We've all seen the story climax where the Bad Guy and the Hero are fighting. The Bad Guy strikes and lands a deep cut to the Hero's sword arm and Hero drops his weapon crying out in pain. Bad Guy cackles in his victory as Hero sinks to his knees in despair. But then(!) at the last minute, as Bad Guy gloats our Hero pushes past the pane, picks up his sword, and runs Bad Guy through. Yay! *dancing in the streets and Hero gets the girl*
Only one problem with that. It's called reality.
Harken back with me to basic biology. Bones are the frame of the body, muscles and tendons are what makes them move. A muscle, basically, is like a bunch of strings all clumped together. When the brain wants them to move, they contract. Simple.
Now lets look back at our story scene. Bad Guy has just cut Hero's arm. Let's say just below the elbow. All the muscles in the forearm are what control the hand. A deep cut to the arm will sever any of the muscle groups in its way.
A good way to feel this is to hold out your right arm, take your left hand and grip it around your forearm just below the elbow (you'll feel it more if you grip from the underside of your arm). Now make a tight fist with your right hand, then spread your fingers wide. Keep opening and closing your hand and you should feel the muscles flex under your left hand. Those are the strings that control your movements.
Now imagine that your arm has just been cut. Still gripping your arm with your left hand, try to pick something up and hold it tight without moving those muscles. Hard, isn't it? Cutting a tendon is even worse. There are tendons all through your wrists, elbows, shoulders, hips, knees. All of theses are very vulnerable targets. If Hero gets a nasty slash in the shoulder, that whole arm will be useless.
The biggest misconception I see is that it's the pain that is keeping you from using that arm or that leg. If you can just "push past the pain" then you can keep fighting. The truth is, pain has nothing to do with it. If the muscles and tendons are gone then it's like trying to control a marionette puppet with its strings cut. That's why my teacher calls it "de-animating" the limb. There simply aren't any strings/tendons/muscles there to use.
This fact is used in tactics all the time. You want to aim for the tendons and the large muscle groups because, in the heat of a fight, the opponent might not feel pain. But if you've cut all the tendons in the hand, then it doesn't matter. They can't grip the sword to fight back with, no mater how much they want to.
So next time you have a character get a bad slash or stab, be sure to think of what that would actually do to the human body. Use the same technique that we did gripping the forearm and try it on other places. Does he get cut in the leg? Hold the part of the leg that you want to have get injured and then move around. What things can you do without moving that muscle? What things won't work?
Doing this kind of thinking will add a whole new depth to your fight scenes and your story (and will keep my eye from twitching when I read it).