What are sub-concussions and how do they impact long-term brain function?

What is the difference between a concussion and a sub-concussion?

A concussion is a traumatic brain injury that produces noticeable signs or symptoms, such as headache, dizziness, or confusion. A sub-concussion results from an impact of lower force than required to cause a concussion. These lower-force impacts still damage the brain but produce no visible symptoms and go unnoticed.

The absence of concussion symptoms does not mean the absence of brain injury. Sub-concussions still transmit damaging rotational and linear forces to the brain, causing microscopic damage. Because sub-concussions are invisible to players, parents, medics and coaches, they are not being recognised, measured or managed. 

In fact, according to the Rezon Survey 2025, most participants in sport rated their knowledge of sub-concussion as low to moderate. However, only 1% of individuals correctly identified “no signs or symptoms” as the sole accurate answer, aligning with the clinical definition of sub-concussion ^[4]. This lack of understanding around sub-concussions has resulted in a lack of brain protection in sport and more focus being given to visible concussion recognition and management.

Rezon Halos® is designed to reduce the rotational forces involved in both concussive and sub-concussive impacts. Reducing rotational forces to the brain is essential to lower the risk of neuro-inflammation, a common feature in diseases such as CTE, Parkinson’s disease and motor neurone disease.

How common are sub-concussions in sport?

Sub-concussions occur in all impact, contact, collision and combat sports, during both training, game play and competition. They result from routine actions such as tackles, falls, collisions with other players, or contact with equipment or the ground ^[5].

An athlete may experience hundreds to thousands of these impacts across a season, and hundreds of thousands over years of amateur play or professional career ^[6]. This cumulative exposure is particularly concerning for young players. We now know more from research about how the brain responds to rotational forces than in the past. We also know a child’s brain is more susceptible to these forces. 

What we don’t know is which child may have a genetic or metabolic risk factor, which child will continue to play sport into adulthood, or as a professional. Given the damage of sub-concussions are cumulative, every child exposed to repeated head impacts carries a risk of a high cumulative lifetime exposure, and the risk of neurodegenerative disease actually starts in childhood. So, Halos® worn as early as possible is critical across all sports where head impacts occur.

Why are sub-concussions harder to detect and diagnose?

Sub-concussions produce no immediate or visible symptoms, no pain and no automatic removal from play.

The Rezon Halos® Survey 2025 shows that the understanding of sub-concussion is low to moderate among participants in sport. Only 1% correctly identified that sub-concussions involve no signs or symptoms ^[7]. 

There have been significant advancements in measuring head acceleration in sport. Instrumented mouthguards (iMGs) can record linear and rotational forces, impact location, direction, and frequency, and can provide alerts when a head impact exceeds a predefined threshold. However, these systems have important limitations.

First, iMGs do not detect all sub-concussive impacts. Impacts under the threshold of 5–8g are not captured and therefore won’t be considered.

Second, the data and alerts are primarily designed to flag higher-force impacts associated with concussion risk. As a result, attention remains focused on the less frequent, high-magnitude events rather than the far more common low-force impacts that accumulate over time.

While iMGs can provide useful information about some head impacts, they do not offer clinically meaningful conclusions about cumulative brain injury or long-term neurodegenerative risk on their own. This reinforces why rotational force reduction and sub-concussion protection is more proactive than relying on detection after impacts have already occurred.

Detection does not protect the brain. It only identifies when an impact may have exceeded a predefined threshold and injury has likely occurred.

Detection-based approaches focus on recognising what is “too much,” rather than addressing the root cause of brain injury accumulation. Rezon Halos® is focused on reducing the transmission of rotational forces to the brain in the first place, lowering the amount of brain injury sustained across head impacts in sport.

Is ‘If in doubt, sit them out’ enough to protect the brain?

The ‘If in doubt, sit them out’ message, promoted in the UK Government Concussion Guidelines for Grassroots Sport (2023) ^[8] and the Australian Sports Commission Concussion and Brain Health Position Statement (2024) ^[9], plays an important role in identifying and managing symptomatic concussions.

However, this guidance does not apply to sub-concussions, which occur without any signs or symptoms. Because sub-concussions cannot be recognised or diagnosed at the time they happen, they fall entirely outside the scope of symptom-based messaging and removal-from-play guidance.

This can lead to a false sense of security. Players, coaches, parents, and teachers may believe that education around recognising and removing athletes with observable brain injury is sufficient protection. In reality, this approach does nothing to address the accumulated damage from repeated sub-concussive impacts, which are far more common than diagnosed concussions.

Reducing brain injury risk, therefore, requires more than recognition and removal of concussed players. It requires reducing the transmission of rotational forces to the brain in the first place, which is involved in concussive and sub-concussive brain injury. This is the specific problem Rezon Halos® is designed to address.

How do repeated sub-concussions affect long-term brain function?

Repeated exposure to sub-concussive impacts is associated with:

• Changes in memory and concentration
• Cognitive and executive function impairment
• Increased risk of long-term neurodegenerative disease including CTE

The Rezon Halos® Survey 2025 ^[10] also shows that players have a limited understanding of Chronic Traumatic Encephalopathy (CTE), with only 4% reporting high confidence in recognising its signs and long-term effects. CTE is linked to cumulative brain trauma, sub-concussions, rather than concussions.

The risk and severity of CTE is not caused primarily by single big-hit concussions, but by multiple smaller sub-concussive blows to the head and body. Failure to grasp this critical discovery is too common, and presents a huge issue in understanding how to tackle CTE.

Dr Emer MacSweeney – CTE: The silent killer in contact sports | TEDxAthens ^[11]

Why doesn’t traditional head protection, including scrum caps, protect against sub-concussions?

Most traditional head protection is designed to reduce surface injury (cuts and scrapes) and linear force, not brain injury. This includes padded headgear and scrum caps used in rugby.

World Rugby approved headgear is designed to protect against cuts and abrasions only. It is not intended nor expected to protect against any form of mild traumatic brain injury or skull fractures. As a result, World Rugby approved headgear was never designed to reduce rotational forces, which are the primary driver of brain injury. World Rugby approved headgear is therefore not designed to protect the brain ^[12].

Reducing brain injury risk requires reducing rotational force transmission to the brain, not just linear force. This is the key limitation of traditional scrum caps and padded headgear.

Rezon Halos® differs fundamentally. It is intentionally engineered to reduce rotational force transmission to the brain and holds an internationally recognised Virginia Tech 5-star safety rating. Virginia Tech is the leading independent biomechanics test laboratory, using a globally accepted, peer-reviewed methodology to assess headgear performance in reducing concussion risk ^[13].

Virginia Tech tests all types of sports headgear and awards a star rating from 1 to 5. Only products achieving a 4- or 5-star rating are recommended. The Virginia Tech testing programme has been operating for approximately 15 years and is based on more than two million recorded head impact data points.

To date, no scrum cap or rugby helmet has publicly disclosed a 4- or 5-star Virginia Tech safety rating. This includes the N-Pro headguard ^[14]. N-Pro has undergone testing using the Virginia Tech head-to-head methodology, the same methodology used to assess Rezon Halos®. These results are publicly available ^[15]. 

How can the risk from sub-concussions and concussions be reduced in sport?

Sub-concussions and concussions cannot be eliminated from impact, contact, collision and combat sports. Concussion is a statistical risk, and it can be identified through clinical signs and symptoms. Most people recover well from a single concussion, but the brain remains at risk for up to a year after a concussion.

Sub-concussions are in fact the greater risk, and now a key focus of scientific research (especially for the risk of CTE). Sub-concussions still cause a disturbance of neuronal dysfunction and is clinically asymptomatic.

Importantly, while not every sports player will suffer a concussion, they will typically take hundreds, and even thousands of sub-concussions every season. And hundreds of thousands over a career. Whilst an individual sub-concussion is asymptomatic; multiple sub-concussions can adversely affect memory, the ability to focus, learn and think as well as behaviour.  

The most effective strategy is to:

• Better understand sub-concussions in sport, rather than the focus being solely on awareness of concussion symptoms, procedures for recording concussion, and concussion management.
• Focus on reducing rotational force transmission during frequent sub-concussive impacts and less frequent concussive hits. This helps lower cumulative exposure to brain injury and reduce the build-up of neuro-inflammation that contributes to long-term neurodegenerative risk.
• Using protective equipment designed specifically for brain protection, not scalp, and certified as Category II Personal Protective Equipment (PPE) ^[16].

Rezon Halos® is the only CE / UKCA Category II PPE–certified and Virginia Tech five star safety rated protective headband globally, and is proven to reduce concussion risk by 74%, rotational force by at least 60% and linear force by up to 64%*. It is suitable for athletes of all ages and abilities, and is also suitable for day-to-day activity use by those at higher risk of head impacts.

*based on independent testing