Hairsplitting BMI results
Fantasy football metrics run the gamut. Some are applied effectively while others are not very helpful. Even when you have a seemingly valuable metric, it may not be applicable between positions. It is important to understand which pitfalls to avoid.
Running backs have a limited window of fantasy relevance. That is compounded by the NFL primarily using backfields that work as a committee. However, most fantasy leagues require at least two running backs in your starting lineup. This combination of facts contributes to a high level of scrutiny and perhaps a willingness to stretch the bounds of useful information.
I was in the best shape of my life during two-a-day practices in high school. Even on our off days, I was heading out to the gym. I wasn’t packing on any muscle, but I was certainly lean. Unfortunately, my fall physical provided a different view. My BMI (Body Mass Index) listed me as borderline “overweight”. From that moment on, I knew it was an imprecise measurement.
You can’t make it a day on social media without seeing at least one conversation/argument about a player’s BMI. Regarding the running back position, this is pure folly. First, the measurements used are taken during the combine. This occurs months before they ever set foot onto an NFL practice field. A player’s body composition will likely change in the NFL.
So what does an NFL running back’s BMI look like? What is the range? Considering that this is for fantasy purposes, I’m only going to focus on the players that are the most relevant. For each of the past 5 seasons, I have identified the top 24 fantasy performances (0.5 PPR) and added 6 backs that had exceptional per game averages (minimum 8 games played). A sample size of 150 performances over 5 years is more than sufficient.
Average BMI = 30.23
Median BMI = 30.17
Middle 50% range (25th percentile to 75th) = 29.36 to 31.18
This doesn’t tell us much yet, but this middle 50% is full of names you’d expect. Zeke, Kamara, Derrick Henry, Todd Gurley, and Aaron Jones fall within this range.
The lighter fellas in the first 25% range include McCaffrey, Dalvin Cook, Chris Carson, Melvin Gordon, and rookie Antonio Gibson. All are above a BMI of 28 and under 29.3. On the other end of the BMI spectrum, the thicker guys represented their 25% well. Saquon, Chubb, Montgomery, Fournette, and undrafted rookie sensation James Robinson are between 31.5 and 32.5.
There were a few running backs at the furthest reaches of our sample’s range. McKissic and Mostert both clocked in under a 27 BMI. Terrance West, Jonathan Stewart, and CJ Anderson were all above 33. Anderson was the only player who had a BMI that was a mathematical outlier (33.91). However, those 5 players in the outer reaches accounted for only 3% of the 150 performances.
If you absolutely need guardrails, steer away from players under a 27 BMI or over a 33. 97% of productive running backs are distributed rather equally all over the range between the two extremes.
What if you are only concerned with RB1 (top 12) levels of production? No worries, there isn’t much difference. RB1’s have a slightly higher floor of 28 and a lower ceiling of 32.43. They have also had a slightly higher average BMI (30.37 compared to 30.23). That difference equates to a few insignificant pounds at most.
He is probably fast enough
40 yard dash times are spread out as well. Although, there is only one data range direction to monitor. After all, who complains that a running back is too fast?
Average 40 = 4.53
Median 40 = 4.53
Middle 50% = 4.49 to 4.59
Times for RB1s were nearly identical. RB1 had an ever so slightly quicker median time at 4.52. There were many RB1 performances by running backs who actually ran 4.60 or later. Carlos Hyde, Le’Veon Bell, Kareem Hunt, Mark Ingram, David Montgomery, Josh Jacobs, James Robinson, James Conner, and LeGarrette Blount all had great fantasy seasons despite their relatively poor 40 times. 4 of those performances happened just last year.
Only 3 fantasy relevant performances (2%) were by players who ran 4.70 or later. The aforementioned Blount and Rex Burkhead did it twice. So if you need a guardrail for 40 times, 4.70 will work. You could push that up to 4.65, but that loses another RB1 performance (Conner). It also gets a little too close to all of the players mentioned above who had top 12 seasons.
Everything else being equal, I’d rather a time of 4.42 to a 4.62. But their fantasy production has been near the same for the past 5 seasons. Don’t put undue weight on a player’s 40 time or their BMI.
We often talk about players who “don’t have much tread left on their tires” and the cumulative effect of years in the NFL. It is typically a throwaway line about a cagey vet. However, we don’t discuss the fallout after a career season by a running back. “He is one of the best in the league, he isn’t going to decline!” Don’t be so sure.
Since 2011 there have been 10 players who accumulated over 2,000 yards from scrimmage and 350 touches in one season. 9 of those players had a significant dropoff in production the next year. The 10th player, Derrick Henry, will play his next season this fall. The sample size is small, but 9 out of 9 is eye-opening.
On average, the 9 players had a 48.3% reduction in scrimmage yards and a 39.8% drop in touches. However, 3 of the 9 suffered season-ending injuries the following year. If you remove them from the dataset and only use the remaining 6 players, the average drop is 30.9% in scrimmage yards and a drop of 19.8% in touches. That cannot be ignored.
He may still have an RB1 season in 2021, but you should devalue Derrick Henry. After the season he put together in 2020, there will be at least one person in every fantasy league who will overpay. Take advantage of Henry’s perceived value, the probable dip that follows a career season, and the likelihood that his best days are behind him.
What about the players who just missed the thresholds of 2,000 scrimmage yards and 350 touches? In 2017, Todd Gurley exceeded 2,000 yards but did not get 350 touches. He had a very minor drop in production the following season. There were 9 players during this timeframe who exceeded 350 touches but didn’t approach 2,000 yards. Only one player surpassed his previous season, Matt Forte in 2014. Le’Veon Bell was a holdout in 2018 and Dalvin Cook has yet to play in 2021. The remaining 6 players all had reductions in scrimmage yards and touches. So, you want to consider that Dalvin Cook may be looking at reduced production as well.
Selling high can be scary and your roster composition may have you contending for your championship. But consider the ages of the most productive backs and the inevitable dropoff, whether quick or gradual. Then consider the results after a season with an enormous workload. The odds are heavily in your favor to devalue those players, even if you plan to keep them.