Well, the same studies pretty much say that fundamentally there are a sharply limited number of effect-cases in real combat:
1. Adrenaline and general tolerance paves over the injury until the fight is over at least and it sets in;
2. You bleed out.
3. You shock out (either physically or mentally--some people after a certain degree of injury just shut down as they realize on some level it happens).
4. A very small number of critical injuries either disable or outright kill you (though note most lethal injuries are not actually lethal instantly; a lot land in bleeding out, either immediately, short term or long term (and in fact, a lot of these can be prevented with a trained medic immediately on-scene), cause infection, or otherwise produce cascade effects that lead to mortality over time but won't do so instantly). This is actually uncommon when using most man-portable weapons, past or present; the more useful ones trigger case 2 or 3 above.
Most of these are not cumulative in any particularly meaningful way (though you can get some special cases with bleeding); while there are some complicating issues involving fatigue, especially in melee combat, people take multiple, sometimes serious injuries and keep fighting until the fight is over, and people take one half-way serious one and fold up. Very few game systems make even a gesture at representing this; D&D has never been one of them.