There has been a lot of talk on concussions in ultimate, primarily focusing on training and prevention. (Check out some of the conversations between ultimate players on Facebook and Twitter) As someone who has suffered a mild traumatic brain injury (MTBI) as a result of a collision playing ultimate, but mostly as a biomechanics researcher, reading these discussions has refueled my curiosity on the mechanics behind concussions. What causes concussions, and how are they studied in the fields of sports injury mechanics and prevention?
Mechanism of Injury
Let's go through the mechanism of collisions to the head.
The first scenario is an external force is applied to the skull, like a head on head collision. The force is distributed about the skull, because it is a rigid bone, and the skull accelerates. This acceleration, related to the force by F=ma, causes the skull to move quickly and push against the brain before they accelerate together, causing injury to the brain. The greater the change in velocity (speed or direction of motion) the greater the force of impact between the brain and the skull, resulting in a worse injury.
The second scenario is rapid deceleration of the head, like falling to the ground. Here the brain and skull are moving together, until the skull hits an immovable object and quickly decelerates. The brain continues to move until it is abruptly stopped by the skull, causing damage to the brain.
The third scenario is a pure acceleration based injury (non-contact), like whiplash. Though force isn't directly applied to the skull, the change in direction of motion still causes an impact between the head and the skull, causing injury. (This can sometimes be an easy thing to forget. Just because you didn't get hit in the head doesn't mean you couldn't have suffered a MTBI.)
In all three scenarios, the brain is damaged by large physical loads being applied directly to the soft tissue.
Injury Risk Calculations
It shouldn't be surprising that the driving motivations behind head injury research are prevention, both in sports and in transportation saftey restraints, and forensic biomechanics. (Forensic biomechanics is an interesting field in which scientists act as expert witnesses, modeling or recreating accidents examining the motion and forces of the head. This is used to determine how injuries were sustained, think crime TV, or in compensation lawsuits, to determine if the alleged injuries are plausible and/or preventable.)
Traumatic brain injuries (TBI) are studied by looking at these forces and accelerations in different scenarios. For example, acceleration can be determined by looking at head velocities and stopping distances after impact in game footage , or directly measured using an accelerometer (like in a Fitbit). With information from video analysis, animal and cadaver models, and biomechanic testing of impact dummies, the Head Injury Criterion (HIC) was developed. The HIC is an equation using linear acceleration and duration of impact to calculate a score correlating to likelihood of injury (an HIC score of 1000 suggests a 16% probability of severe head injury) . This equation has been validated by multiple studies and is the head injury metric mandated by the National Highway Traffic Safety Administration (NHTSA) (for evaluation of motor vehicle collision injuries).
While this injury risk calculation is helpful, it is limited by a focus on linear, singular direction motion associated with a head on, or rear end impact. Since sports related injuries are generally more complicated motions involving multi-directional motions, studies have started to look at linear and rotational motion of the head. Rotations can play an important role in injury as shear stress can occur between the head and skull, causing injury to the brain. (Note, these rotations can also strain the stabilizers for your head, that is, your neck soft tissues. The forces and moments on these structures have their own injury criteria, which I'm not going to go into. However, from unfortunate personal experience, injury to the ligaments and muscles of your cervical spine can result in a lot of tension and exacerbate TBI symptoms.)
So, how do protective measures work to prevent TBIs? Hard shell helmets distribute forces, reducing the force felt by the area of impact. Softer helmets, think protective headbands/helmets used in soccer, in addition to the inner lining of harder helmets, compress or deform with applied force and act to increase the impact duration (remember how that was important in the HIC equation?), and reduce the final load on the head. Mouth guards also work to distribute internal forces, and while this definitely protects the face, jaw, and teeth, its effectiveness in preventing TBIs is still debated .
Another method of prevention is to improve your body's ability to react to changes in head acceleration. Stabilizing muscles in the neck, trunk, and core can help reduce the acceleration of the head. Reaction trained stabilization techniques can help prevent injury from expected and unexpected motion (same thing goes with knees, ankles, etc.). For somewhere to start, I'd recommend Ren Caldell's playlist for concussion reduction strengthening exercises.
A more ultimate specific consideration for concussion prevention, in terms of collision prevention, is field awareness. Having a solid understanding of the offensive flow, and cutting and clearing lanes can help avoid unnecessary and unintentional contact. On defense, a better understanding of your mark's movements and anticipation of their motion can put you in the best position to prevent them from getting the disc... without being in the exact same place. Collisions are still a part of ultimate, but knowing where your body and others' are can help you how many collision end in injury by being aware and activating those stabilizers.
Other Concussion-related Tidbits
Here are just a few other things that can contribute to the concussion discussion, but don't need lengthy explanations (and are unrelated to each other).
- First, here is a link to the SCAT 3 or Sport Concussion Assessment Tool. This is what medical professionals use to determine likelihood of a concussion. It wouldn't be a bad thing to keep in a team disc bag. On a related note, it's also not a bad idea to get yourself and/or your team baseline tested by a medical professional (one college team I played for got a school sports trainer to organize a few sessions for AT students to practice using the SCAT 3, by giving the ultimate teams baseline tests).
- Concussion monitoring is also a major concern in soccer, especially in younger players. Check out this awesome wearable device and app that lets you know when you've experienced some worrisome acceleration. It even keeps track of how many you've had throughout the season, and can be used for individuals or teams.
- Speaking of incredible women in soccer, a few past and present USWNT players have pledged to donate their brains for concussion research. Since my focus is on biomechanics, I sometimes forget that there are still a ton of unanswered questions concerning concussions on the biological side. These athletes are incredible, not just for recognizing how they can (invaluably) help further research, but for their dedication to the sport and its future generations throughout their lives and beyond.
I need to point out that I am not a medical doctor, physical therapist, or athletic trainer. My knowledge of these subjects stem from my personal experience with MTBI (asking doctors, trainers, and PTs tons of questions throughout my recovery), a few of my biomechanics graduate classes, and a brief academic literature review.
 Barth, J.T., Freeman, J.R., Broshek, D.K., and Varney, R.N., "Acceleration-Deceleration Sport-Related Concussion: The Gravity of It All", Journal of Athletic Training. 2001; 36(3):253-256.
 Hayes, W.C., Erickson, M.S., and Power, E.D., "Forensic Injury Biomechanics", Annu Rev Biomed Eng 2007. 9:55-86.
 Daneshvar, D.H., et. al. "Helmets and Mouth Guards: The Role of Personal Equipment in Preventing Sport-Related Concussions", Clin Sports Med. 2011. 30(1):145-163.
Image via Diffen.com