Editor—McCrory says in his editorial that heading a soccer ball results in head accelerations of less than 10 g (or less than 1000 rad/s2), whereas the minimum values for the development of sports related concussions are 40-60 g (or 3500-5000 rad/s2),1 with a reference to an article published by our group.
To mix or equate linear acceleration measured in g (1 g = 9.8 m/s2) with angular acceleration measured in rad/s2 is not appropriate. Also, although several of our earlier studies indicated linear accelerations near 10 g during soccer heading, our most recent studies at higher speeds (12 m/s or 26 mph) have shown average linear accelerations of more than 20 g and average angular accelerations of 1500 rad/s2 for direct frontal impacts.2 Soccer ball speeds notably higher than 12 m/s (26 mph) are reached on the soccer field; speeds up to 54 m/s (80 mph) have been reported.3 At these speeds, the linear and angular accelerations would be higher with greater potential for injury.
We do not believe that the minimum values of acceleration at which concussion occurs are known. In any head injury the linear and angular acceleration of the skull may combine to cause brain injury. Angular acceleration is assumed to be more important because the brain is more susceptible to the ensuing shear deformation.
In the original experiments by Gennarelli et al it was reported that monkeys that were immobilised by cervical collars were much less likely to receive concussions than animals whose heads were allowed to rotate.4 This leads us to speculate that factors that affect angular acceleration (head position, angle of heading, location of impact) will affect the potential for injury.
Perhaps the most important predictor of concussion is a history of prior concussion. A person who has had one concussion is three times more likely to have a second, and eight times more likely to have a third concussion than a member of the general population.5 This cumulative effect may be present at subconcussive levels as well. For this reason we do not believe that the cumulative effects of soccer heading are fully known.
Competing interests: None declared.
References
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