Weighing In On NFL Running Backs: Is Bigger Always Better?

By Jake Fisher and Elizabeth Fryman

From rootzoo.com

NFL running backs come in all shapes and sizes. Relative to other backs, Darren McFadden is tall (6’2) and skinny (210 lbs). Jerome Bettis was average (5’11) and large (255 lbs). The question we seek to answer is this: does the size of a running back affect performance over the course of his career.

We find that from 1978 to 2000, running backs with higher BMI (body mass index) rushed for more yards per season over their careers. But when controlling for omitted variables, we learn that there’s not enough evidence to say that BMI increase causes the increase in rushing yards. Instead something about NFL culture has been changing since 1978 that favors a simultaneous increase in BMI and rushing yards.

In our study’s data set, we include running backs drafted in the first and second round of the NFL draft from 1978 to 2000. We do not use players drafted in the past 10 years because these players have not had full careers and will bias the sample. For the most part, players drafted from 1978 to 2000 have finished their careers or are very close to the end.

To test the impact of size, we measure running backs with a BMI (body mass index) evaluation. BMI is a relationship between weight and height.

BMI= (703 x lbs.)/(inches^2)

An individual with a large weight and a small height will have a higher BMI. Recent examples of low BMI backs include Reggie Bush, Chris Johnson, and Matt Forte. Recent examples of high BMI backs include Jerome Bettis, Jamal Lewis, Jonathan Stewart, and Ricky Williams. In medical circles, a high BMI is used to measure obesity. But for running backs, where player weight for the most part comes in the form of muscle and not fat, a high BMI should be an indication of compactness and strength. BMI will be the independent variable in this study. OLS regression analysis shows that weight has increased significantly and height has decreased significantly over time.

Decade

Average BMI

1980s

29.1

1990s

30.2

2000s

30.8

The dependent variable in this study must be a measure of rushing success. One good way to measure rushing success is to measure a player’s total rushing yards per season in the league. This statistic does not take durability into account, but our data shows no evidence of a relationship between longevity and BMI. Total rushing yards per season will judge how effective a back was when he was in the league.

It turns out that higher BMI has a significant positive relationship with average season rushing totals with 90 percent confidence.

The summary statistics show that the median BMI in our data set is 29.38. The average season rushing total for players in the top half of BMI is 503.5 yards. The average season rushing total for players in the bottom half of BMI is 385.9 yards. Players with higher BMI seem to rush for more yards per season. This trend is significant but variable, as indicated by the R2 of .0185. Higher BMI running backs also show a significant increase in carries per season. For players above the median in BMI, the average carries per season is 125.3. For players below the median, the average is 95.4 carries per season

Before we say that running backs carry the ball more and gain more yards because of higher BMI, we need to control for some variables. One omitted variable could be what draft pick the player was. It could be that teams favor running backs with higher BMI in the draft and thus select them higher. These backs could also rush for more yards as a result of being drafted earlier. However, when controlling for pick, the effect of BMI is still significant with 90 percent confidence for both rushing yards per season and carries per season.

Another potential omitted variable comes in the form of time. One explanation is that over the past 30 years, first and second round running backs have simultaneously increased their BMI and rushed for more yards. From 1991-2000, the average BMI was 30.47. From 1978-1990, the average BMI was 29.02. It looks like player BMI has been going up. From 1978-1990, the average rushing total per season in the league was 405.4 yards. From 1991-2000, the average rushing total per season in the league was 499 yards. From 1990-2000, a player averaged 125.52 carries per season over a career and from 1978-1990, a player averaged 99.33 carries per season. Thus time, not BMI, could be a cause of the observed trend of an increase in average yards per season and carries per season.

When controlling for draft year using a time-fixed effects model, BMI becomes insignificant for both rushing total per season (p-value .315) and carries per season (p-value .149). Considering the year a player was drafted eliminates the predictive power of BMI on rushing success.

There are several possible explanations for this trend of increased rushing yards in our 20-year data set. It could be that the NFL culture has become more focused on running over time, with teams putting a stronger emphasis on their best running back. Or perhaps backs were getting injured more in the 1980s than they were in the late 1990s.

The findings can also be explained by an increase in quality of first and second round backs relative to other picks. This hypothesis gets support from the following fact. Over time, teams have become more selective when it comes to early round running back draft picks. The 1980s saw 80 backs picked in the first two rounds, the 1990s saw 65 and the 2000s saw 53.

Decade Yards/Season Carries/Season  RB Picked in 1st/2nd Round

1980s

413.3

101.1

80

1990s

480.3

121.5

65

2000s

608.1

142.0

53

In summary, it seems like an increase in BMI just happens to be correlated with an increase in yards gained and carries because rushing yards, carries, and BMI have all increased since 1978. This conclusion is reaffirmed after looking at the effect of BMI within each draft year. High BMI players are not obviously performing better compared to others running backs selected in the same draft.

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10 Responses to Weighing In On NFL Running Backs: Is Bigger Always Better?

  1. David Roher says:

    Really thorough job and a great read. It would be interesting to look more into that “culture” concept, and also to see if this fits in at all with the tackling problems that defenses are having this year.

  2. dadler3 says:

    Nice work, guys. One other thought is that it may be that today’s backs are running for more yards because they have higher BMI (as a group). Maybe today’s backs are more durable because they’re more solid.

    If every back in the 1990s ran for 100 more yards/season and they were also larger, we don’t necessarily know if they’re doing better because it’s the 1990s and teams are running more/using running backs differently or if they’re just bigger and teams are able to use them more.

    Take an extreme example…each player has the same BMI and yards for each draft class. For every class, BMI goes up by .5 and yards goes up by 50. This would be captured by year fixed effects, but it could be BMI driving the result.

    Something to think about. Great work.

    • Leonardo Miranda says:

      I agree with you. The BMI is increasing and the carries and yards per players are increasing, concomitantly with the durability, so I think there is a relationship here. (I’m assuming that the study is valid not only for 1st and 2nd rounds RBs, but all of them.)

      In other view, there is no player that is far better than the others top players of him generation. There is a relative “equality” between the players of each time. So, you can’t see the differences of productivity relating them with the BMI, because there are so many variables, like speed, tenacity, aggressiveness, and so on.

      But ignoring all other variables, you can see that for a regular player, higher BMI means more productivity. But I agree with the hypothesis that over time teams have become more selective when it comes to early round running back draft picks. Nevertheless, I think that it don’t invalidate my thoughts above.

      PS: I really loved the job, guys.

      (Sorry for the bad English.)

  3. E says:

    Yes, good work. I’m sure you’re aware but just in case: the guys at Football Outsiders have a metric called “Speed Score” to identify who will make a good NFL RB, or, as they put it:

    “… takes into account both a player’s time in the 40-yard dash at the Combine as well as his weight. Speed Score bears a significant correlation to NFL carries (a correlation coefficient of .46 and a sign of a player’s durability), yards (.46), and DYAR (.37), all at a level superior to that of strictly using the 40-yard dash time alone.
    The metric is calculated by multiplying the player’s weight by 200, and then dividing that figure by his 40 time, taken to the fourth power. Although it sounds like a bizarre calculation, the whole thing is pretty simple. 40 time is multiplied to the fourth power because of the huge difference there is in hundredths of a second for a player running the 40. The weight is multiplied by 200 to scale the metric so that an average Speed Score is just about 100. The average for first-round picks in the NFL Draft is 112.9.”

    An article describing it in with reference to the 2009 drft class is available at http://views.washingtonpost.com/theleague/predraft/2009/04/in-the-cattle-call-that.html , though it was covered in more detail in their 2008 book.

  4. NFL draft says:

    Here is a bad news:
    An NFL player died Thursday.
    Police investigations and sober reaction from stunned teammates will now mark the close of Chris Henry’s life. But here’s the finding that no official report will carry.

    Some tales, for whatever reason, seem destined for unhappy endings.

    There might be second chances, hopeful signs, promising trends. Redemption could truly be in sight. Henry had them all with the Cincinnati Bengals, and he was still only 26. “He was thankful for what he had in life, and proud of what he had overcome,” a statement from his agents would say.
    You can look for more NFL Draft about him.
    But sometimes, fate will not be deterred.

  5. dc says:

    I was going to mention Football Outsiders’ Speed Score as well. It’s basically just a modified form of they way you’d calculate Kinetic Energy in Classical Mechanics (1/2MV^2), rather than just mass, as in BMI, and thus tells you a bit more.

  6. snomm1s says:

    I think it would be beneficial to look at career numbers as well. In a given season I wouldn’t expect there to be that much difference between healthy lower BMI RB vs higher BMI RB drafted in the 1st two rounds. Healthy being the key word.

    I would expect that, other than RBs and *maybe* CBs, average height has increased or stayed constant for all positions on the football field. RBs have definitely shrunk over the last 50 years. Short RBs (below 6′) dominate the career rushing list now when 25 years ago the reverse was true.

    I am convinced that the reason is because shorter backs (regardless of weight and BMI) are quicker (due to lower center of gravity) which helps them avoid big hits and shorter legs decrease the chance of injury. These two factors lead to much longer careers. Also, the increase in size of Dlineman, LBs, and Safties means that power running is much more difficult so taller heavier backs are not as desirable.

    I have no doubt that high BMI backs aren’t obviously performing better for a season. However, I would be willing to bet that *short* backs (BMI low or high) perform better over a career than tall backs.

  7. Pingback: No Less Worth Despite Their Girth: A Study of Overweight Coaches « Harvard Sports Analysis Collective

  8. Brian Battle says:

    I’m very much enjoying your site.

    Quick thought: Do NFL teams have to report official height and weight of their roster every year? Though the weight changes might be slight, (and perhaps inaccurate) wouldn’t they provide insight on how successful individual runningbacks are at varying BMI?

  9. Aarnout says:

    Regarding the speed score: wouldn’t it be smart to add the 20 yard-shuttle time to the calculation. I ha

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