What is a concussion?
In the last few years we've gotten a lot more info on them, and literally, they are brain damage. What makes them especially dangerous is that concussions can be extremely unpredictable in terms of cause to effect - sure, getting hit harder in the head is worse, but sometimes lighter hits can cause severe concussions or heavier hits not one at all.
Concussions build up over your lifetime. Earlier concussions make later ones worse and more damaging. Receiving a second concussion while still under the effects of a first one can literally kill you. (High school football deaths are mostly from this.)
The only way to avoid concussions is don't hit your head at all.
That said, you can do a few things if you're going to be in an activity where these happen.
Wear headgear, use gloves. Preferably, go light. Really preferably, don't hit each other in the head and train well to take falls. Obviously, this runs counter to a lot of self defense training and sports, so you have to figure out where on the scale you want to be and how many head shots and how hard is going to be necessary for you to develop and keep your skills and conditioning.
Strengthen your neck muscles. The stronger your neck, the better you can take those shots because you'll have better stabilizer muscles. You can still roll with punches, you just don't want the whiplash effect happening. Stats show sports you wouldn't necessarily think of as being that brutal to the head having high rates of concussions (soccer, volleyball), primarily because they don't do a lot of strength training for the neck and supporting muscles.
Learn what the signs of a concussion are. Don't ignore them. Don't try to tough it out.
When do you have a concussion?
The old way of recognizing a concussion was to ask people things like - "How many fingers am I holding up?" "Who is the President?" "What is your name?" These are valid in the loosest sense - if you lose the ability to visually focus, if you cannot remember things (including short term memory loss - "What were you doing 2 minutes ago?"), you have a concussion.
Do you have a headache? Do you feel nauseous? Do you feel dizzy? Does light seem too bright? Do you slur your speech or mix up your words? You also have a concussion.
If someone's eyes look in different directions, or the pupils do not match in size, there is a concussion. If you shine a light in their eyes and they don't react, there's a concussion.
Sports teams now have a computerized word test they run people through while they're healthy and after they take a hit, run them through again - if they drop below a certain performance level, this indicates a concussion as well.
What do you do if you have a concussion?
If you have a headache that seems to be getting worse? Go to the hospital RIGHT AWAY. The worst case scenario is internal brain swelling/bleeding. People who die from concussions typically die 30 minutes to a few hours after taking the hit, as their brain slowly swells and crushes itself inside their skull.
If not, the answer is it's time for you to stop training. Take a rest. Go home.
Now comes the other unfun part of recovering from a concussion. You're looking at 3-10 days of recovery IF you rest. Rest means:
- Don't take any other head hits
- Try to engage your brain as little as possible - no reading, no studying, no figuring out the tough problem at work...
Non-rest can increase your concussion recovery time to months.
The concussion has damaged the neural connections in your brain, and you basically are stuck letting your brain connect things around again, starting from the most basic autonomic processes. Things like "pupils focus like this", "We control our tongue and mouth using these neural connections", etc. Trying to read or intake heavy information ends up causing the brain to prioritize high level processes and leaves the low level ones unrepaired.
Wow that sounds totally unreasonable!
Yep! The brain is not well designed to take hits. It helps if you remember that for most of human evolution we weren't designed to last quite as long as we do these days, nor did we need to do much more than organize hunting food or gathering it as a social group.
It's up to you to decide for your own health how much risks you want to take with head shots and how you want to train around that.
You can see a lot of traditional martial arts where sparring only involves body and leg shots. That's an adaptation, but not necessarily as good for self defense. You can also see arts where people do controlled drills most of the time, and only once in a while break out the head gear and go with any force. That's more useful. It really depends on what risks you plan on taking for you.
Just be aware - muscle heals easiest, bone heals ok, joints heal hard, the brain barely heals.