A concussion (or mild traumatic brain injury) is a type of traumatic brain injury (TBI) that occurs when the brain experiences sudden movement or impact. This can happen due to a direct blow to the head, a fall, or other types of forceful impacts. Concussions are quite common and can affect people of all ages, from young children to older adults.
Concussions are often referred to as “mild” TBIs because they are usually not life-threatening and don’t typically cause permanent brain damage or impairment. However, they should always be taken seriously and treated properly to prevent any potential long-term effects.
As mentioned before, concussions can occur from a blow to the head or violent shaking of the head and neck. This sudden movement causes the brain to move back and forth within the skull, potentially damaging brain cells and disrupting normal brain function.
Some common causes of concussions include:
The signs and symptoms of a concussion can vary from person to person.
Common symptoms include:
It’s important to note that not all concussion symptoms may be present immediately following the injury. Some may appear hours or even days (up to 48 hours) after the initial injury. For more information regarding concussion signs and symptoms, see our post on How to Know if you Have a Concussion.
According to the available scientific literature, it takes approximately 70-120 G’s of linear acceleration, or 5,582-9,515 radians per second squared of angular acceleration, to cause a concussion injury in adults [1]. In children and adolescents, this number appears to be lower, around 32-92 G’s or 1,018-4,200 radians per second squared [2], though this is based only on two studies with small sample sizes, and more research on this topic would help draw firmer conclusions. For perspective, research from automobile accidents has demonstrated that a subdural hematoma, a more serious brain injury resulting in bleeding outside of the brain, occurs around 316 G [3].
How the brain gets exposed to levels of gravitational force and acceleration can vary heavily, and it can happen with no direct impact to the head at all. Yes, that’s right, you can get a concussion without hitting your head, and this fact busts one of the most common myths about concussions.
Some other common concussion myths include:
This is not true, and in fact, most concussions (approximately 90%) do not involve a loss of consciousness.
There is no such thing. Similarly, there is no such thing as a “severe concussion.” Grading concussions is based on outdated information and standards of practice. Either a concussion occurred, or it didn’t.
This is false and can be detrimental to recovery, with new research suggesting that rest is not indicated beyond the first 48 hours after injury. If you suspect you’ve had a concussion, it’s important to seek care from a healthcare provider with experience in concussion management. Interventions like physical therapy can help ensure you recover from your concussion effectively.
G’s (or g-force) is a unit of measurement used to quantify the amount of force exerted on an object due to acceleration. In the context of concussions, G’s are used to measure the amount of force exerted on the brain during impact or movement. One G equals the force of Earth’s gravity on a stationary object. Or, put more simply, standing still on Earth would result in experiencing 1 G of force on your body.
There are three types of G’s that can affect the brain and potentially cause a concussion:
In the context of concussion research, G forces are used to describe forces producing linear acceleration, and radians per second squared are used to describe angular or rotational acceleration.
So what actually happens when the brain is exposed to the G forces or angular acceleration required to cause a concussion? As we just discussed, G forces cause some form of rapid acceleration and deceleration of the brain within the skull. When this occurs, neurons (the nerve cells in the brain) are stretched or sheared, causing a deformation of the neuron’s membrane. This membrane deformation causes ions (sodium, potassium, and calcium) to rush in and out of the cell rapidly [4]. The concentration of these ions is normally kept in a particular balance on the inside and outside of the cell for normal functioning. This rapid exchange of ions results in large numbers of neurons firing unintentionally and is called the “excitatory” phase of concussion [4].
Once this delicate balance of ions inside and outside of the cell has been disrupted, the brain has to use large amounts of ATP, the body’s energy molecule, to move these ions back in and out of the cell to restore the correct concentration levels and bring things back into balance. Following a concussion, the body’s ability to produce ATP is impaired at a time when the demand for ATP increases. This creates an energy deficit that is responsible for what’s referred to as the “spreading depression” phase of concussion [4] [5].
G forces aren’t always harmful, and we are constantly exposed to G’s in our everyday lives as we move within and manipulate the world around us.
Common activities and their corresponding G forces include:
Researchers have looked at the G forces and radial acceleration that players are exposed to in various sports in an attempt to better understand their involvement in the context of a sports injury.
We know that high levels of G forces can cause injury to the brain, but they can also cause injuries to other parts of the body, including the neck. While the brain has to be exposed to 70-120 G to experience a concussion injury, whiplash injuries to the neck occur at only 4.5 G. Since the head and neck are effectively inseparable, this analysis of G forces would suggest that it is impossible to sustain a concussion without suffering a corresponding neck injury, and indeed a 2006 study of hockey players found that this was the case [16].
If the mechanism behind concussion injuries is the acceleration and deceleration of the brain within the skull, then perhaps it is possible that those with stronger neck muscles can prevent the head and neck from moving as much or as rapidly through space, thereby reducing the risk of concussion. Researchers have examined this hypothesis in several different studies on hockey players and found that there does not appear to be any significant correlation between neck strength and the likelihood of sustaining a concussion after being hit [17] [18]. Additionally, neck size or circumference did not seem to play a role [18] [19].
What researchers did find in a 2014 study was that neck stiffness, rather than neck strength, reduced the likelihood of suffering a concussion injury after a hit. Those who could tighten their neck muscles to increase neck stiffness before an impact had a reduced likelihood of sustaining a head injury. Additionally, those who could tighten their neck muscles quicker before an unanticipated impact had a lower likelihood of sustaining a concussion [20]. A 2018 study on NCAA hockey players found that players delivering a hit were far less likely to sustain a concussion than players on the receiving end of the hit [21]. This makes sense, as the player delivering the hit anticipates the contact and can brace for it well before it occurs. This finding has important implications in the context of sport, as it would suggest that training athletes to better anticipate a hit and brace their neck musculature, rather than training their neck strength, may better help to reduce the risk of concussion. Exercises and drills designed to improve visuospatial and game awareness can help athletes better see and anticipate contact, allowing them greater opportunity to stiffen their necks before impact.
Researchers have found that in-game fatigue appears to play a role in predicting the risk of concussion after contact. Several studies have demonstrated that more concussion injuries occur late in games or toward the end of a period, suggesting that player fatigue is a predominant factor [22]. Fatigue likely reduces the ability to contract the neck muscles to generate enough stiffness to offer a protective effect from an impact. It is also possible that as fatigue sets in, game awareness decreases, and players cannot anticipate contact as quickly and thus cannot increase neck stiffness quickly enough ahead of an impact. Therefore, training endurance, including that of the neck musculature, may also be beneficial, particularly for those who play contact sports.
Several studies have suggested that children and adolescents are more susceptible to concussion injuries than adults in their 20s and 30s and often have longer recovery times. One of the possible explanations for why this may be is that children tend to have weaker neck muscles relative to the size of their heads. At age 4, a child’s head is already 90% of its full size, while they are only 19% of their full body weight! Even at age 12, these figures are 95% and 47%, respectively. Because of the size of the head relative to the rest of the body, it’s possible that children and adolescents aren’t as easily able to generate enough neck stiffness to prevent the acceleration of the head and neck and are thus more likely to suffer concussions after a lower magnitude impact relative to adults.
Other studies have also demonstrated that women are, on average, more susceptible to concussion injuries than men, though the exact reason for this is not currently known. One hypothesis comes from a study that found that men and women use different techniques to generate neck stiffness when bracing for an impact, with men relying more on muscle strength and women relying on greater neuromuscular activation (as determined by EMG) to generate the same amount of stiffness [23]. The use of greater neuromuscular activation to generate neck stiffness may result in more rapid fatigue of neck muscles, leading to an increased risk of concussion with repeated impacts in women as compared to men [23]. However, there are other possible explanations for why women are more likely to suffer concussions than men, including hormonal fluctuations and being more likely to report the injury. Further studies are needed to help fully understand sex-related differences in concussion susceptibility and recovery.
It has been a common misconception in the medical field that once you have suffered one concussion, future concussions occur more easily. However, it is presently unclear what the impact of repeated concussions is on future susceptibility. The most up-to-date research would suggest that if an individual recovered fully from a previous concussion, they should not necessarily be at any greater risk of a subsequent concussion than someone who has never experienced one. However, individuals who have not fully recovered, including those for whom a long period has passed but are still experiencing persistent post-concussive symptoms, may be more susceptible to future concussions. In light of this, athletes who have suffered multiple concussions and are experiencing longer recovery times with each concussion or are finding new concussions are occurring with less and less force may need to consider ending their participation in contact sports. Importantly, those who are still in the period of physiologic recovery (22-45 days post-injury) after a concussion are at risk of more serious injury, including significant brain swelling, which in some cases can be fatal, should they suffer another head injury.
While helmets are important to protect against more serious head injuries like skull fractures, their ability to protect against concussion appears to be limited, as they do not act to reduce the amount of acceleration of the head and neck, which is the primary mechanism behind concussion injuries. However, newer technology has been developed that can allow some helmets to detect how much acceleration was experienced by the head following an impact, and this can be helpful for on-site medical personnel to know when an athlete should be evaluated for a possible concussion.
Concussions and other traumatic brain injuries occur when the brain is subjected to acceleration and deceleration, often measured in G’s or radians per second squared (depending on whether it is linear or angular/rotational). This can occur with or without contact to the head, so patients with any symptoms after an impact should be properly evaluated, even if they weren’t hit in the head or it seemed the impact wasn’t that hard (remember, “mild concussions” don’t exist!) It’s also important to have the neck assessed, as the forces required to cause a concussion injury are significantly greater than those needed to cause a whiplash or strain injury to the neck. As such, every concussion comes with some type of accompanying injury to the neck. The ability to stiffen the neck muscles before an impact can reduce the amount of acceleration and deceleration experienced by the head (and thus the brain) and decrease the likelihood of suffering a concussion and it does not matter how strong your neck is if you aren’t able to activate the muscles quick enough. Coaches and athletes looking to reduce the risk of concussion in contact sports should focus on training game awareness to allow athletes to better anticipate impacts and brace for them accordingly.
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