by Zach Binney

The expression "no days off" is as synonymous with Bill Belichick as cut-off hoodies. So when the 2011 collective bargaining agreement (CBA) between the NFL and the NFL Players' Association (NFLPA) restricted offseason and in-season practice days, the coach was not pleased. He and others grumbled that their players were no longer getting the reps they needed to perform well in the regular season.

But Belichick did not stop there. He also blamed the practice restrictions -- which cut organized team activities (OTAs) from 14 weeks to nine; eliminated padded two-a-days during training camp; increased off days during training camp; and limited regular-season padded practices from whatever-the-coach-wants to 14 -- for increasing injuries.

The practice restrictions were designed in part to improve player safety by decreasing stress on players and giving them more opportunities to rest and recover. Belichick's dueling theory was that with fewer practices, players were physically unprepared for the rigors of an NFL season, leading to more injuries. He was not alone in his view, nor was his theory entirely without merit. There are a number of scientific studies that show that training either too little ("undertraining") or too much ("overtraining") can raise your injury risk. The idea is to find the sweet spot in the middle that minimizes injury risk. What was unknown was whether the NFL's practice regimens resulted in overtraining in the pre-CBA era, undertraining in the post-CBA era, both, or neither.

Fortunately, we don't have to rely on Belichick's theories since we have the data to examine what happened to NFL injuries before and after the 2011 CBA. To do this we counted the number of injuries by year from 2007 to 2016 and observed how they changed in the pre- and post-CBA periods. We also counted games missed due to injury each year to account for any changes in injury severity rather than just the number of injuries.

An injury was defined as any new physiologic event appearing on the NFL's weekly injury reports. We included only non-head injuries that caused a player to miss at least one regular-season game since we know reporting of minor and head injuries has skyrocketed over the last decade. We did not want those trends mixed up with any effects of the CBA. We excluded preseason injuries because the CBA also raised preseason roster sizes, which may have caused injury counts to rise just because more players were at risk. This left us with 7,425 injuries in approximately 19,000 player-seasons from 2007 to 2016.

More details about our methods are available in this pre-print paper.

All-Injury Counts Over Time

We began by looking at total injury counts and games missed before and after the CBA. Figure 1 does not show any appreciable increase in injuries after the CBA (the black vertical line between 2010 and 2011). If anything we see a decrease in injuries for the first three years under the new practice scheme (2012 and 2013) before they rebound to historical levels.

Figure 1. Number of Regular-Season Game-Loss Injuries and Games Missed Due to Injury, 2007-2016, with 95% Confidence Intervals.

There also seem to be some modest increases in injury counts over time before the CBA ever took effect. This suggests that it could be unfair to simply compare injury counts in the pre- and post-CBA periods. Such a comparison could make the CBA look bad when an increase in injuries was due to ongoing trends that started well before it took effect.

Another way to think of this is if we were to try to draw a line through our data points, that line would be going up from 2007 through 2010. It would be a bad sign for the CBA if after 2010 there was either a sudden and persistent increase in injuries (a jump up in the line after 2010) or if the pre-CBA increase in injuries sped up (an increase in the line's slope after 2010). If the line just continues on as before for 2011-2016, we could say the CBA had no impact. Hold that thought.

Injuries Stratified by Conditioning Over Time

Looking at all injuries together may not tell the full story. Belichick's theory was that less practice time left players insufficiently conditioned for the NFL season. Perhaps under the new CBA there was an increase in injuries most dependent on an athlete's physical conditioning -- damage to soft tissues such as the calf, hamstring, and quadriceps. We can contrast this with what happened among traumatic injuries such as fractures that have little to do with an athlete's physical conditioning.

When we do this in Figure 2, we see no appreciable change in either conditioning or non-conditioning injuries when the new CBA takes effect. The closest thing we identified was a temporary spike in conditioning injuries in 2011. The 2010-2011 offseason featured a 4.5-month lockout that kept players away from their teams in addition to the CBA's new practice restrictions. This may have led to players training less than at any other time in our study, leading to poorer conditioning and a temporary one-year bump in conditioning-dependent injuries. That said, it's impossible to draw any sound conclusions from a single data point.

Figure 2. Number of Regular-Season Game-Loss Injuries and Games Missed Due to Injury, Stratified by Conditioning Status, 2007-2016, with 95% Confidence Intervals

Statistical Models

Plotting injury counts over time does not suggest the CBA had any impact on injuries. However, it would be useful to fit a statistical model to our data in case our eyes are not picking up on some subtle trends.

We fit something called a Poisson interrupted time series model to the data. This allowed us to do a couple useful things:

Account for the pre-CBA increase in injuries mentioned earlier.

Control for changes in player age or other injury risk factors across the study period.

The details are available in our full paper, but suffice it to say the model found no evidence of an increase in injuries or games missed due to injury under the new CBA's practice restrictions.

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Sensitivity Analyses

We conducted several sensitivity analyses, which involve tweaking some analytical decisions and seeing how that changed our results. We tried including preseason injuries, including minor injuries, re-classifying ankle and knee injuries to be "non-conditioning," and looking at only hamstring injuries. None of them changed our conclusions.

Limitations

Our analysis has a major limitation: there were several other changes designed to enhance player safety and reduce injuries at the same time as the CBA and throughout the study period. Changes made simultaneously with the CBA's new practice restrictions included moving the kickoff up to encourage more touchbacks and the expansion of the "defenseless players" list to cover, among others, receivers who have not re-established themselves as runners. It is difficult to disentangle the effects of all these various changes in our data, but it is most likely these effects would have biased injuries in later years downward. This would in turn have made the CBA look better than it was.

Conclusions

There is no evidence that the 2011 CBA led to increased injuries in the NFL. It does not appear that the practice restrictions pushed the NFL, on average, from a state of optimal training that minimizes injury risk to one of undertraining. Of course restricting practice time may have other negative effects such as degrading player skill or game quality, but that is beyond the scope of this article.