Unfortunately, even though you may feel invincible, your brain is not being protected from concussions or sub-concussive injury. In fact, your reduced sensations of pain may prompt you to take bigger hits than you would without any protective gear at all, resulting in a net increase in damage to the brain by putting on a helmet.
In order to understand how football helmets fail to protect our brains while simultaneously making us feel safe, we will need a little bit of physics.
Modern helmets tend to be made from a hard plastic shell with compressible foam lining, and each component helps to manage the energy of impact in a different way. The hard shell takes a localized impact like a coffee mug hitting the side of your head, and disperses the energy over the surface of the helmet. It does not reduce the energy of impact, but it does spread the energy out over a large area, so each square inch of surface only gets a small fraction of the impact energy. The foam lining in the helmet spends energy by deforming and compressing on impact, which leaves less energy to do things like deforming and compressing your head.
When you feel pain, special cells called nociceptors detect compression or damage in the surrounding tissue, and send a warning message to the brain. When you get hit wearing a helmet, the hard shell disperses energy, and the foam lining absorbs energy. Depending on how hard you get hit, there may not be enough energy left to set off these sensors, so you will feel impervious to pain.
Deforming and compressing tissue does much more than just causing sensations of pain. Broken bones, black eyes, cuts, bruises, and skull fractures occur as a result of localized tissue deformation and compression, and you can protect yourself against all of these injuries by wearing a helmet.
Head Rotates on Impact
If helmets do such a good job of dispersing and absorbing the energy of impact, why do we still have a problem with concussions and Chronic Traumatic Encephalopathy (CTE)? It turns out that energy is only one of two-important quantities transferred to your head during a collision. The other quantity is momentum, which you can think of as the ability to make an object move. Momentum cannot be dispersed or absorbed like energy, so even though modern helmets can protect you from a black eye or a skull fracture, they cannot stop your head from rotating on impact.
In order to understand why a forced rotation to the head is so bad for the brain, take a look at your shoes. If you untie your shoelaces and extend them, they might reach 12 inches above your shoe. If you start twisting the laces together, you will notice this length contracts by an inch or two, and fighting against that contraction as you twist takes a lot of force. Your brain is connected to the rest of your body through the brain stem and spine, just like your shoelaces are connected to your shoes, so when your head turns on impact, axons throughout your brain are stretched. This is, of course, an over-simplified analogy that breaks down upon deeper inspection, but I hope the mental picture helps make the nature of the injury seem a little less cryptic.
Boxing Gloves Today
It is interesting to note that boxing gloves and Mixed Martial Arts (MMA) gloves work on the same basic principals football helmets do. Boxing gloves use compressible foam padding to absorb the energy of impact just like the inner lining of a football helmet, and the large surface area of a glove compared to bare knuckles disperses the energy of impact, just like the hard shell of a helmet. Momentum is unaffected, so the brain suffers the full effect of the blows while the gloves protect the face and hands from cuts, broken bones, and other superficial injuries. It is not a coincidence that professional boxers and professional football players both suffer from the same neurodegenerative disease.
All the safety gear in use today, including football helmets and boxing gloves, were developed long before we had a sophisticated understanding of the permanent and cumulative damage done to our brains during contact sports. Now that we know better, ignorance is no longer an excuse, and we should all expect better. The commonly repeated mantra today is "no helmet can protect from concussions or CTE," but as a physicist, I would like to call "bullsh*t," and give you a fun experiment you can try at home to demonstrate why I refuse to buy into that claim.
To get started, you will need a friend and a medicine ball. If you don't have a medicine ball, feel free to grab your spouse or roommate's pillowcase and stuff it tightly with your dirty laundry. Stand tall, with both heels together and your arms crossed in front of you. Ask your friend to throw the medicine ball at your chest.
When the ball hits you, it transfers momentum to you, and if your friend throws it hard, it should give you enough momentum to force you to take a step back. Do this once or twice so your friend can get a feel for it, and ask them to throw the ball the same way every time. If it is true that no helmet can protect against concussions and CTE, then there should be nothing we can do to stop you from taking a step back when your friend throws the medicine ball. If, as I suspect, this claim is more like a child claiming a homework assignment is "impossible" just because they do not want to do the work, we should be able to find a way to keep you from taking that step.
Now it is time to start testing. Grab something hard, like a cookie sheet, and hold it against your chest. This is like the hard shell of a helmet, and it does nothing to stop the transfer of momentum. Try again with something compressible, like a pillow. This is like the foam lining in the helmet, and again, it does nothing to stop you from taking that step back. Try holding the cookie sheet and the pillow together, or try holding any other material you can dream up. Finding different materials to hold in front of your chest is analogous to the full scope of research and innovation explored by the NFL and helmet manufacturers, and it is guaranteed not to work.
Now that we have tested the conventional options, we can try a few that are not doomed to fail by the laws of physics. One simple trick to reduce the effects of the transfer of momentum is to reduce the leverage applied at the point of impact. Instead of hitting you with the medicine ball at your chest, ask your friend to throw it at your crotch. This reduces the leverage by moving the impact closer to the point of rotation, your heels. Of course, you do not have the option to tell someone where to hit you in a game, but if you look at helmets available today, their large size means they move the point of impact further away from the point of rotation at the base of the skull, giving the impact more leverage than it had before.
Another trick you can try is to add some mass to your body. Try wearing a backpack full of textbooks, or holding free weights or jugs of water in your hands. If you have more than one friend, ask a second, preferably smaller friend to climb on your back. The more weight you add, especially when you add it far away from your heels, the less velocity the medicine ball will give you on impact. Of course, if you add more mass than your body can support, the whole thing might backfire on you, but it does not take much mass to make a difference.
The last trick I will suggest may seem like cheating, but I assure you it is not. Pick up one of your feet and take a very large step back, bracing yourself with it. You can think of this as your leg providing an outside force that changes the momentum, but I prefer to think of it as borrowing a little mass from the Earth. This is extremely effective, and there is no reason we cannot do the same with helmets, borrowing a little mass from our bodies.
If these tricks seem familiar to you, it might be because you have taken some martial arts classes. Front push kicks and side kicks transfer a lot of momentum, so martial artists use these same methods to keep from getting pushed back when they hold a large kick pad for their partner. Good kick pads are intentionally heavy, and an experienced pad holder will take a low stance with one foot back.
Now that we know it is possible to reduce the effects of momentum transfer, the first step we can all take is to stop being complacent about safety. Concussions and CTE were an unavoidable part of the game back before we understood the nature of the injury, but the only reason they continue to plague our athletes today is because making real changes to our safety gear is a hard homework assignment we do not feel like doing right now.
Full disclosure: The author has filed a patent application for headgear using these methods to protect the brain from trauma during impact.
The above is an article by Jason Thalken, author of Fight Like A Physicist: The Incredible Science Behind Martial Arts by Jason Thalken PhD