American football is one of the most physically demanding sports in the world. The combination of speed, mass, and frequent collisions creates an environment where head impacts and events on a head are common. While public attention often focuses on diagnosed concussions, researchers increasingly recognize that a broader spectrum of head impacts and events on a head -including those that do not cause immediate symptoms-may also affect brain health. These are known as sub-concussive events.
Understanding Concussions
A concussion is a mild traumatic brain injury (mTBI) caused by a sudden force transmitted to the brain. Contrary to popular belief, a concussion does not require a player to loose consciousness. In fact, most sports-related concussions occur without a knockout.
When a player’s head experiences a rapid acceleration, deceleration, linear and rotational movement, the brain moves within the skull. This movement can stretch and disrupt nerve cells, alter brain chemistry, and temporarily impair normal neurological function. Symptoms may include likes of:
Headache
Dizziness
Confusion
Memory problems
Sensitivity to light or noise
Balance difficulties
Nausea
In American football, concussions typically occur during tackles, blocking, or collisions involving the helmet, shoulder, or body. Importantly, the impact does not need to be directly to the head. A forceful hit to the torso can generate enough acceleration to cause the brain to move inside the skull.
The Physics Behind Football Concussions
Several factors contribute to concussion risk in football:
Linear Acceleration
Linear acceleration occurs when the head is driven forward, backward, or sideways. This type of force can cause the brain to collide with the inside of the skull, leading to tissue stress and injury.
Rotational Acceleration
Many experts consider rotational acceleration to be particularly damaging. When the head rotates rapidly, brain tissue experiences twisting forces that can strain nerve fibers. These rotational forces are strongly associated with concussion symptoms and more severe neurological effects.
Repetitive Exposure
Football players may experience hundreds or even thousands of head impacts throughout a season. While only a small percentage result in diagnosed concussions, the cumulative exposure remains a growing concern among scientists and medical professionals.
What Are Sub-Concussive Events?
A sub-concussive event is a head impact that does not produce noticeable concussion symptoms and therefore does not meet the clinical criteria for a concussion.
Examples include:
Routine blocking at the line of scrimmage
Minor helmet-to-helmet contact
Repeated impacts during practices
Frequent collisions during special teams play
Because these impacts often go unnoticed, they can accumulate over time. An offensive or defensive lineman, for example, may sustain dozens of low-level head impacts during a single game without ever being diagnosed with a concussion.
Why Sub-Concussive Events Matter
For many years, sports medicine focused primarily on preventing and treating diagnosed concussions. However, advances in brain imaging and biomechanical monitoring have revealed that repeated sub-concussive impacts may also produce measurable changes in the brain.
Research has shown that athletes exposed to numerous sub-concussive impacts may demonstrate:
Changes in white matter structure
Alterations in brain connectivity
Temporary changes in cognitive performance
Biomarkers associated with neural stress or injury
Importantly, scientists are still investigating how these changes relate to long-term neurological health. Not every athlete exposed to sub-concussive impacts and events develops lasting problems, and many questions remain unanswered.
The Connection to Chronic Traumatic Encephalopathy (CTE)
One reason sub-concussive impacts have attracted significant attention is their potential relationship with Chronic Traumatic Encephalopathy (CTE), a neurodegenerative disease associated with repeated head trauma.
CTE has been identified in some former football players and is characterized by abnormal accumulations of tau protein in the brain. Symptoms may include cognitive decline, mood disturbances, and behavioral changes.
Researchers increasingly believe that lifetime exposure to repeated head impacts-not just diagnosed concussions-may contribute to CTE risk. However, the exact relationship remains an active area of scientific study, and many factors likely influence who develops the disease.
Measuring What We Cannot See: The Role of Head Impact Monitoring
One of the challenges in understanding brain injury in sports is that not all potentially significant impacts and events produce immediate symptoms. A player may feel completely normal after a collision, yet still have experienced a substantial force to the head. As a result, researchers, sports scientists, and medical professionals have increasingly turned to head impact monitoring technologies to better understand athletes’ exposure to both concussive and sub-concussive events.
Modern head impact monitoring systems use wearable sensors that record the magnitude and characteristics of head movements during sports participation. These systems can measure linear acceleration (often expressed in g-forces) as well as rotational or angular motion, which is considered particularly important because rotational forces are strongly associated with stress on brain tissue.
The goal of impact monitoring is not to diagnose a concussion. Instead it is to provide objective data about the real-life events on the pitch, when and how often impacts and event occur, the forces acting on a head in the events, and how exposure accumulates over time and more. This information can help researchers study injury mechanisms, assist coaches and team staffs in following the events on the pitches in real-time, evaluating athletes, training practices and procedures, and support team personnel more data to help in identifying athletes who may require further assessment.
One example of this technology is the ACT Head Impact Tracker Head Sensor Pro. The sensor is designed to measure both linear acceleration and angular forces acting on the head during sports activities, providing a more complete picture of impacts and exposure. The lightweight sensor can be attached to a headband or other tightly fitted headwear and transmits data through a mobile application and browser access for analysis.
Importantly, technologies such as the ACT Head Impact Tracker are measurement tools. They are not medical devices, nor concussion assessment tools. ACT Head Impact Tracker does not diagnose concussions, provide medical advice, or determine whether an athlete is injured. Instead, the data can complement event assessment and evaluation, and contribute to a broader understanding of an athlete’s event and impact history, number of concussive and sub-concussive events, the physical brain load, time of the events, frequency and proximity of them.
As research continues, head impact monitoring may play an increasingly important role in improving athlete safety. By quantifying both major collisions and the many smaller impacts and events that occur during practices and games, these technologies offer valuable insights into the cumulative exposure that athletes experience throughout a season and across an entire career. Understanding this exposure may ultimately help sports organizations develop better training methods, improvement in equipment designs, validation of actions taken, and more effective injury-prevention strategies.
Advances in head impact monitoring technology are helping researchers and sports organizations better understand not only diagnosed concussions but also the cumulative burden of sub-concussive impacts, providing new opportunities to improve player safety and long-term brain health.
Reducing Risk in Modern Football
Football organizations have implemented numerous strategies to reduce both concussive and sub-concussive exposure:
Rule Changes
Leagues such as the NFL and college football organizations have introduced rules targeting dangerous hits, particularly those involving the head and neck area.
Practice Modifications
Many teams now limit full-contact practices to reduce the total number of head impacts and events players experience during a season.
Better Detection and Medical Protocols
Sideline evaluations and return-to-play protocols help to reduce the risk of returning to competition before recovery is complete. _________________________________________________________________
Looking Ahead
The understanding of brain injury in football has evolved significantly over the past two decades. While concussions remain an important concern, researchers increasingly recognize that the cumulative effect of sub-concussive impacts and events may also influence brain health.
Future advances in impact and event monitoring technology, medical screening, and coaching practices may help reduce exposure to both concussive and sub-concussive forces. Continued research will be essential to better understand how repeated head impacts and events affect athletes over the course of a career and throughout their lives.
Conclusion
Concussions occur in American football when forces transmitted to the head cause the brain to move rapidly within the skull, disrupting normal neurological function. Yet the story does not end with diagnosed concussions. Sub-concussive events-repetitive head impacts and events that produce no immediate symptoms-have emerged as an important area of study because of their potential cumulative effects on the brain. As science continues to advance, protecting athletes will require attention not only to obvious injuries but also to the thousands of smaller impacts and events that can occur throughout a football season and career.
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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