It’s not so much special training against pain, so much as it’s contextualized by training and experience. What you do becomes normal over time, so the more used you are to getting injured then the more inured to it you become. This happens to athletes and soldiers all the time. The more dangerous the job, the greater the chance of injury, then the more used to it they are.
@readingwithavengeance once told a story on her blog about a soldier in Iraq who got a gut wound, grossed out his regiment by going to the dining area with his guts hanging out while waiting for pickup so he could get taken to surgery. His exposed intestines were protected by a plastic bag, but, you know, perfect time for shit on a shingle.
Remember, what might sound horrific to you is normal for someone else. Normal is relative to events in your life and your experiences. A lot of the problems that come with martial arts and training in fiction are born off the idea that because the author’s normal is normal then their character must need or be doing something special. When, in actuality, all their character has done is adjust to a new normal that most who go through the same would experience.
The ability to endure pain is a lot more common and a lot less special than fiction makes it out to be.
Someone who has gone through a great deal of emotional trauma will be better at enduring it than someone who has not.
The same is true for physical pain.
As with everything, practice makes perfect.
One part of training is learning to work through pain because training is painful. Developing endurance, training up your muscles is painful. It hurts. But to complete your training or do your techniques, you have to force yourself through it and overtime you improve.
Part of training is learning the difference between good pain (my muscles are lazy) and bad pain (my muscles are seriously injured), but in the beginning it’s all just pain.
Pain is the body’s way of speaking to you. The problem is that your brain instills a lot of false limits, so there’s what your body’s used to doing, what you believe you can do, and what you can actually do. Most people underestimate themselves, they listen to their body and stop when it hurts. Physical training teaches you about the different kinds of pain your body experiences, how to work with it, how to ignore it, how to suppress it, and how to overcome it. It’s not a special lesson, it’s one you learn by consistent repetition.
You got to be tired, you wake up hurting, but still you have to go out and train. You’re not injured, your body just hurts. Physical training involves either breaking your muscles down (such as weight lifting and body building, you tear your muscles to build muscle) or stretching and lengthening them with endurance training like running. It hurts, but it’s not an acceptable excuse to stop.
A large part of training is acclimatizing, you’re getting your body and mind ready and/or used to the strain they will be put under when actively participating in their future endeavor.
Army Drill Sergeants, particularly yell at recruits in Basic in order to simulate and stimulate stress in the trainees with the goal to acclimatize them to continue working, thinking, acting, and fighting while they are under stress.
You want to try to put together a rifle with some asshole screaming in your ear and telling you that you’re the worthless scum of a bootlicking cocksucker? Well, if you can do that then you’ll probably be able to do it when your friends are dying around you and bullets are flying over your head.
For people used to receiving minor physical injuries (scrapes, cuts, muscle pain, bruises), injuries just start to sort of roll off them. They still hurt, but you keep working despite them. You acclimatize, you create a new sense of normal, and adapt. You figure out the limits of your injury and then start managing around it.
The question for soldier’s with injuries isn’t just whether or not it causes them pain. Does it significantly impact their ability to do their job? Can they still meet the physical requirements? That’s not just an executive decision you make as an author, but one that happens in conjunction with the rules you’ve set up for your world and the organizations in it.
This is the part where we talk about bureaucratic rules. If you’re writing a mercenary then it doesn’t really matter. It’s possible, depending on the era which you draw your inspiration for your military, that it won’t matter in the regular army either. However, every army has standards and those standards must be met by the participating soldiers. There’s a lot more to an army than fieldwork.
Soldiers do not assign themselves, soldiers are assigned by a higher authority figure.
Your soldier has to meet their military’s physical fitness standards in order to continue operating as a soldier in the field. If they can’t meet those standards then they are a liability to the men they serve with and will be placed elsewhere. (Soldiers are not lone wolves, they are cooperative beasts. They are part of a unit, that unit works together. If you haven’t spent time thinking about the Company and the cadre of characters your soldier serves with, then it’s probably best to start.)
If his recurring injury does not significantly impact motor function or his ability to perform/meet the necessary standards then it’s just pain.
Pain is suppressed. Pain is dealt with. Pain is ignored.
For him, that’s normal. It won’t be odd to anyone in his unit unless they’re new and he certainly won’t be the only one suffering from an old injury/scar that is a reminder of his mistakes.
It’s sort of along the same lines conceptually as the old man whose bones ache every time there’s a change in air pressure or a storm moves in.
The key to selling this kind of injury in your fiction is to present as normal. Maybe not normal to you or your reader, but it is for him. He didn’t go through any special training to achieve the ability to ignore his pain. That’s ultimately a false note.
If he can’t work through his pain, then he can’t do his job. If he can’t do his job then people die. He prioritizes what has to be done to do his job over his physical well-being. That’s part of his training.
All training involves pain.
He has pain, he got used to it.
It really is as simple as that.