Does Hard Ground Increase Concussion Risk in Sport? The Brain Injury Science Explained

Rotational force occurs when the head is struck or accelerated at an angle, causing the brain to rotate. Unlike straight-line (linear) impacts, rotational motion causes the brain to lag behind the skull due to inertia.

This relative movement creates shear strain within brain tissue. This is associated with tearing of tiny blood vessels and brain cells, resulting, over time, in the breakdown of the blood-brain barrier and the creation of damaging neuro-inflammation.

Shear strain is now recognised as a primary mechanical driver of brain injury in sport ^[6]. This is why simply thinking about how “hard” the initial contact feels does not tell us everything about brain injury risk. What matters is the movement and acceleration that the impact produces within the brain, and the accumulation of damage to the brain over many head impacts, games, drills, training sessions, and years.

Rezon expert advisor, Professor Michael Grey, explains this distinction in more detail in How Repeated Head Impacts Change the Brain, Even Without Diagnosed Concussion.

As he explains, concussion is a clinical event, while tissue strain is a biomechanical event. They overlap, but they are not the same thing. An impact can produce biomechanical loading without producing the signs and symptoms required for a concussion diagnosis ^[7].

Concussion is not the only brain injury risk

Sport’s brain-injury conversation has traditionally concentrated on concussion. That is because concussion produces recognisable clinical signs and symptoms ^[8]. But concussion represents only a small part of total head-impact exposure and brain injury risk in sport. Players also experience many head impacts that do not result in diagnosed concussion. These are commonly described as sub-concussive or non-concussive impacts.

They may involve a clash of heads, contact with another player’s shoulder or hip, heading a football, a tackle, a fall, or the head striking the ground. Sub-concussions do not cause visible symptoms, but they still transmit linear and rotational forces to the brain, and they happen far more often than diagnosed concussions ^{[7, 9]}.

Rezon’s brain-health expert, Dr Emer MacSweeney, describes sub-concussive impacts occurring more than 500 times more frequently than concussion in sport ^[10]. Unlike diagnosed concussion, these repeated impacts often attract no medical attention because the athlete does not experience the clinical symptoms that would trigger removal from play. That makes cumulative exposure particularly important.

Systematic reviews of repetitive sub-concussive head impacts have identified changes across neurological, imaging and biomarker measures in some athlete populations ^{[9, 11]}. The central point is that brain injury risk does not begin only when a player crosses the threshold for a diagnosed concussion. A player can experience hundreds or thousands of lower-force head impacts across training, matches, seasons and a sporting career. Each adds to the athlete’s total head-impact exposure.

A major 2023 study published in Nature Communications estimated lifetime repetitive head-impact exposure among 631 deceased male football players. Measures incorporating cumulative linear or rotational acceleration were more strongly associated with CTE pathology than reported concussion count or estimated number of impacts alone ^[12].

That does not mean every sub-concussive impact will result in long-term disease. It does show why the number and intensity of repeated head impacts matter, rather than concussion count being the only measure of concern.

Hard ground adds another variable to that exposure. If a routine fall creates a more severe head-to-ground impact because the surface provides less attenuation, the brain injury risk is increased. The responsible response to brain injury risk in sport is to protect the brain from the transmission of rotational forces when head impacts do occur.

The best brain-injury risk mitigation is reducing force transmission during head impacts

For athletes participating in sports where head impacts occur, the best mitigation is head-worn brain protection designed to reduce the rotational forces transmitted to the brain during impact. That protection should not matter only when the pitch is unusually hard, but at all times where concussion and sub-concussion risk is present.

A player’s head can hit normal grass, drought-hardened ground, artificial turf, a basketball court or ice. It can hit another player’s head, shoulder or knee. It can come into contact with a football post, hockey stick, baseball, or other equipment. The impact can be concussive or sub-concussive. But brain protection needs to be present when those unpredictable impacts happen. The condition of head impacts changes; the need to reduce rotational force transmission does not.

This is also why brain protection should be assessed differently from generic sports headwear. The important question is not simply whether something covers or cushions the outside of the head to protect from superficial injury. The important question is whether testing demonstrates a reduction in the head accelerations associated with brain injury; importantly rotational as well as linear acceleration.

That is the principle behind the testing of Rezon Halos®.

How Rezon Halos® are tested for head impacts

Rezon Halos® have been independently tested in two scenarios particularly relevant to brain injury in sport: head-to-head impacts and extreme head-to-ground impacts ^[13].

The tests measure both linear and rotational acceleration.

Virginia Tech head-to-head testing

The Virginia Tech Helmet Lab independently evaluates sports headgear using its STAR methodology. For soccer headgear, Virginia Tech conducts 24 laboratory impacts across the side and back of the head at low, medium and high impact energies. It measures both linear and rotational head acceleration and uses those results to calculate a STAR score based on relative concussion risk for the impacts tested ^[14]. Virginia Tech recommends soccer headgear achieving a four- or five-star rating ^[14].

Rezon Halos® achieved a five-star rating and concussion risk reduction of 74% ^{[13, 14]}.

Across individual Virginia Tech impact conditions, Halos® reduced:

• peak linear acceleration by up to 64%
• peak rotational acceleration by up to 61% ^[13].

What the testing demonstrates is that Rezon Halos® can materially reduce both linear and rotational acceleration during the tested head-to-head impacts.

Extreme head-to-ground testing

In addition to Virginia Tech testing, Rezon commissioned independent testing at the Centre of Excellence for Sports Engineering and Research at Sheffield Hallam University to simulate an extreme head-to-ground impact ^[13]. A freely suspended instrumented headform was dropped from 98.9cm onto a rigid plane inclined at 45 degrees. The surface was designed to create high friction and substantial rotational loading.

The bare-head condition produced average peak linear acceleration of 133.7g and rotational acceleration of 4,798.4 rad/s². With Rezon Halos® fitted, those values fell to 79.3g and 3,510.2 rad/s² respectively ^[13].

Across the extreme head-to-ground testing, Rezon Halos® reduced:

• peak linear acceleration by an average of 41%
• peak rotational acceleration by an average of 27% ^[13].

These tests were deliberately designed as an extreme laboratory head-to-ground scenario and they directly address the central biomechanical question raised by hard ground: Can head-worn brain protection reduce the forces transmitted when the head hits a hard surface?

In the tested extreme head-to-ground impacts, Rezon Halos® substantially reduced both linear and rotational acceleration ^[13].

Hard playing surfaces exist across sport

Hard-surface brain injury risk is not confined to rugby, football or weather-affected grass.

Outdoor sports including rugby, football, hockey, lacrosse, cricket and Gaelic games involve falls, collisions and head-to-ground impacts. Natural ground conditions can change with rainfall, drought, frost, drainage, compaction and repeated use. A grass pitch that provides relatively good impact attenuation at one point in a season may behave very differently after prolonged dry weather or when frozen.

Indoor sports make hard surfaces part of the normal playing environment. Basketball and netball players jump, contest balls, change direction at speed and collide with other players. When balance is lost, the head can strike a rigid court rather than a compliant grass surface. Gymnastics, cheerleading, indoor climbing and other sports also expose participants to head-to-ground or head-to-wall impacts.

Sports on ice make the issue even clearer. Curling and figure skating take place on a deliberately hard surface. Slips, edge loss and uncontrolled backwards or sideways falls can produce direct head-to-ice impacts without another player or participant being involved.

The sports are different. The playing surfaces are different. The mechanism of a fall is different. But whenever the head accelerates rapidly during impact, the brain can be exposed to linear and rotational loading. That is why brain protection is relevant far beyond traditional collision and contact sports.