Football Outsiders

Innovative Statistics, Intelligent Analysis

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.

Most Injury-Prone NFL Positions, 2007-2015
Position Injury Rate per 1,000 AEs (Standard Error)
RB 20.7 (0.5)
DB 17.4 (0.3)
WR 17.1 (0.4)
LB 17.1 (0.3)
TE 16.9 (0.5)
DL 15.1 (0.3)
OL 12.8 (0.3)
QB 8.6 (0.4)
ST 4.4 (0.3)
Total 15.1 (0.1)

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.

Comments

16 comments, Last at 21 Mar 2018, 12:03pm

1 Re: Weight and Injuries

by Aaron Brooks Good Twin // Mar 15, 2018 - 1:28pm

DB will have positional confounding, too.

Small DBs are more likely to be corners, whereas bigger ones will be box safeties (small LBs, basically).

I'm curious how many of the jumbo LBs are really DE/LBs.

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5 Re: Weight and Injuries

by MC2 // Mar 16, 2018 - 3:17am

Good point. I suspect that most of these results (at all positions) may be tainted by usage, or at least by playing style. For example, big QBs (like Roethlisberger, Newton, or Luck) often seem willing to take bigger hits, presumably because they have less fear of injury, whereas smaller QBs are often much more proactive about throwing the ball away, sliding when they scramble, and so on.

The one exception might be at RB, where conventional wisdom (that big backs are "more durable" than small ones) seems, at least anecdotally, to be true. Consider the case of Chris Thompson. Last year, he got off to a great start, and appeared to be by far Washington's best option at RB. Nevertheless, Jay Gruden stubbornly refused to give him more touches, citing his lack of size and, presumably, durability, as the reason. Eventually, however, injuries to the Redskins' other backs forced Gruden's hand, and just as he predicted, the increase in touches eventually led to Thompson suffering a season-ending injury.

One more note: If you look at the chart listing injuries by position, is there any wonder why the lowest rates are for QB and "ST" (which, I assume, refers to kickers and punters)? After all, there are specific rules against "roughing" the passer (and "roughing" the kicker/punter). I'm sure injury rates for RBs would be much lower if they enacted a penalty for "roughing" the ball carrier!

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2 Re: Weight and Injuries

by ChrisS // Mar 15, 2018 - 1:40pm

Good article. Glad you did not use BMI because it is a very simplistic tool with limited use, especially for athletes. “The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals,” explained Dr. Carl Lavie, a cardiologist..."

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3 Re: Weight and Injuries

by IrishBarrister // Mar 15, 2018 - 7:36pm

As a former offensive lineman, I have a pretty clear hypothesis why the heaviest linemen don't get injured as often: usage. Guys that heavy are generally not expected to move in space as much as their (relatively) lighter counterparts. So they are less often expected to pull, trap, take out a DB on screens, etc. as much as the 300 pound guys. And it's those types of plays that can really take a pounding on your knees and ankles. Outside of getting stepped on by a teammate or something like that, most of your injuries come from plays where you were running.

And the graph seems to confirm my idea that the ideal weight to play the position long-term is about 300 pounds. There's no way the extra weight is doing any favors to your knees while at a full sprint.

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7 Re: Weight and Injuries

by jtr // Mar 16, 2018 - 8:51am

Those pull and trap plays are also the biggest collisions, since the blocker gets a chance to get to full speed rather than hitting someone who lined up just a few inches away. It definitely makes sense that pullers would get hurt more often than non-pullers.

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12 Re: Weight and Injuries

by IrishBarrister // Mar 19, 2018 - 4:30pm

To be clear, everyone pulls or flies out on screens. It's just how much. I mean, if you've got a 360 pound right guard, how often are you calling power to the left and having him come across to take a linebacker in space? You'll do it, sure. Just not much. Mostly, you're just gonna let your 360 pound hippo guard do what he does best: maul and drive.

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4 Re: Weight and Injuries

by Zach Binney // Mar 15, 2018 - 7:58pm

Interesting. I wasn't familiar enough with the intricacies of OL play to feel comfortable making such a statement, but it's consistent with the data. Thanks for your insight!

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6 Re: Weight and Injuries

by Jerry // Mar 16, 2018 - 3:30am

Maybe looking at snap counts (Where they're available) will help with some questions. It could be that the blocking-specialist tight end/fullback is on the field for fewer plays, which would explain incurring fewer injuries.

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8 Re: Weight and Injuries

by jtr // Mar 16, 2018 - 8:54am

When I clicked on this, I already expected that bump around 210lbs. That's the weight range that running backs and safeties tend to play at, and those are the players involved in the most frequent collisions and the most violent collisions, respectively.

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9 Re: Weight and Injuries

by brian30tw // Mar 16, 2018 - 4:04pm

It will be difficult to control for usage because of the endogeneity of the system here. The assumption you'd be tempted to make is that players who are used more are more likely to suffer an injury. The problem is there is also causation in the opposite direction: players who are injured are less likely to be used. Just something to keep in mind as you continue down this road.

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10 Re: Weight and Injuries

by Zach Binney // Mar 16, 2018 - 4:19pm

Indeed. It's a really tricky problem, which is part of the reason why I stopped my analysis where I did. You could look at something like the number of snaps prior to first injury each year. Pre-injury usage can't be affected by a player's injury rate (except insofar as teams purposely restrict their usage of players they consider injury-prone, which, sure, it probably happens, but I'm guessing not as much as people suspect and mostly just among players 30+).

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11 Re: Weight and Injuries

by Badfinger // Mar 19, 2018 - 11:43am

I'm curious how big the buckets are when you've broken down into position groups and then by weight. The big tails at the heavy side of LB, WR, TE, and OL both up and down don't make he think it's safer or less safe to play that position at that weight, but that there are only a few players in those brackets, so even one more or less player losing time to injury causes the swing when there's virtually no change across all groups of that weight between the 230-290lb brackets.

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13 Re: Weight and Injuries

by Zach Binney // Mar 20, 2018 - 8:02am

Sure. So I will say that every bucket in these graphs has at least 20 player-seasons underlying it (without that filter the graphs get REALLY noisy on the ends). You can get an idea of the likely variation from random chance using the error bars. Whether that's too much variation or not for you to have confidence in the results is absolutely a personal call!

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14 Re: Weight and Injuries

by justanothersteve // Mar 21, 2018 - 1:24am

I'm putting this here because it may also relate to injury. According to Peter King's Mailbag, this was on the agenda at the upcoming league meetings in Orlando:

"The Niners, Cards and Chargers are proposing that West Coast teams have no more than three 10 a.m. body-clock games on their schedules; in other words, when Pacific Time teams travel to the Eastern or Central time zones, they’d only be allowed three games starting at 10 a.m. PT."

I know this has been discussed by some fans here. It seems the teams agree.

Other items include subjecting personal fouls, roughing the passer, and defenseless receiver to replay review, and limiting defensive pass interference to 15 yards.

https://www.si.com/nfl/2018/03/21/draft-top-five-picks-mmqb-mailbag-pete...

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15 Re: Weight and Injuries

by Zach Binney // Mar 21, 2018 - 7:06am

This'll be my next article, promise!

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16 Re: Weight and Injuries

by justanothersteve // Mar 21, 2018 - 12:03pm

There's not much more to this item beyond what I wrote here. Mostly which teams are behind which change. It's a few paragraphs in a much larger article. I think we'll hear more about this next week.

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