the NFL's Injury Surveillance System (ISS)

Over the last two decades, suggested links between playing in the NFL and brain disorders have encouraged detailed assessments of the possible long-term health impacts of American football. Neurodegenerative disorders, such as mild cognitive impairment (MCI), Alzheimer's disease, amyotrophic lateral sclerosis (ALS), and Parkinson's syndrome have been found to be associated with concussion history in retired professional football players.([ 7]) Additionally, although previous research suggests that NFL players exhibit lower overall mortality rates than the general U.S. population, their rates of mortality due to neurodegenerative causes are significantly higher.

Additionally, mounting MTBI concerns have prompted the league to enact rule changes aimed at protecting the head, such as the 2010 rules eliminating helmet-to-helmet contact with a defenseless player and requiring play stoppage when a runner's helmet is removed.([14])

Prior to the 2011 season, the league implemented amendments to the Free Kick rule, whereby the kickoff restraining line was moved from the 30- to the 35-yard line, and kicking team players other than the kicker were required to line up no more than 5 yards behind the restraining line.([15]) The intention of the rule was to increase touchbacks (where the ball is not returned) and to eliminate the customary 15–20 yard running "head start," thereby reducing injury frequency during special teams play.

Because shifting the kickoff restraining line to the 35-yard line might be expected to increase the number of touchbacks (kicks that travel beyond the opponent's goal line and are not returned by a ball carrier, resulting in stoppage of play), a separate evaluation of injury rates was performed for only returned kickoffs to evaluate the potential effects of the rule changes unassociated with reduced gameplay. In order to evaluate whether differences in injury occurrence across seasons could be attributed to the Free Kick rule amendments vs. other extraneous factors (e.g., increased awareness), a similar analysis was performed for injuries occurring during punt plays, as these plays would have been unaffected by the kickoff rule amendments.

These findings indicate a systematic decrease in kickoff injury rates from 2010 to 2011 as a result of implementing the rule amendments, as anticipated by the NFL

Meteorological factors have also been shown to influence sports injury risk. Wet surface conditions, whether on natural grass or first generation artificial turf, have been associated with decreased risk of injury, and greater rates of injury have been documented with higher temperatures on both natural grass and second- and third-generation artificial turf.([22]) In our study, average temperature, humidity, and wind speed of games during which kickoff or punt injuries occurred did not change significantly from the 2010 to 2011 seasons.

The success of injury prevention efforts is dependent upon evaluating the effectiveness of specific interventions, as practiced in other high-risk occupational settings. The field of industrial hygiene, focused largely on optimizing safety and health in the workplace, categorizes strategies for hazard control according to a hierarchy of effectiveness.([ 9]) At the top of this list is eliminating or substituting the hazard. An example would be the abolishment of kickoffs, which would eliminate the potential for kickoff injuries

Edgeworth Economics utilized the NFL's ISS, which captures all players diagnosed with a concussion based on sideline evaluation of suspected symptoms; such injuries may not have required a stoppage of play.([26],[27]) Because NFL gamebooks, the data source for our analysis, capture injury information based on visual identification, injuries that require on-the-field medical attention and delayed gameplay are most likely to be recorded. The appearance of concussive symptoms and cognitive deficit can occur several hours following an episode, and may not be identified until sometime later (e.g., during a subsequent visit to the doctor).([28]) Thus, our analysis likely captured only more severe concussions that required immediate medical attention.

paradigms

More specifically, competing incentives can alter the effect that rule changes, such as those implemented on kickoffs in 2011, have on injury risk. For instance, special teams is commonly referred to as a "proving ground" for players trying to break into the ranks of the first team, and aggressiveness is routinely awarded.([31]) This mentality may incentivize players and coaches to avoid touchbacks, and thus undermine the purpose of the rule change. This is supported by the large increase in the percentage of injuries that occurred on kickoffs where the ball traveled into the endzone, shown in Figure 1. Given all of the factors at work, the NFL must continue not only to play an integral role in the control of hazards in the workplace that it manages, but also to continuously monitor and evaluate the effectiveness of its player safety initiatives.

To evaluate whether injury rate changes could be attributable to the rule amendments, comparisons were made with punt injury rates (presumably unaffected by the Free Kick rule changes) and distributions of potential confounders were assessed across seasons. Incidence of injuries occurring on kickoff plays fell from 2010 to 2011 (RR: 0.45, 95% CI: 0.28–0.73), although on kickoff plays where the ball was returned, this decrease became non-significant (RR: 0.67, 95% CI: 0.41–1.08). While the incidence of head injuries decreased by approximately 3-fold during kickoff plays, this change was not statistically significant (RR: 0.33, 95% CI: 0.09–1.21). No difference was observed in injury incidence during punts between the two seasons, and the distribution of confounding factors was largely uniform across seasons. The observed decrease in injuries occurring during kickoffs was likely directly attributable to the Free Kick rule amendments, principally from the increased frequency of touchbacks. The absence of a significant change in head injuries during kickoffs was unexpected, but may be attributable to small sample size. Despite the injury rate reductions, the rule changes likely had little effect on player safety during active gameplay.

Only two injuries occurred on kickoffs that resulted in a touchback, further suggesting that injury risk may be driven primarily by upfield blocking and/or tackling, which intensify when the ball is returned.