When the human heart rate increases from stress alone (as opposed to exercise), certain predictable effects take place.
Note a trend in this chart. A certain level of elevation of the heart rate, between 115-145 bpm, benefits everything a human being can do, except fine motor skills. (If you need to thread a needle, it's best to be at resting heart rate.) Beyond that heart rate, human performance generally deteriorates, except for gross motor skills and movement, which are best over 160. The heart rate that supports best the activities of, say, swinging a sword with a full adrenaline power boost, generally shuts down the higher brain functions that are good at conducting strategy. One of the effects of training is to be able to enable a warrior to act appropriately in combat by rote memory, since higher brain functions tend to shut down.
Another form of training, probably more ancient that Zen Buddhism, teaches a warrior to calm himself or herself, in particular through controlled breathing. Note that an archer needs calmness more than a front-line swordsman. And a bomb-disposal technician needs to be even calmer than an archer. In stories, human characters who are good at fine motor skills under stress (like bomb disposal) should not be sweating, with a pulse throbbing in the neck--the person good at that kind of job will strike others as being unnaturally level-headed, eerily calm, like someone with "ice water in the veins." Being a berzerker works for a swordsman. A berzerker archer--or fighter pilot--is not a realistic character.
The higher heart rate chart only tracks some of the effects combat stress can have on a human being. The following chart is from On Combat, based on a study done on police officers in kill-or-be-killed encounters:
(Here's that link to the Warrior Science site.)
One of the difficulties concerning this chart is it's hard to match it up with the heart rate chart above. It's not as if police officers in a shootout--or soldiers in combat--are wearing a heart monitor. But other studies seem to indicate these perceptual changes occur when the heart beats over 160 per minute.
Another difficulty with the chart is that not everyone experiences the same effects. As the author of this blog post, I should make it clear that my personal high-stress experiences do involve some combat situations (such as a gym rocketed when I was in Iraq, with two people killed and 17 wounded, and I helped with the wounded), but I have never myself been in a shoot-out with an enemy. My primary personal experience with high stress actually stems from being an eye witness to a number of accidents, including a gun accident caused a relative carelessly pulling a rifle trigger and shooting my younger sister in the face (she survived). People who read my descriptions of combat will find characters having strange random thoughts in the midst of stress, or making odd, out-of-place observations. According to the chart above, only 26% of people under circumstances like mine reported "Intrusive Distracting Thoughts." I personally have also experienced diminished sound, tunnel vision, automatic pilot (where you can act but cannot speak), slow motion time, and a degree of dissociation. I write what I know myself, but studying information like in the chart above has helped me understand my own reactions are not necessarily the only ones--and are not necessarily typical.
Some of these perceptual reactions can be very adaptive. For example, slow motion time, caused by the brain increasing its processing rate, gives the ability to react to something happening much more quickly than a person normally could do. Yet often a person experiencing slow motion time is not able to make a good independent decision and will only usually be able to repeat something learned by rote in training. Though Lt. Col. Grossman points out that a person can repeatedly train in high stress situations and learn to think and act more rationally, while still reaping the benefits of greater gross motor ability and perceptual benefits such as slow motion time. This is part of the secret of success of special forces units, according to Lt. Col Grossman. They train in high stress so often, they are very nearly fully functional mentally with heart rates over 160--as opposed to not having an increased heart rate at all. (Though very little change in heart rate is probably what happens for certain highly unusual individuals in combat.)
This topic is too large to fully handle in a single blog post. For more detail on these effects you can follow the links, read the books, and round out your own research on what has been observed to happen to human beings in combat.
Let me part with a few observations as a speculative fiction writer on how all this relates to stories. First, these are natural human reactions. A science fiction story could easily feature designer drugs that let one effect, say slow-motion time, be turned on without the negative side effect of, say, being unable to speak. (Of course these drugs could go horribly wrong somehow.) Clearly a cyborg does not operate by these rules at all and could be completely calm in battle. Which is in fact portrayed often enough in science fiction.
Second, these are natural human reactions. Lord of the Rings Dwarves might go straight to a high heart rate, getting gross motor benefits, while nonetheless being able to think. Elves may stay icy calm, but still be able to fight. Aliens from other worlds might have high stress reactions quite unlike humans altogether. For example, especially if they were to have some form of natural armor (say on the back), an alien race might have to fight a natural tendency to roll up into a ball, which would not be adaptive if an enemy was shooting bullets.
Overall, combat stress reactions are a reality any fiction writer who portrays combat should be thinking about. And the topic can provide some especially interesting story opportunities for the speculative fiction writer.
Questions? Comments? There's a lot more that could be said on this topic and I'd be happy to address any particular issues in the comments below.