Say you don't own one of those cars where the seatbelt automatically slides back over your shoulder when you close the door. Say you forget to buckle yourself in, and thanks to terrible bad luck, you accidentally step on the gas and ram into a brick wall at 25 miles per hour (40 kilometers per hour). Scientists would say your head hit the windshield with an approximate g-force of 100.
Now let's say none of that happened. Instead, you're a defensive lineman playing college football. On a typical morning you suit up for practice, trot out to the field and go through drills with the rest of your team. Soon enough you crash headfirst into another player. As you do so, six sensors inside your helmet remotely feed information to a laptop sitting on the floor of a small building near the field. The laptop records a hit of 80 gs to the front of your head. Ten minutes later in another drill you hammer into a teammate and get a 98-g blow in the same place. That's the equivalent of two car crashes so far, but thanks to good technique and a thickly padded helmet, you're fine. It's just another typical day on the field.
Then there's an evening practice. The laptop shows you sustained a 64-g hit to the front of your head again. You're fine. Up you get, and it's back to the fray. A little later during a routine play, you get a bit of an elbow from a passing offensive player. Just 63 gs, but down you go with a concussion.
Nine weeks later you're over the concussion and back in play. During warm-ups you get hit with 76 gs, but you're OK. Then it's game time. First play, and you get hammered in the ear with 102 gs. You stagger off the field. For days light hurts your eyes, and you have a hard time staying awake. You've had another very serious concussion.
When it comes to chronic traumatic encephalopathy (CTE), concussions aren't the only problem. CTE is a degenerative neurological disease that has symptoms resembling Alzheimer's and Parkinson's, but unlike those conditions, CTE is caused by repetitive injury to the brain. That day in practice, you didn't just get four big impacts; if you add up all the small, medium and large hits, they come to 31 in total. Now multiply that by a couple thousand. The real issue is all those sub-concussive hits you take day in day out, week in week out, year after year throughout your time playing football [source: Gladwell].