How a character reacts to a hit to the stomach will depend on whether or not they had the presence of mind to tighten their abdominal muscles before the impact occurs. This is where the concept of the breath being knocked out of the body comes from, it’s a natural reaction to being hit. If you exhale all the air from your lungs, you’ll notice that your abdominal muscles automatically tighen and the body, particularly the shoulders, curl inwards pulling back. This is the concept behind the kihap, or the loud shout, that occurs in most martial arts when hitting the target. The kihap forces all the air from the lungs, making the muscles of the attackers body tighten at the key moments before impact with their victims body. The problem, of course, with the reaction is that it won’t really help to mitigate the effects of punch to the stomach if the exhalation occurs after the punch is thrown.
As with almost everything to do with combat, timing is key.
Basically, the character is going to feel like they want to throw up. They may actually throw up. A hit to the stomach will force it back into an unnatural position, one that is very uncomfortable. Dizziness, dropping the head as the body comes forward to protect the stomach, arms automatically moving to protect (i.e. wrap their arms around) the injured area. You can also expect a sudden flood of adrenaline if the victim is taken by surprise and sometimes even if they’re not as the body kicks over into fight or flight mode.
So, there could be a sudden increase in heart rate, a loss of fine motor control, a bitter taste in the mouth, etc. And of course, because all the air has suddenly left the body, they’ll be attempting to suck it down like there’s no tomorrow. The effects will be more immediate if the attack is unexpected, so: shock, surprise, anger, fear, panic, all these mental reactions can be used to stun lock the mind and leave the victim incapable of fighting back. If the person in question is unused to experiencing that kind of pain, the effects will be greater and the recovery much more slow. The more used to this particular variety of pain they are, the more hardened they will be to it.
Don’t think of it as an immunity, but rather something more easily ignored. It’s similar in concept to the idea of working out. In the beginning, your muscles are screaming and you feel like you’re going to die. But, as time passes and you keep working at it, it gets easier and the pain of your muscles doing things they don’t want to do becomes more familiar and more easily ignored. Taking a hit is relatively similar, though much more immediate and difficult to overcome.
When getting hit in the face, such as the nose, expect rapid swelling and possibly blood. So, a warm, wet feeling on the face, a taste of copper in the mouth, a sharp stinging pain right between the eyes, it will interfere with vision. Tasting your own blood is a rather surreal experience. People, for the most part, do not react well to it. The head snaps back and will again, drop forward right into the next hit if the victim isn’t careful. Any hit to the face (or really at all) invites the possibility of biting the tongue, especially if the victim isn’t wearing a mouth guard. If that happens, there will be more blood in the mouth, pain, panic, and gagging. For a hit below the eye expect rapid swelling, stinging pain, and loss of vision. There will be visible bruises that will last for, oh, a good week or more afterward.
Bruises are common in all parts of the body when they get hit and they last a long time. If your character fights constantly, they will show that wear and tear in all it’s glory on their body. It can last for a month, depending on how deep the bruises go. When I was training it wasn’t uncommon for me to find small welts all over my body, so much so that when I see a bruise now I just shrug it off.
During my third degree test, I took a roundhouse to my forearm and it became one, big mass of a bruise. I had a matched set for about two weeks, because I’d used the other arm for brick breaking.
The hand of the attacker will also bruise and possibly cut the skin, both on the victim’s body and the attacker’s knuckles. It’s worth remembering that a proper punch is necessary to keep the hand from breaking many of the small bones on impact. But hitting someone else is going to sting. Attacking better protected places on the body, like the rib cage, or the face, will be more obvious as opposed to hitting in the soft places like the throat or the stomach, still the hands will show signs of being in a fight regardless.
This is why the concept of “I don’t want to hurt anyone” is a nice sentiment, but complete bull. Want has nothing to do with it. Combat is a choice. If you fight or fight back, you’re going to hurt someone even if that person is just yourself. The question is not really “do I hurt them at all” but how far do you go and can you live with the consequences.
In specific instances, there’s the possibility of friction burns from the clothes rubbing against the body.
And of course, the most important and long lasting effect on the mind: shame. Also, guilt.
There’s more to it, but at that point it’s a good idea to start looking through medical and forensics textbooks on the subject. This is a little morbid, but in order to generate the right kind of feeling, you may want to stop and look at images of people who have been battered. Hollywood is very clean and combat is ugly. If you want to know how to describe something, you need to know what it actually looks like and decide whether or not it’s something you want to bring into your story.
(Edit: I should also point out that there is no “best” way to do anything, just the best that you’re capable of while working with the scene and how the themes there fit within the overall narrative. Violence is an excellent way to evoke emotion, but readers do have a threshold. How realistic you are is going to depend a lot on what you want them to be seeing and feeling when they read that scene. A sequence that is too vicious and too raw without properly being set up by the narrative runs the risk of knocking the reader out of the moment. This isn’t me saying don’t do it, just make sure you’re balancing realism with the needs of your story. A brutal beating is a key moment for a character, but it shouldn’t happen on the page more than once in a book that’s not dealing with abuse and brutal beatings (and even sometimes when it is). Work with what you’re capable of writing and marry that to what your comfortable with, after you’ve assessed what those limits are, feel free to push away at them as needed.
In the end, you’re the only one who can really figure out what your story needs to function.)