Do Thursday Games Cause More Injuries?

by Zach Binney It's not hard to find players going on the record about how much they hate Thursday Night Football (TNF). Their concerns center around player health, safety, and fitness to play after just a four-day rest period. These concerns were on display in last night's TNF matchup, where Doug Baldwin attributed his groin injury to short rest and Richard Sherman reportedly ruptured his Achilles after nursing soreness in the tendon for at least two weeks (when his Achilles first popped up on the injury report). Would Baldwin have avoided injuring his groin and Sherman avoided exacerbating his Achilles if the game had been played on Sunday instead? It's impossible to know in any individual case, but I was curious if these concerns would show up in NFL-wide injury reports. Are there higher rates of reported injuries for weeks with short rest, as opposed to normal or even extended rest? The injury rates below are given per 1,000 athlete-exposures (AEs). One AE is defined as one player participating in one practice or game. AEs are calculated based on the 53-man active roster, with reductions made for players held out of practices or games for ongoing injuries (not including IR, which is separate from the 53-man roster). An injury is defined as any new issue appearing on the NFL's weekly injury reports, and the week the injury occurred is taken as the week before it was first reported. 

As an example, in Week 8's reports, Karl Joseph of the Raiders popped up with a new groin injury. This injury was assumed to have occurred in Week 7's TNF game. (It could also have occurred in Week 8's practices, but that's less common.) Because the Raiders played on Sunday in Week 6, this injury is said to have occurred on four days' rest. We excluded injuries occurring in Week 1 because there is no well-defined rest period, and Week 17 because data for that week is unavailable for non-playoff teams. Additionally, the data below is for 2007 to 2015; 2016 and 2017 are excluded because the removal of "Probable" from game status reports makes comparisons with earlier years difficult.

All Injury Rates

There were no substantial differences in injury rates across different rest periods (Figure 1, blue bars). TNF and other short-rest rates were not elevated; nor were the rates after bye weeks lower, as one might hypothesize if additional rest protected against injuries. Surprisingly, injury rates for TNF games (14.5 per 1,000 AEs, 95 percent CI 13.5-15.6) were lower than those for the typical seven-day rest period (16.1 per 1,000 AEs, 95 percent CI 15.8-16.4). It's possible this difference could be due to players only playing at 900,000 rather than a million miles an hour on short rest, leading to less violent collisions, but there's a more likely alternative explanation for the difference: it's a quirk of public NFL injury reporting.

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Teams typically issue their first injury reports on Wednesday of each week; that's three days after a Sunday game but six days after a Thursday game. Any injury that resolves in that three- to six-day period (to the point the player can fully practice) then wouldn't be reported after a TNF game, but would after a Sunday game. If just 10 percent of injuries (16.1-14.5/16.1) resolve in that period, that would fully explain the lower TNF rate we saw among all injuries. That seems plausible, especially since there's no clinical reason I can imagine for shorter rest to be associated with fewer injuries. If it's substantially more than 10 percent that resolve in this three- to six-day period, then injury rates might actually be elevated with short rest and our analysis missed it, but without better quality (non-public) data that's just speculation. It's possible that only certain types of injuries may be more common on less rest, though. A recent paper in the other football found around 20 percent higher rates of "muscle injuries" during shorter rest periods (three or fewer days' rest, versus six or more). We decided to look at leg muscle injuries to see if we could replicate this study for the NFL.

Leg Muscle Injury Rates

We defined leg muscle injuries as any calf, hamstring, groin, quadriceps, or thigh injury. Once again, there were no substantial differences in injury rates across different rest periods (Figure 1, red bars). TNF and other short-rest rates once again were not elevated.

Figure 1: Injury Rates with 95 percent Confidence Intervals (CIs) by Rest Period for All Injuries (Blue) and Leg Muscle Injuries (Red)

Limitations

This analysis only includes injuries that were severe enough to merit an injury report listing, which is a threshold with some arbitrariness and variation across teams. It does not account for "normal wear and tear" or how ground down and beat up a player feels after a Thursday versus a Sunday game. I'll trust players who tell me it's rougher on their bodies, even if it doesn't show up in injury report counts.

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It's also important to note that the injuries we're attributing to a given rest period could have happened either during the game following that rest period or the subsequent week of practice. This is a time during which players could have been in a weakened state from shorter rest, so our analysis makes sense. We were unfortunately not able to account for any effect shorter rests have on injuries in the practices during the shorter week itself, but previous studies have shown only around 15 percent of NFL regular season injuries occur in practices, limiting these issues.

Conclusions

There's little evidence from injury reports to suggest TNF games and other short rest periods are associated with different injury rates from the typical Sunday-to-Sunday schedule. However, this study is limited by the use of public injury report data, which only accounts for discrete reported injuries that limit players in practices or games rather than wear and tear. Everybody's hurt, but only a few of those are reported as injured. I am in no position to tell an NFL player how they feel after four versus seven days of rest, and analyses that better measure the players' actual health before and after these games are needed.