Helmet-to-helmet impacts in football cause more concussions in players getting hit than in players doing the hitting. While the occurrence of blindside hits may seem like a reasonable explanation for this pattern, the effects of player awareness and neck muscle tension on concussion are likely too small to fully account for this pattern. Another possible explanation is that a striking player tends to use the front of their helmet to strike a wide range of sites, like the side and rear, on their opponent’s helmet. These different helmet impact locations may be important for two reasons.
First, we know from prior research that the brain is more vulnerable to rapid head twists (“no” motions of the head) than to rapid head nods (“yes” motions of the head). This difference means head impacts that cause twisting motions can lead to more brain injuries than head impacts that cause nodding motions.
Second, helmeted heads aren’t spherical and identical like billiard balls, so the striking and struck players’ helmets and heads can experience different types of motions during the same impact. Some impact locations produce more linear motion while others produce more rotational motion.
To test if these two phenomena could explain why struck players are concussed more than striking players, we ran two computer models of the skull and brain using head motions taken from laboratory football helmet impacts to 12 different locations on the helmet. Our results showed that for the same initial impact energy, the brain experienced lower strains for impacts to the forehead and crown of the helmet than for impacts to other regions of the helmet. Less brain strain means less potential for brain injury.
These findings suggest that striking players are choosing impact locations that minimize their own risk of brain injury. Struck players, on the other hand, often don’t have this choice and thus see impacts over a wider range of impact locations. As a result, struck players’ brains can see higher levels of strain than striking players’ brains. Across all impacts, these higher strains will manifest as more concussions in players receiving the hit than in players delivering the hit.
Figure: Laboratory impact locations (top row) and heat maps of the peak brain strain in three orthogonal slices through the brain (bottom three rows) for two impacts with the same input energy, i.e., the same impactor speed. The forehead impact represents a common location for a striking player’s head impact and the jaw pad impact represents a location more commonly associated with a struck player’s head impact. The heat maps, which are generated using the SIMon brain model, show higher brain strains from the twisting motion that develops in the jaw pad impact.
For more information, see: Elkin BS, Gabler LF, Panzer MB, Siegmund GP (2018). Brain tissue strains vary with head impact location: A possible explanation for increased concussion risk in struck versus striking football players. Clinical Biomechanics. doi: 10.1016/j.clinbiomech.2018.03.021.