with regular band-aids (replacing at least every time you box) or better yet, moleskin bandages for more padding and wrapping.
Solution b - You are not landing the knuckles flush and are instead, sliding off the bag on contact, pulling on the skin. You never want to skim or partially hit the bag and if you find that happening, slow things down a little.
Problem 2 - You have blisters on the other 2 knuckles (ring finger and pinkie knuckles) and/or the inside, door-knocker knuckles.
Solution a - You are hitting with the wrong knuckles! All contact should happen with the front, outside big knuckles where you index finger and middle finger begin in the hand. If you are hitting with the smaller knuckles or worse, the "door-knocker" knuckles that are right in the middle of your finger tips and hand knuckles. You will likely have other issues if you continue to punch this way so the abrasion blisters are actually a useful warning sign! Just like the issue above though, you are going to need some kind of bandage until the blisters heal.
Knuckle Pain
Problem 1 - Your knuckles hurt when you hit the bag. Generally in both hands but more so on the dominant hand and generally, when throwing all the punches though again, some hurt a little more than others.
Solution a - You do not have a tight fist upon impact. And sometimes this is a little tougher wearing wraps and gloves but you really have to make sure those fists are as tight as possible. Don't squeeze all the time though, or you'll burn out your forearms, but from the moment you decide to throw that punch, squeeze those fists tight. There are schools of thought that preach not making that tight fist up until the exact point of impact, which is certainly logical but could cause an injury if you are even slightly wrong about the timing of that point of impact. i.e. if the bag is moving and you connect a little before you expect.
Solution b - You may need new gloves. If your gloves are older and the padding has started to wear, it is time for new ones. How long does this take? I have found that it totally depends on the glove and how hard you hit. Sometimes, expensive gloves are more economically smart, as well as safer, because they won't wear down as quickly. You just have to make sure you get the right ones. If you are a heavy hitter, you are going to likely need heavier gloves. 14-ounce may be okay but 16-ouncers are often better. Some brands are definitely better than others and some styles are too. For the TITLE Brand, I have said many times that the World Bag Gel 2.0 gloves are better for hitting the heavy bag than any other TITLE style, even those which are more expensive.
Additional Note About Gloves: In all the time I have punched a heavy bag, I have only hurt my knuckles one time. I was trying out some weighted gloves and my fingers were too crammed together so the tendon pulled a little bit upon impact. This was kind of an edge case scenario but my point is just that if you do buy or try new gloves and something doesn't feel quite right, those are not the gloves for you! New gloves should always feel pretty good. Maybe a little stiff at first, slightly tight but if the hand feels wrong or very uncomfortable, try something else.
Problem 2 - Your knuckles hurt only on one hand but not so much, the other.
Solution a - You had a specific, acute injury. Maybe a punch landed wrong (not a tight enough fist, wrong knuckles, not wrapped properly, using old gloves, etc.) and created an injury that now leads to pain. If it's really bad, get it checked but most of the time, it just needs to heal in these cases. You can still box during this time but I would recommend a couple things- just not going so hard for a little bit and/or use to something to help absorb some of the shock from your knuckles. A few different things can work. The first is a toe separator used primarily for drying painted toenails. But if you put it over your knuckles when you wrap, it will provide some padding and made all the difference for me that first and only time I hurt my hand while boxing. If it doesn't help you, the next step is to try something a little better. I have found that Silipos hand pads provide plenty of shock absorption. Just like the toe separators, you place them over the top knuckles and then wrap over them. There are also knuckle shields made specifically for boxing that you can use too.
Solution b - Your punch is not landing properly on that hand. Likely it is your power side so while you might have the same technical issue on your non-dominant hand, the issue isn't as prevalent as you don't hit as hard on that side. Take more time to see how your hand is landing and make sure you are hitting with A) a straight wrist, B) a tight fist and C) making contact with the top two, big, hand knuckles only. The ones your index finger and middle finger attach to. To find out for sure, throw each punch on that hand and freeze so that you can see exactly where where you're making contact. Then make some adjustments and quickly get out of this habit of freezing at the end of your punches.
Problem 3 - Your knuckles hurt specifically on particular punch.
Solution a - This means you are almost definitely not throwing and landing that particular punch properly. Just like above, practice throwing it, freezing on impact, and checking to make sure that A, B and C are true. If you're not sure, have someone else look at it. Just remember that even when you fix it, there may still be some residual pain as the initial injury heals. You can also take the steps in “Problem 2, Solution” a while it does.
Wrist Pain
Problem 1 - Your wrist hurts when you punch the bag.
Solution a - Your wrist is not straight upon impact. It is bent forward or backward, which puts immense pressure on those wrists, the harder you hit. You may not even realize that this is happening. Before even throwing a punch, try straightening your wrist and see what it looks like. Then throw a slowed-down punch and take a close look.
Solution b - Your wrist bends upon impact. Maybe it's straight coming in but bends once it hits the bag. Oftentimes, this is because either the fist is not tight enough on impact or the elbow is not behind the punch. Or both! We've talked about keeping a tight fist but the elbow is really important too, not just for your wrist but your shoulder is well. We see this a lot on hook punches. A good rule of thumb is to make sure your elbow is never below your fist on hooks.
Solution c - Just like with the knuckles, it is possible to generally maintain good form but on one particular punch, you manage to tweak your wrist. Or you have an unrelated injury that makes your punches hurt until that injury heals. A more nuclear option here is to take something like masking tape or duct tape and literally tape over your wrist after wrapping (and maybe down the wrist as well). I would not necessarily wrap the hand wraps tighter, as a loss of circulation will only compound the problem but taping over will add support as well as remind you that there is an injury, and not to go too crazy.
Problem 2 - Your wrist hurts particularly on hook punches.
Solution a - Besides not keeping a generally straight wrist, another very common problem we notice on hook punches are people hitting with those inside, smaller, "door-knocker" knuckles mentioned earlier, so the hook is delivered more like a slap. This could be due to being too far away from the bag when throwing the hook or a slap just feeling more natural. Open palm-style strikes (hitting with the hard part of the bottom of the hand) have their role in self-defense and MMA but when punching a heavy bag, particularly with boxing gloves on, this is a dangerous thing to do. You are putting a lot of pressure on that wrist and you are very likely losing power and speed to do it anyway. Hooks are compact, powerful punches that should be delivered quickly and with low probability of being successfully blocked. If this is you, try getting closer to the bag, maintaining a right-angle L-shape with your arm and throwing hooks, and slowing things down to make sure you're landing in the right spot.
Hitting Too Hard? No!
One final thing I want to say here is about hitting to hard. I have also heard some people report issues these hand or wrist issues and wonder if they are just punching too hard. Unless you have a pre-existing injury or aren't sufficiently warmed or wrapped up with proper gloves, it is impossible to hit too hard. If punches are causing pain and the harder you hit, the more pain you feel, it is because you are not hitting correctly, not too hard. Sure, you don't want to necessarily go all out when you first start boxing. It is better to start slow and learn the technique. And you generally don't want to go all out when you first begin a heavy bag session either but once you are warm and your hands are used to it and you're wrapped properly with appropriate gloves and you've got proper form, let loose!!! A great thing about the heavy bag in general is that it is the best way to really train your power. You don't have to worry about hurting your training partner and again, as long as you are smart about it, you don't have to worry about hurting yourself either!