Over the past two decades, public awareness of sports-related concussions has grown dramatically. Athletes, coaches, parents, and medical professionals now recognize the importance of identifying and properly managing concussions to protect brain health. However, concussion research has also revealed another important area of study: sub-concussive impacts and events.
These lower-level head impacts may not cause immediate symptoms, but researchers increasingly believe they may play an important role in understanding cumulative brain exposure and long-term neurological health.
So, what exactly is a sub-concussive event, and why are scientists paying so much attention to it?
Understanding Concussions
To understand sub-concussive impacts, it is first helpful to understand what a concussion is.
A concussion is a mild traumatic brain injury (mTBI) caused by biomechanical forces acting on the brain. These forces occur when the head experiences rapid acceleration, deceleration, or rotation, causing the brain to move within the skull.
Common concussion symptoms include:
Headache
Dizziness
Confusion
Memory difficulties
Balance problems
Nausea
Sensitivity to light or noise
Difficulty concentrating
A concussion occurs when these forces disrupt normal brain function and produce recognizable symptoms.
What Is a Sub-Concussive Event?
A sub-concussive event is a head impact or head acceleration that does not produce symptoms severe enough to meet the clinical criteria for a concussion.
In other words, the athlete experiences a measurable force acting on the head, but:
No concussion is diagnosed
No obvious symptoms occur
The athlete continues participating normally
The individual often has no awareness that the event occurred.
This is why sub-concussive events are sometimes referred to as “invisible”.
Sub-Concussive Does Not Mean Insignificant
One of the most common misconceptions is that a sub-concussive event is simply an impact or event that “doesn’t matter.”
In reality, the term only means that the impact did not result in a diagnosed concussion.
Researchers are increasingly interested in these events because athletes may experience hundreds, thousands, or even tens of thousands of sub-concussive impacts throughout their sporting careers.
Examples include:
Routine contact in football
Minor collisions in basketball
Ball impacts in soccer
Falls during cycling or skiing
Repetitive head acceleration in equestrian sports
Contact during cheerleading stunts
Player collisions in handball
Individually, these events may be too small to produce symptoms. However, their cumulative effects remain an active area of scientific investigation.
How Do Sub-Concussive Events Occur?
Sub-concussive events can result from either direct or indirect forces.
Direct Impacts
Direct impacts occur when an object or another person contacts the head. Examples include:
Helmet-to-helmet contact
Ball impacts
Head-to-head collisions
Contact with the ground
Impacts against equipment or obstacles
Indirect Impacts
A direct blow to the head is not required. Rapid body movements can also generate head acceleration. Examples include:
Body and torso contacts
Sudden stops
Whiplash-like movements
Falls
Abrupt changes in direction
Hard landings
In these situations, the brain may still experience biomechanical stress despite the absence of direct head contact.
Linear and Rotational Forces
Scientists generally describe head impacts and events using two major components:
Linear Acceleration
Linear acceleration occurs when the head moves rapidly forward, backward, upward, downward, or sideways. These forces are often measured in g-forces (g).
Rotational Acceleration
Rotational acceleration occurs when the head rotates around its center of gravity. Modern concussion research suggests that rotational forces may be particularly important because they create twisting and shearing stresses within brain tissue. Many researchers believe that understanding rotational motion is essential for evaluating both concussive and sub-concussive events.
Why Are Researchers Concerned About Repetitive Exposure?
Historically, sports medicine focused on diagnosed concussions because they are visible and clinically apparent. However, advances in neuroscience and biomechanics have revealed that repeated sub-concussive impacts may also influence the brain.
Research in various sports has identified associations between repetitive head-impact and event exposure and:
Temporary changes in cognitive performance
Alterations in brain connectivity
Changes in white matter structure
Biomarkers associated with neural stress
Importantly, scientists are still investigating what these findings mean for long-term health. Not every athlete exposed to sub-concussive impacts experiences neurological problems, and many questions remain unanswered. Nevertheless, cumulative exposure has become an important focus of research.
The Dose Matters
Many experts now view head-impact and event exposure similarly to how other forms of exposure are considered in health sciences. A single low-level event may have little effect. However, repeated exposure over time may be more important than any individual impact.
Researchers therefore study factors such as:
Number of impacts and events
Impact and event magnitude
Frequency of exposure
Recovery time between impacts and events
Duration of athletic participation
This concept is often referred to as cumulative head-impact and event exposure.
Measuring What We Cannot Feel
One challenge in studying sub-concussive events is that athletes often do not notice them. As a result, researchers increasingly use wearable sensor technologies to objectively measure head movement.
Modern head impact monitoring systems can track:
Linear acceleration
Rotational acceleration
Angular velocity
Impact frequency
Cumulative exposure over time
These systems allow scientists to quantify impacts that would otherwise go unnoticed.
One example is the ACT Head Impact Tracker head sensor Pro. The sensor is designed to measure both linear and rotational head movements during sports and physical activities. By collecting objective data, such technologies can help better understand how athletes experience head acceleration in real-world environments.
It is important to note that head impact monitoring devices are not concussion diagnostic tools. A concussion diagnosis must always be made through appropriate medical evaluation. However, monitoring technologies can provide valuable information regarding impact exposure patterns and support ongoing research into brain health and injury prevention.
Which Sports Involve Sub-Concussive Events?
Sub-concussive events may occur in many different sports and activities. Examples include:
Contact Sports
American football
Rugby
Ice hockey
Lacrosse
Team Sports
Basketball
Handball
Football/Soccer
Cheerleading
Individual Sports
Cycling
Alpine skiing
Snowboarding
Equestrian sports
Skating
Combat Sports
Boxing
Mixed martial arts
Wrestling
The specific mechanisms vary, but the common factor is exposure to head acceleration and impact forces.
What We Still Do Not Know
Despite significant advances in research, important questions remain. Scientists are still working to determine:
How much exposure is too much?
Which types of impacts and events are most important?
How age influences vulnerability?
How sex differences may affect risk?
Which athletes are most susceptible to long-term effects?
How cumulative exposure interacts with diagnosed concussions?
Answering these questions will require continued research across many sports and populations.
Looking Ahead
The study of sub-concussive impacts and events represents one of the most important emerging areas in sports medicine and neuroscience.
Advances in wearable sensors, biomechanics, neuroimaging, and biomarker research are providing new opportunities to understand how the brain responds not only to major injuries but also to the vast number of smaller impacts and events experienced throughout a lifetime.
As knowledge grows, sports organizations may be able to develop more effective strategies for monitoring exposure, improving safety, and protecting long-term brain health.
Conclusion
A sub-concussive impact or event is a head impact or acceleration that does not produce symptoms severe enough to be diagnosed as a concussion. Although these events often go unnoticed, researchers increasingly recognize that repeated exposure may play an important role in understanding brain health and cumulative neurological stress. By combining advances in research, athlete education, and head impact monitoring technology, the scientific community is working toward a better understanding of how both visible and invisible head impacts affect athletes across a wide range of sports and activities.
ACT Head Impact Tracker is a measurement device. ACT Head Impact Tracker is not a medical device. It does not provide medical advice, diagnostics, or treatment suggestions. In case of suspected head injury, immediately and safely remove the athlete from activity and seek assessment from a licensed medical professional.
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