How do sub-concussions and rotational forces contribute to CTE risk in sport?
Sub-concussions damage the brain without producing the visible symptoms required for a concussion diagnosis. Every sub-concussive head impact involves rotational forces, which twist brain tissue and can shear axons, damage fine blood vessels and trigger neuro-inflammation [1, 2, 3, 4, 5].
The greatest long-term concern is the accumulated burden of repeated sub-concussive brain injury. Over time, repeated exposure to these impacts contributes to the biological pathway of neuro-inflammation associated with Chronic Traumatic Encephalopathy, or CTE [5, 6, 7].
Exposure begins when repeated head impacts occur, which is commonly in youth sport, and long before an athlete is perceived to be at risk [8, 11].
CTE risk in sport is not a concussion problem. It is driven by the accumulated damage of repeated sub-concussive head impacts and rotational forces transmitted to the brain.
The CTE Problem in Sport is Bigger Than Concussion Alone
Sport has traditionally focused on diagnosed concussion because concussion is visible, recordable and clinically recognisable. A player is dazed. They report symptoms. They fail an assessment. They are removed from play.
Concussion is the visible clinical diagnosis of a brain injury. Concussion protocols that intervene after brain injury has occurred are important, but they do nothing to protect the brain from the impact that caused it, nor from the far more frequent sub-concussions that inevitably preceded it.
The deeper brain-health risk in sport is not limited to impacts that produce visible concussion symptoms. Sub-concussions damage the brain without producing symptoms. Repeated sub-concussions transmit force to the brain again and again; usually without stopping play, triggering medical review or appearing in injury statistics [5]. These head impacts may not look serious from the outside, but they are still causing measurable damage to the brain, which accumulates over time [5, 6, 8, 10].
This accumulated burden is central to the CTE problem in sport: cumulative exposure matters. A player may experience only a small number of diagnosed concussions, or even no diagnosed concussions, yet receive hundreds or thousands of sub-concussive impacts across years of training and competition, beginning in childhood [5, 6]. Changes to white matter have been recorded in children after just one season of contact sport, even where no diagnosed concussion was recorded [8].
The most important long-term exposure is therefore the accumulated damage from repeated sub-concussions, not simply the number of diagnosed concussions recorded in an athlete’s medical history.
That is why the conversation must move beyond concussion alone.
Read a concise definition of CTE here.
Dr Emer MacSweeney: The expert behind Rezon Halos®
The science behind Rezon Halos® is informed by the clinical work of Dr Emer MacSweeney, Global brain expert, Consultant Neuroradiologist, and Rezon Expert Advisor. Dr MacSweeney’s work focuses on identifying objective evidence of chronic and progressive brain injury in living patients exposed to repeated sub-concussive impacts [9].
This clinical perspective is central to Rezon’s position: brain protection in sport must focus on the rotational forces transmitted to the brain and on reducing the accumulated damage of sub-concussions, not purely protecting the skull or scalp from superficial injury.
Watch: Dr Emer MacSweeney explains why sports head protection must focus on protecting the brain from rotational forces.
What is a sub-concussion?
A sub-concussion is a brain injury caused by an impact that transmits rotational forces to the brain without producing the visible signs or symptoms required for a concussion diagnosis.
Sub-concussions damage the brain without producing symptoms.
That distinction matters.
The prefix “sub” refers to the clinical threshold for diagnosis. It does not mean the brain has not been insulted. It does not mean no damage has occurred. It does not mean the impact is safe.
Research on repeated sub-concussive head impacts has reported measurable effects on cognition, brain function and brain integrity [5, 8, 10]. Other research has shown that sub-concussive impacts can alter brain network function even when no concussion is diagnosed [10].
The key point is simple: The absence of concussion symptoms does not mean that no brain injury has occurred.
A player can sustain a head impact, remain conscious, continue playing and they will still have experienced biomechanical force transmission to the brain. In some cases, non-concussive impacts have been demonstrated to involve head acceleration greater than impacts that did cause a diagnosed concussion [1]. Exposure to sub-concussive impacts begins from a child’s earliest years of participation in sport.
This is why sub-concussions are so important to CTE risk in sport. They are frequent. They are invisible. They accumulate over years of play. The brain changes associated with repeated exposure can be measurable and persistent. And the accumulated damage contributes to the neuro-inflammatory and neurodegenerative processes associated with CTE [5, 6, 8, 10].
Why rotational forces are the critical mechanism in CTE risk
Rotational forces are present in sub-concussive brain injury. When an impact causes rapid rotation of the head, that movement is transmitted through the skull to the brain, where soft brain tissue twists and shears.
Rotational brain injury occurs when the head rotates rapidly during impact, causing the brain to rotate inside the skull. The brain is soft, suspended in fluid and highly vulnerable to shear strain [2, 3].
Linear force moves the brain in a straight line. Rotational force rotates the brain.
That rotational motion is especially relevant in sport because impacts are rarely perfectly straight. A head can be struck by another head, shoulder, elbow, knee, ball, stick, board, floor or ice at an angle. That angled impact can rotate the head and transmit rotational acceleration to the brain [2, 3].
Rotational forces contribute to:
In sub-concussion, this brain damage occurs without visible symptoms. The player continues, while the accumulated burden of rotational brain injury increases.
This is why Rezon focuses on brain protection in sport. Conventional sports headgear and scrum caps are designed to reduce cuts, abrasions and superficial head injury. Brain protection requires a different focus: reducing rotational forces transmitted to the brain, and reducing the accumulated damage from highly frequent sub-concussions [2, 16].
How does Rezon Halos® reduce the transmission of rotational forces and CTE risk?
Rezon Halos® is engineered to reduce the rotational forces transmitted to the brain during sub-concussive and concussive head impacts. Halos® use patented Rotection® technology, which is made up of nine layers. The nine independent layers are engineered to move independently from the head on impact, and three specific layers deform to further reduce impact energy, helping redirect and reduce the rotational acceleration that would otherwise be transferred through the skull and into the brain [16].
When the head rotates rapidly during impact, the brain can rotate inside the skull. That motion creates shear strain within brain tissue, damaging axons, disrupting neural communication and contributing to the biological pathway associated with CTE [2, 3, 4, 7].
By reducing the rotational forces reaching the brain during each impact, Halos® aim to reduce the tearing and shearing of brain cells and fine blood vessels, reduce the triggers for brain inflammation, and lower the cumulative burden of rotational force exposure across repeated sub-concussive impacts [2, 5, 6, 7, 16].
Watch: Rotational forces shear axons in the brain, disrupting neural communication and contributing to neuronal cell death.
This accumulated exposure matters. Repeated sub-concussive brain injury can increase the burden of neuro-inflammation over time. The aim of consistent brain protection is to reduce the accumulated damage and help keep the inflammatory burden below the level at which damaging neuro-inflammatory processes become chronic and contribute to longer-term neurodegenerative consequences.
Wearing Halos® will reduce the risk of a one-off hit that could trigger a career-ending or life-changing concussion. Wearing Halos® consistently in training and games reduces the accumulated burden of sub-concussive brain injury. This reduces the potential risk of triggering longer-term neurodegenerative consequences.
CTE risk exists wherever repeated brain impacts occur
CTE risk exists wherever there is repeated exposure to sub-concussion . Risk also starts at the point in a player’s lifetime when repeated head impacts begin, meaning youth sport and school sport are where many players are first exposed to sub-concussive brain injury.
A child’s brain is still developing. Their neck strength, coordination, anticipation, balance and ability to brace for impact are not the same as an adult’s. Their opponents are also developing, unpredictable and technically inconsistent.
Many adult risk-reduction measures are not fully practical in youth sport. Perfect tackle technique cannot be expected from a child or from every opponent. Neck strengthening is limited by age, development and practicality. Reducing contact in training can help, but for children who train once a week, the reduction may be limited compared with the unpredictable impacts that happen in play.
The most important point is this: A child can receive hundreds or thousands of sub-concussive brain-injury events before anyone perceives them as “at risk”.
A Boston University-led JAMA Neurology study of 152 young contact-sport athletes who died before age 30 found CTE pathology in 63 donors, or 41.4% [11]. Most of the young athletes with CTE had played at amateur levels, including high school and college sport [11].
CTE risk is not limited to professional sport. The accumulated exposure that contributes to CTE can begin in childhood [8, 11].
CTE risk is also not limited to traditional contact sports. Risk extends beyond contact and collision sports such as rugby union, rugby league and Australian rules football. Brain-injury risk can also exist in sports where head impacts with hard surfaces or other players occur, including cheerleading, figure skating, curling, hockey, netball, basketball, and other sports involving falls or collisions.
In some of these sports, the risk may not come from repeated player-to-player contact. It may come from high-speed falls, head-to-ground impacts or impacts against hard surfaces.
Can Rezon Halos® brain protection prevent CTE?
Rezon Halos® is brain protection for sport.
Rezon Halos® can reduce brain injury risk in sports players by reducing the forces transmitted to the brain during sub-concussions and concussions.
No head-worn protection can prevent CTE or remove every risk from sport. CTE is associated with accumulated exposure to repetitive brain trauma, particularly the repeated sub-concussive injuries that can occur across years of training and competition [5, 6, 7].
The objective of Rezon Halos® is to reduce that accumulated burden.
Every time Halos® reduce the rotational forces transmitted to the brain during an impact, the brain is exposed to less of the mechanical loading that tears and shears brain tissue, damages fine blood vessels and contributes to inflammatory processes.
Reducing damage in a single impact matters. Reducing damage consistently across hundreds or thousands of impacts matters even more.
Wearing Halos® will reduce the risk of a one-off hit that could trigger a career-ending or life-changing concussion. Wearing Halos® consistently in training and games reduces the accumulated burden of sub-concussive brain injury. This reduces the potential risk of triggering longer-term neurodegenerative consequences.
Rezon Halos® is not like conventional sports headgear or scrum caps that are designed to protect the scalp from cuts and scrapes. Halos® is a head-worn brain protection system designed to reduce rotational force transmission using patented Rotection® technology [16].
Independent testing shows that Rezon Halos® reduces rotational acceleration by up to 61% in head impacts [16]. Rezon Halos® is rated 5-Star by Virginia Tech, with testing showing a 74% reduction in concussion risk using Virginia Tech’s peer-reviewed methodology [17].
Rezon Halos® also hold CE and UKCA Category II PPE certification for head protection for sports [18].
Frequently Asked Questions
- “Subconcussive” is a dangerous misnomer: hits of greater magnitude than concussive impacts may not cause symptoms. Nowinski CJ, Rhim HC, McKee AC, et al. British Journal of Sports Medicine. 2024.
- What Is Rotational Brain Injury? Definition, Causes & Risks. Further Reading. Rezon.
- Biomechanics of Concussion. Meaney DF, Smith DH. Clinics in Sports Medicine. 2011.
- Axonal Pathology in Traumatic Brain Injury. Johnson VE, Stewart W, Smith DH. Experimental Neurology. 2013.
- Repeated Sub-Concussive Impacts and the Negative Effects of Contact Sports on Cognition and Brain Integrity. Ntikas M, Binkofski F, Shah NJ, et al. International Journal of Environmental Research and Public Health. 2022.
- Leveraging football accelerometer data to quantify associations between repetitive head impacts and chronic traumatic encephalopathy in males. Daneshvar DH, Nair ES, Baucom ZH, et al. Nature Communications. 2023.
- The Neuropathology of Chronic Traumatic Encephalopathy. McKee AC, Stein TD, Kiernan PT, Alvarez VE. Brain Pathology. 2015.
- Subconcussive Head Impact Exposure and White Matter Tract Changes over a Single Season of Youth Football. Bahrami N, Sharma D, Rosenthal S, et al. Radiology. 2016.
- What a world-leading Neuroradiologist wants you to know about CTE. Re:Cognition Health. Featuring Dr Emer MacSweeney, Consultant Neuroradiologist.
- Effects of Subconcussive Head Trauma on the Default Mode Network of the Brain. Johnson B, Neuberger T, Gay M, et al. Journal of Neurotrauma. 2014.
- Neuropathologic and Clinical Findings in Young Contact Sport Athletes Exposed to Repetitive Head Impacts. McKee AC, Abdolmohammadi B, Stein TD, et al. JAMA Neurology. 2023.
- Neurodegenerative disease risk among former international rugby union players. Russell ER, Mackay DF, Lyall D, et al. Journal of Neurology, Neurosurgery & Psychiatry. 2022.
- Neurodegenerative Disease Mortality among Former Professional Soccer Players. Mackay DF, Russell ER, Stewart K, et al. New England Journal of Medicine. 2019.
- CTE diagnosis and research. Australian Sports Brain Bank.
- Chronic traumatic encephalopathy in Australia: the first three years of the Australian Sports Brain Bank. Suter CM, Affleck AJ, Lee M, Pearce AJ. Medical Journal of Australia. 2022.
- Rezon Halos® Testing.
- Rezon Halos® Virginia Tech Testing.
- Rezon Halos® CE and UKCA Category II PPE Certification.
Author: Alex Reily
