Weight and Injuries

by Zach Binney

A majority of NFL players are obese. Of 64 players on the 2017 Dolphins roster, for example, 35 (55 percent) were technically obese using a common measure called body mass index (BMI). All but one would be considered overweight. Quarterback Matt Moore -- at 6-foot-3 and a svelte 192 pounds -- was the lone player to fall into the normal range.

Of course these guys have to be huge to play in the NFL, and BMI does not account for weight from muscle versus fat. NFL players are some of the world's most elite athletes, and their bodies have to strike a delicate balance – too big and they might be too slow to make that tackle; too small and they might be too fragile to survive it. The right balance requires weighing a lot of factors, but I was curious about one in particular: are heavier players any more or less likely to get injured?

It's possible that injuries could be more frequent in heavier players due to the greater impact forces they suffer. For example, every step a 320-pound lineman takes puts more stress on his knees and ankles than the same step for a 210-pound receiver. But it's also possible injuries could be lower for heavier players since, in general, larger body mass is associated with more stable joints and a greater ability to absorb the extra forces to which they are subject.

To examine this question I calculated injury rates for NFL players by 20-pound weight buckets. I also used BMI, but the results were nearly identical, and weight is a little easier to interpret. Injury rates are presented per 1,000 athlete-exposures (AEs); one AE is one player participating in one practice or game. An injury was defined as any new physiological event appearing on the NFL's weekly injury reports. I included injuries from Weeks 1 to 16 of the regular season for 2007 to 2015. I excluded injuries occurring in Week 17 because data for that week is only available for playoff teams. 2016 and 2017 are excluded because the removal of "Probable" from game status reports makes comparisons with earlier years difficult.

Weight and Injury Rates Among All Players

Our first look at the data (Figure 1) suggests that heavier players have lower injury rates. But there is a major problem with this analysis.

Figure 1. Injury Rate per 1,000 AEs by Weight Category, All Positions, With 1-standard Error Bars

Weight and injury rates both vary by position. Specifically, linemen tend to be the heaviest players, and they also suffer lower injury rates due in part to what they are asked to do: less high-speed running and quick directional changes with fewer violent collisions. The table below shows injury rates by position from 2007-2015.

This means Figure 1 is not just depicting the effect of weight on injury rates. It is depicting a mixture of the effects of weight and the effects of position on injury rates. Epidemiologists call this mixing of effects "confounding." Specifically, we would say that the effect of weight on injury rates is confounded by that of position. Because linemen are heavier and suffer lower injury rates for reasons besides their weight, Figure 1 may show a decrease at higher weights for reasons that have nothing to do with weight.

To look at the effect of weight separate from position we could examine the association between weight and injury rates within each position. Epidemiologists say this kind of stratification "controls" or "adjusts" for confounding.

Weight and Injury Rates Within Each Position

When we do this in Figure 2, the association between weight and injury rates reverses. For most positions, higher weight is now associated with a higher injury rate. This provides some evidence for our hypothesis that the greater impact forces heavier players experience lead to higher injury rates.

There are two major exceptions to this trend. Heavier running backs exhibit lower injury rates than their lighter counterparts. This may be because of differences in the way heavier running backs are used. Our running back category included both halfbacks and fullbacks, so instead of an effect of weight we may simply be seeing that fullbacks or heavy halfbacks receive fewer carries and thus have fewer chances to get hurt. Supporting this hypothesis of "confounding by role" is a small negative correlation between weight and rush attempts among running backs in 2016 and 2017.

Injury rates also drop among the heaviest tight ends (271 to 290 pounds). Similar to running backs, confounding by role may be an explanation. The heaviest tight ends may be primarily blockers rather than receivers, and blocking is likely a lower-risk activity than running routes and being tackled post-reception.

The heaviest offensive linemen (more than 350 pounds) also exhibit lower injury rates than their lighter counterparts, but confounding by role is less likely to apply here. I do not have an explanation for this decline besides random chance.

Figure 2. Injury Rate per 1,000 AEs by Weight Category, Stratified by Position, With 1-standard Error Bars.

Conclusions

This preliminary analysis found that heavier players have higher injury rates after controlling for position. Because of the residual confounding by role present at least among running backs, a good next step would be to further control for usage and see if the association of higher weight with higher injury rates remains. If it does, what should teams do with this information? Fielding a team of 195-pound offensive linemen is probably unwise, but focusing on players at the lower end of the weight range for their positions may offer teams some modest advantages when it comes to injury prevention. All else equal, if a team has a choice between a 180-pound defensive back and a 210-pounder, they may want to go for the lighter guy.