Exploring the effect of size on NFL combine events: Defensive Players

In a previous post, we explore the relationship between weight and 40 yard dash rates by position.  Now we'll attempt to delve into this a little deeper with some more analytical methods.

Still, we have a small problem that height and weight tend to correlate, and that the strength of correlation depends largely on the amount of body fat a player has (a statistic that is regrettably undocumented in the NFL combine).  To account for this, we analyze by similar position, with the understanding that, for the most part, players with similar roles have similar builds.

Let's begin with the secondary (Corner backs and Safeties).  We'll start by plotting their dash rate, shuttle rate, cone rate, vertical leap, broad jump, and bench press numbers by both weight and height with a simple linear regression line fit for each position (Cornerback = CB and Safety = S).  In a handful of subsequent posts, we will examine the degree of correlation between height and weight for other grouped positions.

Hmm, with the exception of bench press (by weight) and broad jump (by height) there doesn't appear to be much going on in terms of linear increase/decrease.  Still, to be rigorous, let's check the strength of correlation (-1 to 1) and test the hypothesis of a correlation of 0 (flat line) with p-values (0 to 1) with an adjustment to account for differences between the Safety and Cornerback position:

Pearson Partial Correlation Coefficients, N = 502 Prob > |r| under H0: Partial Rho=0
	dash_rate	shuttle_rate	cone_rate	VertLeap	BroadJump	BenchPress
Weight	-0.05566 0.2136	0.02257 0.6142	0.01350 0.7631	-0.00719 0.8724	0.03596 0.4218	0.24033 <.0001
Height	0.00592 0.8949	0.01942 0.6646	0.04144 0.3546	0.06905 0.1227	0.21288 <.0001	-0.06053 0.1762

So after adjusting for differences between the two secondary positions, we see that a significantly non-zero correlation between Bench and Weight (p < .0001) and Broad Jump and Height (p < .0001) in the secondary.  The former is easily explained: safeties and corners with high bench press likely tend to be more muscular, and therefore weigh more.  The latter is a little more difficult to explain, but it seems reasonable that taller players can create greater leverage in generating the force to propel themselves forward in a more explosive manner than their shorter counterparts.  It's very surprising that the various rates of timed drills are essentially independent of weight, but there's sampling bias at work here: if you're not fast, you don't play in the secondary.

Moving out of the secondary, let's see what this analysis looks like for the larger linebackers:

With the added height/weight required to play linebacker, there certainly appear to be some speed/explosiveness trade-offs.  Let's break it down by the partial correlations (adjusted for any potential inherent differences between the two positions with respect to these drills):

Pearson Partial Correlation Coefficients, N = 361 Prob > |r| under H0: Partial Rho=0
	dash_rate	shuttle_rate	cone_rate	VertLeap	BroadJump	BenchPress
Weight	-0.17722 0.0007	-0.12900 0.0143	0.00916 0.8625	-0.07954 0.1320	-0.11813 0.0250	0.18034 0.0006
Height	0.00875 0.8686	0.05278 0.3180	0.09952 0.0593	0.04337 0.4120	0.10404 0.0486	-0.18321 0.0005

We see a similar relationship as that observed in the secondary: taller linebackers jump further and heavier linebackers pressing more.  However, it also appears that taller linebackers tend to press less (longer arms perhaps) and heavier linebackers can't jump as far (more weight to propel).  It also appears that the straight-line events (Dash & Shuttle) are inversely proportional to linebacker weight, though the three cone drill is not.  With larger frames and heavier weights, these all seems consistent with the fact that it takes additional force to accelerate the additional mass that linebackers tend to carry.

So let's see what happens for the linemen, the heaviest of defensive players:

Hmm, with the possible exception of the bench reps, there's not a lot happening with height, but weight seems to have some serious trade-offs:

Pearson Partial Correlation Coefficients, N = 456 Prob > |r| under H0: Partial Rho=0
	dash_rate	shuttle_rate	cone_rate	VertLeap	BroadJump	BenchPress
Weight	-0.29577 <.0001	-0.33465 <.0001	-0.31085 <.0001	-0.19282 <.0001	-0.30258 <.0001	0.18226 <.0001
Height	0.00522 0.9117	-0.08350 0.0752	-0.03914 0.4049	0.00402 0.9318	0.07016 0.1351	-0.07981 0.0890

So we see the same tendency in all timed distance drills for the heavy linemen:  more weight means a slower rate.  In a similar fashion, the leaping distances (vertical and broad) are decreased in heavier individuals, with the heavier individuals tend to bench more (possible related to "barrel-chestedness" be advantageous to one's bench reps).

To summarize, bench and weight are correlated regardless of defensive position, which may have to do with additional muscle increasing both of those attributes.  Taller players tend to have longer jumps, unless you are a heavy lineman whose leaping ability is inhibited by weight.  Dash and Shuttle rates are slower for heavier players, unless you're a safety/corner because you have to be fast regardless.  Finally, weight only affects cone rate in the defensive line due to their bulkier builds.