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Exploring Running Back Size

Rotoworld

Jonathan Bales is the author of the Fantasy Football for Smart People book series. He recently launched RotoAcademy - a fantasy football training school.

I was having a conversation with my dad the other day and I asked him to name the top four wide receivers in football with whom he’d want to start a team. He said Calvin Johnson, Josh Gordon, Dez Bryant and A.J. Green. I agree with three, subbing in Demaryius Thomas for Green (although it’s close).

Then I asked him to name the top four running backs with whom he’d start a team. LeSean McCoy was an immediate choice. He contemplated Jamaal Charles and Adrian Peterson, but in running back years, they’re basically dead. Matt Forte was on his list, as was Marshawn Lynch - Marshawn Lynch! - and Le’Veon Bell.

If that list sounds ridiculous to you, it’s because 1) it kind of is and 2) it’s really, really hard to figure out which running backs are the best ones. We can immediately identify the top wide receivers in the NFL, but that task is exponentially more difficult for running backs. If we’re incorporating age into our assessment - which we should since it matters in both real life and fantasy - who the hell are our starting four? I’d likely still throw Charles and perhaps AP in there with McCoy, but I seriously have no idea who is the fourth-best back in the NFL.

In short, we know that Julio Jones and Dez Bryant and those studs are better than second-tier receivers like Jordy Nelson and Eric Decker, who are in turn better than guys like Brandon LaFell. But outside of a couple obvious talents at running back, can we really know that the consensus No. 25 running back is any worse than the No. 5 player at the position? How good is a player like C.J. Spiller? I legitimately considered putting him in my top four, but I have a feeling that most people might not have him in their top 15 or 20.

When it comes down to it, running backs are just so dependent on their teammates for success that it’s really difficult to isolate their play and figure out how good they are. They’re difficult to project out of college and their NFL stats are only loosely tied to their actual talent.

That’s one reason I’ve been going against the grain in advocating a late-round running back strategy as of late, but it’s not like we can just draft only rookies at the position (maybe). Running back is still very important in the fantasy realm, so what can we make of the position?

Running backs are always going to be dependent on heavy usage for substantial production, but we still need to figure out some way to predict which ones are going to be useful when they get opportunities.

Running Back Size

Heading into this analysis, my hypothesis was that shorter, heavier running backs would have more success than taller, lighter ones. I was really interested in the height data because I’ve had a suspicion that shorter backs are generally better; they’re usually more agile, quicker and have a low center-of-gravity which is crucial at the position.

Related to both height and weight is body-mass index (BMI). Using BMI to judge overall health is absolutely asinine - pretty much every NFL player falls into the ‘obese’ category - but it’s still a good measure of how much bulk a running back possesses; a 5’10” back who weighs 210 pounds is bulkier and has a higher BMI than a 5’10” back who weighs only 190 pounds. The taller a player, the more difficult it is for him to have a high BMI. So I’m really looking to test three things - height, weight and body mass - and how they affect NFL performance.

The first thing I did was test the correlation between those three traits and a trio of stats - carries, yards-per-carry and touchdowns - for all backs since 2000 with at least 300 yards rushing. Here are those r-values.

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The initial numbers suggest that height doesn’t affect NFL performance all that much for running backs (more on that later). Taller backs get a few more carries than shorter ones, they have a few more touchdowns, but they’re generally a bit less efficient in terms of YPC.

Meanwhile, take a look at weight and BMI. Neither affects workload very much - meaning we’ve already determined we can’t use running back size to predict rushing attempts - and they’re both weakly correlated to touchdowns. We’d expect bigger, heavier backs to score more touchdowns just because they see more goal line attempts, though.

But take a look at the effect of weight/BMI on rushing efficiency. They’re pretty strongly negatively correlated, meaning as weight increases, rushing efficiency decreases. This was at first a surprise to me because we wouldn’t expect heavier backs to be worse.

I overlooked an important fact, though; generally, heavier backs are slower than light ones, and speed is incredibly important to running backs. I’ve shown that the 40-yard dash matters more for backs than for any other position. Here’s a refresher that breaks down running back approximate value according to combine 40 times.

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I value speed in running backs even more than other stat geeks; if a running back doesn’t run sub-4.55 (regardless of his weight), there’s very little chance I’ll draft him. If he’s very light, his little frame better be accompanied by blazing speed.

The fact that heavy running backs are much worse than lighter ones isn’t surprising when you consider the differences in speed. So does extra weight hurt a running back? No, with two caveats: 1) he isn’t just eating Wendy’s and the “extra” weight is lean muscle and 2) his speed remains unchanged. I’ll take a 220-pound back with 4.45 speed over a 200-pound back with the same speed all day. Weight is good, it’s just not as important as straight-line speed.

Now, let’s get back to height…

Height and Running Back Success

The correlations suggested that, as a general rule-of-thumb, taller might not be worse for backs, but there’s another way to analyze the data - sorting it into buckets. I like to do this because it helps see how performance changes at certain thresholds. And when we do that with height, we see a different story for running backs.

I sorted all of the data into quantiles for height, weight and BMI.

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We see a steady decline in YPC with weight; the lightest quarter of all backs has been the most efficient, followed by the 26th-50th percentile, and so on. The BMI effect is similar, with a very rapid decline after the bottom quarter.

Very quickly, I want to mention that there’s probably a little bit of a selection bias here with lighter backs getting a higher quality of carries. When a running back gets a carry on third-and-10, he usually gains a decent number of yards, and he’s also usually a fairly light third-down back. However, such carries are pretty uncommon and the results are very strong, suggesting that the real culprit is simply heavy backs being slower.

I’m going to bold this sentence and say it again: the best running backs are fast.

Now, look at height. Remember that the correlations showed that height might not matter too much for running backs, but what we really see is that the bottom 75 percent of running backs in terms of height perform right around the same in terms of efficiency. Meanwhile, the top 25 percent - the tallest quarter of running backs - have rushed for fewer than 4.0 YPC since 2000. That’s horrific.

To me, this is clear evidence that height doesn’t matter all that much for running backs … to a point. And that point is right around 73 inches, or 6’1”.

Now let me just address the criticism sure to come to everyone’s minds: BUT ADRIAN PETERSON IS 6’1”! Yes, that’s true. As is the fact that Arian Foster ran a 4.68 in the 40-yard dash, Jerry Rice was also quite slow, Warren Sapp dominated inside despite shorter-than-average arms and lots of other cases of players becoming exceptions to the rule.

My goal isn’t to get every prediction correct, but just to tilt the odds a little bit. And the numbers suggest that, as a general rule-of-thumb, we should prefer shorter backs over those who stand well above 6’0”. That doesn’t mean we need to avoid every tall running back. In the case of Adrian Peterson, it’s kind of difficult to hate a 217-pound back with 4.40 speed. But if another back were similar to Peterson - 217 pounds with 4.40 speed and a comparable skill set - but he checked in at 5’10”, we’d be smart to favor the shorter back.

All other things equal, we should seek running backs 6’0” or shorter.

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