In almost any story, the hero faces challenges that are dangerous to life and/or limb. This isn’t always the case, obviously, but I think it’s obvious that there are a large number of instances where the hero gets beaten up, shot, stabbed, tortured, or injured in some other way. For people who aren’t physiotherapists, doctors, or other medical professionals the types of injuries that people can suffer can be a daunting task to look into. Because really, a person can be injured in an infinite number of ways.
Still, there are certain constraints that should* be placed on our heroes.
(*should, but often aren’t. Like when a hero gets thrown from a car in a motor accident and then walks over to the bad guys’ car and shoots them repeatedly. While adrenaline can do many wonderful things to keep a severely injured person moving, there are limits to what a body can withstand)
In basic terms, there are acute or sudden injuries (like getting punched, stabbed, shot, burnt, etc.), over-use injuries (like strains), and re-injuries (which I hope are fairly obvious).
Now, acute injuries are probably what most writers are going to want the details on. It’s much more heroic sounding to say that “the firefighter dashed into the burning building, pulled the woman out from under a burning beam and as he was running her to safety the floor collapsed and the fall broke his legs”. Rather than “as he swung the woman over his shoulder and ran her out of the building he felt his back twinge and suddenly moving hurt a whole lot more than it used to.”
Still, don’t just dismiss over-use and re-injuries. They can help make a character more rounded out and believable. Re-injuries give them a past that might have been daring and adventurous, or perhaps just unlucky. And over-use injuries add realism.
There are great sources out there to discover the details of injuries and the limits of the human body. I personally really enjoy the books written by D. P. Lyle. Specifically, I only own More Forensics and Fiction: Crime Writers’ Morbidly Curious Questions Expertly Answered. I have also contacted the author with a question and he was very helpful (the stipulation for his help is that he is allowed to use my question and the resulting answer in any books he might write. I thought that was a more than fair trade off). Finding sources like this are great and very important to realistic writing.
Psychologically, I absolutely adore The Writer’s Guide to Psychology: How to Write Accurately about Psychological Disorders, Clinical Treatment and Human Behaviour by Carolyn Kaufman. Even with a BA in Psychology, I find myself referring back to this book as I write. It’s very easy to read and points out what is common perception versus actual fact (which, even those of use who studied the topic in school can sometimes forget or mix up).
There are, of course, lots of online sources that you can refer to. WebMD can be a fun source to assign symptoms and injuries to your characters, though it works best for figuring out what illness would likely cause the specific symptoms you want your character to display. I have seen other sources that claim to explain the details of injuries, but so far none of them have impressed me like the works of D. P. Lyle. Possibly because they are free, online sources that may or may not be written by somebody with any actual knowledge.
If you can at all manage it, find websites that are created to aid nurses, doctors, and similar in their studies. I honestly cannot figure out how I stumbled across a pre-test preparation quiz for nurses that focused on the treatment of fractures, but it was immeasurably useful for describing the treatment of such an injury in a post apocalyptic story I wrote last year for NaNo. I tried to re-find it so I could link it for this article and I can’t 😦 I do remember it had a green and grey kind of colour scheme on the page, but my googling is not turning up results. If you know what site I’m talking about (or stumble upon it yourself) let me know so I can link it properly!
Also, don’t discount things like taking a First Aid course. At the very least, you’ll get a basic understanding of how characters might deal with injuries before they get to a doctor or hospital. A First Aid course might also give you great ideas of torturous things to do to your characters.
Finally, personal experience/anecdotal stories can be surprisingly helpful. Just remember, that everyone is different and that just because something happened a certain way for someone doesn’t mean that that is the most common way things go down. (Example, it is a family trait that we know how serious an injury is by how quickly and how long the injured part is numb… to all sensation. For most people, severe injuries do not go numb without some sort of numbing agent. Like drugs, ice, or tourniqueting. There are exceptions, of course. And complications, like going into shock, that can cause a numbing in the injured area of the body…)
So try to remember that your characters are going to have limitations (even the immortal, unkillable ones). Whether that’s a worry about blood loss, going into shock, the ability to perform a task with a certain injury, or just the need for recovery time. Don’t sacrifice realism for ‘more heroics’ or ‘badassdom’.