Touching Bases July 20, 2010
How Does Time on the DL Affect Fastball Velocity?

Rotobase's injury database contains disabled list data dating back to 2002. Incidentally, that is as far back as FanGraphs carries Baseball Info Solutions' velocity data. So my question is, how long does it take a pitcher to get back up to speed?

First, I've plotted the number of days a player spends on the DL against the difference in velocity between the month he was put on the DL and the month he returns.

Joe Martinez, returning from three hairline fractures caused by a line drive off his skull, displayed the biggest jump in velocity, as you can see in the 2009 section of this graph.. And Brad Penny in 2008, who was plagued with tendinitis in his right shoulder, took the biggest hit of any pitcher, as demonstrated here.

To the point, there's no correlation between the two variables. That's not to say that the severity of an injury has no bearing on fastball velocity-it most certainly does. It means that the sampling biases in this study may overwhelm the effects of an injury. No pitcher will return to Major League Baseball if his injury is too debilitating. The pool of players who do return from injury is strongly biased towards those players who were not cripplingly injured. Even so, perhaps pitchers continue to show effects after they return from the DL to the Majors. Below, I present a table showing the average difference in fastball velocity between the month he hit the DL and all subsequent months after coming off.

Return Month Velocity Difference
First 0.02
Second 0.07
Third 0.23
Fourth 0.41

Velocity increases the further removed a pitcher is from the DL. Players are continually recovering. Still, velocity is generally higher in months after hitting the DL than the immediate month before. What about if we look at the month before that. If a player was on the DL from June 1 to June 30, then how did he throw in April as compared to July?

Return Month Velocity Difference
First -0.20
Second -0.12
Third 0.07
Fourth 0.12

Not surprisingly, this shows that pitchers exhibit symptoms of injury (diminished velocity) in the immediate month prior to hitting the DL more so than in the preceding months.

This effect was exacerbated when I looked at pitchers recovering from Tommy John surgery. Because recovery from Tommy John takes over a full year, this was the only time that I used data from different seasons for a single pitcher, but I still identified over 50 cases where a pitcher recovered from TJ.

Return Month Before (1) Difference Before (2) Difference
First -0.12 -0.51
Second 0.08 -0.29
Third 0.41 -0.02
Fourth 0.26 -0.15

Tommy John alumni pick up velocity the longer they are allowed to stay in the Majors. But most of them do not find the velocity they had in the months before the surgery.

Back to the original 826 players who made the trip to and from the DL in a single year. The injury database is set up in such a way that there are many binary variables indicating whether the injury was to this body part or that, so what else to do but run a linear regression? Nothing was statistically significant, but upper arm injuries seem to exhibit the greatest negative effect on velocity.

Methodological Details:

FanGraphs provides monthly velocity splits, so, for every pitcher who hit the DL, I found all the months they pitched before coming off the DL and all the months they pitched after going on the DL. So if a pitcher's stint was from June 15-July 15, I used his June month as before (1) and his July month as return (1). May and August would therefore be before (2) and return (2), respectively. If a pitcher was on the DL from June 5-June 25, then I excluded June, and used May and July as the before and after months. I adjusted each pitcher's fastball velocity reading by the month and by his team. More pitchers go on the DL in April than come off it, which could have skewed results, as seasonal temperature effects could throw off velocity by a full MPH. So the two Chicago teams and the Indians were bumped up nearly a percent in fastball velocity in April, while the Angels in July were knocked down a bit, for example.

Again, thanks to Rotobase and FanGraphs.