Science:

In the know

It’s not just your shins that need protection

The brain has a jelly-like consistency and sits within a hard-cased skull; a direct or indirect impact to the head or a whiplash effect will cause the brain to move inside the skull. An impact to the head will involve both linear forces and rotational forces.

  • Linear forces are direct straight-line forces that compress or stretch the brain within the skull.

  • Rotational forces are angled forces that cause the brain to rotate inside the skull, brain cells to shear, tiny blood vessels in the brain to be torn and the protective ‘blood brain’ barrier to be disrupted. This results in an abnormal and harmful uncontrolled inflammation which damages the brain and increases the risk of longer-term neurodegenerative consequences including Chronic Traumatic Encephalopathy (CTE).

Professional footballer injury - in game

Repeated rotational forces from concussive and sub-concussive impacts cause more significant brain damage than linear forces.

Rotational brain cell damage diagram

Rotational Damage

Nature did not design the brain for rotational forces and injury, with the brain being badly designed for sudden rotational acceleration and deceleration.

In sport, the brain is subjected to sudden acceleration and deceleration because it is rotated from impacts at the side and back of the head e.g., a head-to-head, head-to-ball, head-to-ground contacts and hits.

The white and grey matter of the brain is made up of differing densities, which means the jelly-like brain sections move at different speeds when rotated, this causes twisting of the brain, shearing of brain cells, brain cell death and the disruption of the nerve cell connectivity networks in the brain.

Damage to the brain cells from an impact can be both immediate (damage to the cell structure – as shown in diagram) and delayed over hours and days (blood flow changes or neural inflammation). Each impact to the brain has the potential to cause an injury that is as visceral as a torn hamstring.

Brain injury happens at the microscopic level, 4,000 times smaller than the eye can see on a regular brain scan. About 40% of brain injuries are classified as mild, in that they don’t register concern based on a brain scan, results or dictate a need for a scan when the individual attends Accident and Emergency immediately after a head injury. Brain injury is not identified because it cannot be seen and recognised by conventional scanning technology.

It’s not all about concussion

Repetitive, sub-concussive impacts (i.e. impacts that do not produce overt signs or symptoms) cause damage to the brain when experienced repeatedly, and research proves that these lead to neurodegeneration later in life. Repetitive, sub-concussive impacts cause injury to the tiny blood vessels in the brain. This in turn results in damage to the ‘blood brain’ barrier, a structure designed to protect the brain.

When this structure is damaged by repetitive trauma, an abnormal ‘immune” mediated inflammatory response is triggered, with the production of neurochemicals. The neurochemicals and inflammatory response should be protective; however, the problem arises when the brain is subjected to repetitive impacts, before the protective neurochemicals and inflammatory changes, from the initial head injury, have had time to return to normal. The subsequent, repetitive head injuries can then result in an abnormally exaggerated further production of neurochemicals and an exaggerated inflammatory response which is harmful to the brain, rather than protective. This response damages the brain tissue and eventually leads to the irreversible death of brain cells. Over time, this abnormal inflammatory pathway, triggered by repeated head injuries in contact sports, eventually leads to changes in a brain protein called tau.

The tau protein found within cognitive brain cells normally stabilises these brain cells to ensure they work efficiently and communicate effectively with other cognitive brain cells, so an individual can think and behave normally. When the tau protein becomes damaged, it can no longer stabilise the brain cells and they lose their ability to function efficiently and effectively. As the tau protein spreads around the brain, more and more brain cells, needed for thought and control of emotions and behaviour, are killed and the symptoms of cognitive impairment and changes in behaviour become increasingly apparent.

The risk and severity of CTE is caused primarily by multiple sub-concussive impacts, and not by one hit concussions. The force to the brain to cause a concussion is 2-4 times greater than for a sub-concussion, but sub-concussions are over 500 times more frequent.

Females are more seriously impacted by brain injury than males, with longer recovery time and more pronounced post concussive symptoms.

Females are at least three times more seriously impacted by brain trauma than males, recovery time longer and post concussive symptoms more pronounced (Association of Sex and Age With Mild Traumatic Brain Injury–Related Symptoms: A TRACK-TBI Study, 2021).

This is because of differences in the microstructure of the brain, the influence of hormones, coaching regimes and the management of injuries. 80.1% of all concussion research data comes from males, female athletes are being put at risk of serious brain injury, and the lack of female data sets mean the true extent of damage is likely to be underestimated.

Research found that women who were injured during the last two weeks of the menstrual cycle – when progesterone was at its highest had worse post-concussion symptoms compared with women injured during the first two weeks – when progesterone was low.

It has been 20 years since sex and gender differences were first seen in brain injury research; yet most women, their coaches and medical support are still unaware.

Children and youths are at a greater risk of brain injury.

Between ages 8-12 is when peak development of the brain occurs, but the brain is still developing through the refinement and rearrangement of pathways and connections between cells until early 20s. Players experiencing head impacts prior to age 12 have been found to have demonstrated worse cognitive, executive and new learning ability as adults, compared to those who were at least 12 years old when they were first exposed to contact and collision sports (Stamm JM, Bourlas AP, Baugh CM, et al. Neurology. 2015;84(11):1114-1120).

Myelination is the process of wrapping a fatty sheath around a neuron’s axon and this process increases from childhood to adulthood. This means children and youths are at a greater risk of brain injury as they have less myelin than adults. Axons with little or no myelin are more exposed, making them more prone to damage from concussion and repetitive impacts, and the chemical damage that occurs in the following hours/days.

Brain injury is not limited to elite or professional sport. The brain does not know if it receives an impact in football or rugby, in training or in a match, in front of global audiences or in the local park. Concussive and sub-concussive brain injury is a real risk for everyone playing sports, regardless of their position, team, fitness-level and experience. Whilst concussive brain injury may be apparent instantly; repetitive sub-concussive brain injury over many years may not be recognised until later, meaning it is never too early to protect the brain in sport.

So, with little pain, and no obvious physical injury, brain injury in sport really is a serious injury that is “hidden in plain sight”. This is very different from other more visible and painful injuries in sport that are given more focus and attention e.g. knee, ankle and shoulder injuries.