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To understand what a subconcussive impact is, we first need to understand what a concussion is.

Simply put, concussions are the hits to the brain that cause symptoms. Concussions have symptoms because the brain is shaken violently enough that brain cells are damaged to the point where they don’t work properly. Subconcussive hits are those that are below that concussion threshold: the brain is shaken, but not so violently that the damage to brain cells is severe enough to see through symptoms.

Examples of these types of impacts include most tackles and collisions on the football field; headers in soccer; checks or collisions with the boards or other players in ice hockey; and body checks in lacrosse. The impacts are there, and the brain is affected, we just don’t notice it right away.

You can think of CTE like an overuse injury. Baseball pitchers who throw too much eventually might need “Tommy John surgery” due to all the microtrauma that leads to wear and tear on a ligament in their elbow. They never had a big injury to the elbow, but it wore out all the same. CTE may be occurring the same way – it’s not a single injury that causes the disease, but instead accumulation of microtrauma from thousands of hits over years. The big difference is that we’ll never have a surgery that will fix your brain, so it’s important to try to prevent CTE by limiting exposure to head impacts – just like pitch counts in baseball help prevent elbow injuries. 
 

In the last several years, research groups using helmet-mounted accelerometer devices have helped us understand how subconcussive impacts add up to brain damage. This technology allows scientists to count how many hits an athlete takes over a period of time, how hard the hits are, and how much rest an athlete gets between hits. Scientists can use this information to compare athletes with lots of hits to athletes with fewer hits, or to people who haven’t been hit at all.

Another benefit of this type of research is that it allows scientists to take out athletes who have had concussions, allowing them to focus only on the effects of subconcussive impacts. By comparing athletes with lots of hits to athletes without hits, scientists can isolate the effect that hits have on a variety of different things, from an athlete’s memory and thinking ability to the strength of the physical connections in the brain. The picture is pretty clear: too many hits is not a good thing. Here’s what we have learned:

Subconcussive hits hurt our memory and attention.
Several studies have shown that athletes who suffer more subconcussive impacts do worse on memory and attention tests than their teammates who suffer fewer hits.

Subconcussive hits damage the connections in our brain.
Different areas of the brain are interconnected by bundles of wire-like brain cells, allowing them to seamlessly communicate. Using a technique called Diffusion Tensor Imaging (or DTI), scientists can see these connections, and measure their structural strength. Too many subconcussive impacts can damage the structure of these wiry connections, making it harder for brain areas to communicate.

Subconcussive hits suppress brain function.
Using a technique called functional magnetic resonance imaging, scientists can see how active different brain areas are during thinking or memory tasks. Studies in contact athletes have shown that athletes with more subconcussive impacts have less brain activity than athletes with fewer impacts.

Subconcussive hits may contribute to later life mood and behavior problems.
Since we’ve only been able to accurately count subconcussive impacts for the last 10 years or so, we’re still in the early stages of understanding what the long-term consequences of these injuries are. The athletes in the first hit-counting studies just aren’t old enough yet to have developed long term problems.

Some researchers have found a clever way to estimate how many hits someone took over their athletic career, offering us a window into what may be in store for athletes today. By cataloging the positions that each athlete played throughout their career, scientists could estimate how many hits athletes took based on how many hits those playing the same positions take today. Their findings are consistent with the rest of the research on subconcussive blows: the athletes who took more were at greater risk for mood and behavior problems later in life.