Aug 2, 2019
Examining subconcussive hits in sports
By Kim Barber Foss, contributing writer

Have you ever taken a close look at a football player’s helmet? Surrounding the decals and logos, you’ll usually see scuffs and scratches that indicate where the player took a hit. Each one of these marks could also represent a subconcussive impact.

Most of you are probably following the coverage of sport-related concussions – the traumatic brain injuries sustained on the “big” hits. However, there is growing concern that lower magnitude, more frequent subconcussive impacts could be the real ticking time bomb that leads to serious long-term repercussions, including chronic traumatic encephalopathy (CTE).

The research surrounding subconcussive hits is still in its infancy, though, which makes them a controversial category of head trauma. Their exact meaning is still evolving, and we don’t completely understand the potential lasting effects.

Still, we know enough to indicate subconcussive impacts have the potential to be dangerous and are something athletic trainers should be aware of and try to prevent. By embracing available information and the strategies to reduce these hits to the head, athletic trainers can continue to be advocates for our athletes’ brain health.

What are they?

A subconcussive hit is an impact that is below the threshold that would result in a concussion. When one occurs, there is still movement (or “slosh”) of the brain, but it’s not severe enough to cause symptoms. At least, not immediately.

There is no definitive impact magnitude that would classify as subconcussive (i.e., 10g, 20g or 50g) because each individual person’s brain may react differently. An impact magnitude of 20g may not affect one athlete yet cause a concussion in another. This makes studying head impact exposure and concussion research very challenging.

As with concussions, football is the epicenter for much of the discussion surrounding subconcussive impacts. Several studies have found that college football players may sustain 950 to 1,500 head impacts in a single season, and a high school player may face up to 1,100.

While it may be the wide receiver or the running back that typically undergoes the “big hit,” it is the linemen that sustain the most impacts per game, as well as the highest average number of impacts in a season. Granted, these hits to the head may have lower magnitudes, but the cumulative higher volume is the concerning factor.

Although football has been the focus of much of the research surrounding subconcussive impacts, they are not isolated to football. From heading the ball in soccer to checking in hockey and lacrosse, any time there is contact or a collision, there is the risk for a subconcussive hit. We need to think of this topic as a brain health issue and not just related to a single sport.

Potential dangers

The chief concern surrounding subconcussive head impacts is the potential long-term neurological consequences. We know that long-term repetitive trauma results in degradation of other soft tissue (think of overuse injuries in baseball pitchers, for example), so it is intuitive that the brain may also experience damage from repeated trauma exposure.

Perhaps the most significant consequence being studied is the relationship between subconcussive hits and CTE. Right now, the best available evidence suggests the cumulative nature of subconcussive impacts, not concussions, is the driving force behind CTE, and some of the athletes discovered with CTE never had a reported concussion.

  » RELATED: A comprehensive approach to concussions

In addition to the connection to CTE, recent research has identified several areas of potential repercussions to players who have been exposed to a greater number of head impacts. Memory, attention/focus, brain activity, and oculomotor performance are all areas thought to be affected by subconcussive hits. Here’s a breakdown of some of the findings:

According to the Concussion Legacy Foundation (CLF), those athletes exposed to a greater number of impacts tend to perform worse on tasks designed to measure memory and attention than athletes who’ve sustained fewer head impacts, and even show decreased brain activity on functional magnetic resonance imaging. Other findings from the CLF noted that imaging studies using diffusion tensor imaging have shown structural damage to connections in the brain, which may make it harder for areas in the brain to communicate. The organization also believes there may be a link between repetitive head trauma and emotional and behavioral problems later in life.

In a 2014 Journal of Neurotrauma study, asymptomatic high school football players showed changes in both neurocognitive and neurophysiological changes over the course of a season. A follow-up study found those changes were correlated by the exposure to subconcussive impacts throughout the season. It is concerning that these participants demonstrated measurable change in neurocognitive function and brain physiology, yet none were symptomatic, which would have promoted removal from activity.

The near-point convergence test measures how well the eyes focus and accommodate as a target is brought toward the subject. A 2016 study in JAMA Ophthalmology examined collegiate football players and found that those athletes who sustained higher frequencies of head impacts had greater convergence insufficiency than teammates with fewer impacts. The differences returned to normal baseline value following three weeks of rest.

What ATs can do

Besides staying current with the research, there are some practical steps athletic trainers can take to be proactive about subconcussive impacts. There are three proposed ways to reduce an athlete’s exposure to them.

The first is to eliminate unnecessary contact. For football in particular, this could be achieved by reducing the number of full-contact practice days. In 2017, the NCAA restricted practice times and eliminated two-a-days in the hopes of reducing routine head impacts, and many high school state athletic associations have taken similar steps.

A second way is to modify contact exposure. This could be by changing drills performed in practices, limiting the number of contact repetitions during each drill, and making sure technique is performed correctly during activities like football tackling.

For instance, a July 2018 article in Annals of Biomedical Engineering explained that making changes to certain college football drills could dramatically reduce the number of head impacts players faced. The researchers said cutting the time spent on the highest-risk drills by just a few minutes per practice could get rid of the equivalent of a years’ worth of head impacts in players.

A third way to limit subconcussive impacts is to actually delay athletes’ exposure to them. An example of this would be substituting flag football for tackle football. A 2018 report from the Aspen Institute’s Sports and Society Program recommends just that. It also encourages athletes to delay the introduction of tackle football until the age of 14.

In other sports, USA Hockey banned body checking at the Peewee level in 2010, and researchers from the Mayo Clinic released recommendations in January 2019 to eliminate body checking in Bantam youth hockey leagues. In soccer, U.S. Soccer’s 2016 Concussion Initiative banned heading for athletes 10 and under and restricted heading in practices for 11- to 13-year-old players.

In addition to impact-reduction efforts, measuring the magnitude, linear acceleration and rotational velocities of head impacts can be beneficial. One way to do this is through the use of accelerometers. These allow researchers to track and quantify the head impact exposures sustained by each athlete over the course of the session or season.

Any subconcussive impact prevention or documentation efforts athletic trainers embark on will likely require getting coaches and parents on board. One way to get them to buy-in is to break down the research – as it can be overwhelming. I would recommend trying to present the information in as “lay-person” a way as possible and not with an alarmist intent.

A final word

Despite the studies we already have, longitudinal research on the consequences of head impact exposure is still being developed, and I predict more research in this area will be published over the next five years. This flurry of new evidence has the potential to overwhelm clinicians.

While athletic trainers need to continue to embrace new research as part of our evidence-based practice, we should be critical of research about both concussions and subconcussive impacts. Critically appraise the study, the sample, and the methods.

For as much as we are still learning about subconcussive impacts, we know the potential is there for them to cause great issue in athletes. Athletic trainers should be informed about them, stay as proactive as possible to prevent them, and remain tuned into the long-term research on the consequences of head impact exposure that will surely come out in the years to come.


Kim Barber Foss, MS, LAT, ATC, is an Athletic Trainer/Research Biomechanist at Cincinnati Children's Hospital Medical Center, through which she provides outreach sports medicine care to Lockland (Ohio) High School. She is also an Assistant Professor in the Graduate School at Rocky Mountain University of Health Professions and an Adjunct Professor with Northern Kentucky University. Barber Foss serves on the Board of Directors of the National Operating Committee on the Standards of Athletic Equipment (NOCSAE) and serves on several local, state, and national committees. She received the 2018 Gatorade Secondary School Athletic Trainer of the Year Award and 2019 NATA Service Award and has participated in numerous studies on concussion and subconcussive impacts.
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