6. Games versus practice (high level of certainty that a risk factor for concussion)

All of the 29 studies included for analysis found a higher risk of concussion in matches/games compared to practices, with the primary reason for the increased risk being the increased risk of high-impact collisions in match play.

While noting that there was a wide range of estimated increased risk, often due to low numbers of concussions in training sessions which skewed the statistics, the fact that all studies found increased risk made it unlikely that future research would alter this finding so as to justify assigning match play a high level of certainty to increase concussion risk.

7. Match/game period (low level of certainty that a risk factor for concussion)

Two studies examined the period of the game as a potential risk factor for concussion, one finding there were significantly more concussions in the second compared with the third period in elite ice hockey, the second finding no difference in concussion rates between the first and second halves of professional rugby league football.

Due to the limited number of studies, Abrahams and her colleagues assigned a low level of certainty to the time of the match as a risk factor.  

8. Mechanism of injury (low level of certainty that a risk factor for concussion)

A collision with another player, either accidentally or illegally, was often the mechanism of injury in most studies, but a major limiting factor is the lack of risk estimation, with often only the percentage or proportion of concussion reported.

Of the two prospective cohort level II studies included for analysis, one study found a significantly different concussion incidence between unintentional and intentional collisions.  Specifically, a hit to the head or helmet was a significantly common mechanism of concussion injury in ice hockey, soccer, and American football players, but no measure of concussion risk could be performed because no risk estimate was measured in either study.

The complete absence of any high-quality studies ascertaining risk based on the mechanism causing concussion, combined with poor data analysis result in a low level of certainty for mechanism of injury and concussion risk, the study said.

9. Playing position (low level of certainty that a risk factor for concussion)

Playing position as a risk factor for concussion has frequently been investigated in American football, ice hockey, and rugby:

• 6 studies (2 of which are high-quality level 1 studies) showed no effect of playing position on concussion risk
• 2 studies (one a level 1, the other a level 2) found an increased and decreased risk for specific playing positions, showing that quarterback in football had the highest risk of sustaining a concussion compared with all other playing positions, with a risk 1.7 to 5 times greater compared with other offensive positions (with the exception of a wide receiver), as well as an almost twofold and threefold increased risk compared with defensive positions.
• 2 American football studies found no effect of individual playing positions on concussion risk.
• 1 of 3 ice hockey studies reported an almost 3-fold decreased risk of concussion among goalies compared with defensemen and forwards;
• Conversely, no difference in concussion risk was observed when comparing goalies agianst defensemen and forwards in the other two studies of competitive youth and professional ice hockey players, although no risk estimation could be determined for the professional hockey study.
• Both rugby studies reported no effect of playing position on risk, although few concussions were reported, which explains the absence of a risk effect.

"Owing to inconsistent findings among the reviewed studies," Abraham and her colleagues concluded that "certainty that playing position is a risk factor for concussion is low."

10. Playing level (low level of certainty that a risk factor for concussion)

The 6 studies assessing concussion risk at different playing levels within the same age-group yielded varied findings:

• 2 prospective cohort studies (levels 1 and II) found concussion rates increased in descending divisions of college football, with one finding that there was a lower risk in Division I football compared to D-2 and 3, which the authors attributed to poor quality of protective equipment, decreased skill levels or players having to play both offensive and defensive positions;
• Conversely, a study on professional rugby leagues found that increasing playing level tended to increase concussion risk, but the difference was not statistically significant.
• 3 studies (professional rugby, 2 ice hockey) found no difference in concussion risk between different playing levels.

"From among the selected studies no consistency in results was found, therefore a low level of certainty was given to playing level as a risk factor for concussion.  Further level I prospective cohort studies are required to improve the level of evidence," concludes the study.