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The Brain is our most complex organ, and unfortunately does not heal very well if it is damaged. Here are some highlights, which will help to understand the science involved in a concussion.

  1. What is concussion?

Instinctively, we all know that single hard hits or repeated impacts on the head are harmful. We now use the word “concussion” to describe the consequences of such impacts. The word concussion comes from Latin “concussionem” meaning a shaking or an earthquake, and from around the mid-sixteenth-century concussion has meant specifically a "brain injury caused by a fall or blow."

Although most athletes, parents, and coaches know concussion, even medical professionals do not have a simple definition for it. Centers of Disease Control and Prevention defines a concussion(1) like this: 
“A concussion is a type of traumatic brain injury—or TBI—caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth. This sudden movement can cause the brain to bounce around or twist in the skull, creating chemical changes in the brain and sometimes stretching and damaging brain cells.”
This includes four very important messages. Firstly, a blow on the head causes the brain to move, or shake, inside the skull. Secondly, this shaking can cause chemical changes. Thirdly, it may cause visible damage to the brain structure, but not nearly in all cases. Finally, the fourth point is that a concussion is always a brain injury, commonly also called TBI.

  1. How much force is needed to cause a concussion?

An impact makes the brain move rapidly back and forth inside the skull and damage. The involved acceleration or deceleration is measured in a unit called “g” and is often, but slightly inaccurately, called g-forces. One g corresponds to the gravitational pull of the earth. Specifically, acceleration is the change of velocity over time. The mathematical formula of acceleration is acceleration = meters divided by seconds squared or a = m/s2. Note, that the formula does not include mass, which means that when a light and a heavy object are dropped simultaneously from the same height, they get the same acceleration when hitting the floor. Mathematically oriented persons immediately recognize from the formula, that if the distance of the velocity change shortens, acceleration gets higher. This is the reason, why it is a good idea to use a padded helmet which gives the head a longer distance to come to a halt if an impact happens. Wikipedia has an excellent article for understanding g and g-forces in more detail.

The highest survived amount of g is said to be 214 g, which was experienced in a car crash by the Swedish race driver Kenny Bäck during the 2003 IndyCar series. A jet fighter pilot may be exposed to 10 g, which is also near the survivable maximum for a pilot, although consciousness may be lost already at 7 - 8 g. What is the explanation? How can somebody die of 10 g and survive 214 g? From the brain’s standpoint, these are quite different situations. The brain is soft, like a mass of jelly embedded with blood vessels and enclosed in a tight plastic bag.  Slowly increasing acceleration (in seconds), like the 10 g in a fighter jet, squeezes the brain and disrupts oxygen supply by pushing blood out of the brain, which may be fatal. However, the brain is elastic and tolerates such acceleration without sharing and tearing. A 10 g impact on the head is almost always harmless. However, things change when the acceleration happens in a short time (fractions of seconds). Instead of just squeezing, the brain violently shakes inside the head, collides onto the skull bones, and may be crushed, sheared, or teared. Head impacts of 40 – 60 g may cause bleeding and hematomas in the brain, and the risk of permanent damage starts to increase when g values further increase.

  1. How do I recognize a concussion?

Traumatic brain injury, or TBI, may occur in many situations, which include accidents at work, in traffic, and during leisurely activities. For practical reasons, concussions during sports activities are called “sports-related concussions”, or SRC. Presently, there are no tests that are a hundred percent accurate for SRC. Tools have been developed for recognizing warning signs of permanent damage. A pocket version of “The Concussion Recognition Tool 5th Edition (CRT5)”(3) can be given by trained lay persons. Another test, “The Sport Concussion Assessment Tool 5th Edition (SCAT5)”(4), which has also a special version for children(5), is meant to be given by medical professionals.

Common immediate symptoms of an SRC include loss of consciousness for a short time, inability to remember events immediately after the impact (post-traumatic amnesia), disorientation and confusion, difficulty in speaking or understanding what others say, maintaining balance and feeling light-headed, headache, as well as a vacant look on the face, sometimes called “glazed eyes”. An athlete with any of these symptoms must immediately stop the activity and seek for medical attention. The athlete must not return to the activity before symptoms have disappeared SRC is an evolving event. It means, that symptoms may be initially mild but get worse during the following hours or even days. This is the reason, why a concussion victim must not be left alone or allowed to drive.

  1. Is concussion serious?

Not all impacts on the head cause permanent damage. Traditionally, it has been thought, that diagnosing a concussion requires at least some sort of disability after having a blow on the head. Still, after decades of medical research(2) it is not entirely clear which symptoms and findings best indicate that there is permanent brain damage after an impact. It is important to understand, that having a concussion does not mean, that the victim will have abnormal findings in their brain scans immediately, nor after many days, months, or even years.

  1. Can one recover from a concussion?

It is not always easy to know when injured has recovered from SRC(6–8), but after a mandatory period of rest, returning to normal activities can be done safely and quickly by following a step-by-step program of increasing physical and mental activity. These programs can be found online in many languages, for example, one for concussed children at

Most athletes completely recover from SRC. But sometimes, long-term difficulties arise after a concussion. In some cases, the after-effects are devastating even causing the inability to study, work, or continue the career in sports. Many stories of such unfortunate athletes can be found in the media.

  1. What happens if I get many concussions?

Medical researchers have discovered, that having several relatively mild concussions repeatedly, may be more dangerous than having a single serious one. This was coined the “repetitive concussion syndrome” and it is associated with degenerative brain changes, similar to that are also found in Alzheimer’s disease.  This is often called “chronic traumatic encephalopathy” or CTE.

A very important study
(9) showed recently, that amateur athletes with multiple concussions had already at teen-age such brain changes. Even more importantly, the same study showed that impacts that did not cause symptoms also cause CTE-like brain findings. This means, that a blow on the head is harmful even if it does not cause concussion symptoms.

  1. Prevention is always better than treatment

An exact limit of a harmful magnitude of g can never be found. This is because humans are different, impacts are different, and measuring devices are different. A recently introduced blood test has been claimed to be able to diagnose a concussion with high accuracy. So, why measure g-forces at all? One must be careful not to mix apples and oranges. Knowing the g-forces is useful because it permits anticipation and prevention of injuries. A tool that helps athletes, coaches, parents, and teams to detect signs of danger and develop safer practices.
Prevention is always better than treatment.   


  1. Centers for Disease Control and Prevention, National Center for Injury Prevention and Control. What Is a Concussion? [Internet]. Available from:
  2. McCrory P, Meeuwisse W, Dvorak J, Aubry M, Bailes J, Broglio S, et al. Consensus statement on concussion in sport—the 5th international conference on concussion in sport held in Berlin, October 2016. Br J Sports Med. 2017 Apr 26;bjsports-2017-097699.
  3. Echemendia RJ, Meeuwisse W, McCrory P, Davis GA, Putukian M, Leddy J, et al. The Concussion Recognition Tool 5th Edition (CRT5). Br J Sports Med. 2017 Apr 26;bjsports-2017-097508.
  4. Echemendia RJ, Meeuwisse W, McCrory P, Davis GA, Putukian M, Leddy J, et al. The Sport Concussion Assessment Tool 5th Edition (SCAT5). Br J Sports Med. 2017 Apr 26;bjsports-2017-097506.
  5. Davis GA, Purcell L, Schneider KJ, Yeates KO, Gioia GA, Anderson V, et al. The Child Sport Concussion Assessment Tool 5th Edition (Child SCAT5). Br J Sports Med. 2017 Apr 26;bjsports-2017-097492.
  6. Haider MN, Leddy JJ, Pavlesen S, Kluczynski M, Baker JG, Miecznikowski JC, et al. A systematic review of criteria used to define recovery from sport-related concussion in youth athletes. Br J Sports Med. 2018 Sep;52(18):1179–90.
  7. Peltonen K, Launes J, Koskinen S, Vartiainen M, Pajunen S, Pertab J, et al. On‐field signs of concussion predict deficits in cognitive functioning: Loss of consciousness, amnesia, and vacant look. Transl Sports Med. 2020 Nov;3(6):565–73.
  8. Putukian M. Neuropsychological Testing as It Relates to Recovery From Sports-related Concussion. PM&R. 2011 Oct;3:S425–32.
  9. Tagge CA, Fisher AM, Minaeva OV, Gaudreau-Balderrama A, Moncaster JA, Zhang X-L, et al. Concussion, microvascular injury, and early tauopathy in young athletes after impact head injury and an impact concussion mouse model. Brain. 2018 Feb 1;141(2):422–58.