Does an April Drop in Velocity Predict An Arm Injury?

Earlier this month I wrote about whether we should be concerned with when we see pitchers throwing slower in April, particularly with regards to CC Sabathia.

If we are trying to predict whether the pitcher has truly lost some zip on their fastball, the answer is somewhat. Pitchers who are down at least 1 mph compared to April of the previous year will go on to finish the season down at least 1 mph about 38% of the time. Essentially, they are over four times as likely to be truly losing velocity compared to those that are not down in April. However, the signal gains in strength as the season goes on. So, if a pitcher is down at least 1 mph in July compared to July of the previous year their likelihood of being down at season’s end jumps to 14 times more than pitchers that are not down in July.

But does being down in April predict an injury? This is something I had not yet investigated. Given the increased discussion about April velocity declines I thought I should take a quick look.

I used the same data set as I used for my earlier analysis of velocity loss. For this analysis, the sample included 875 individual pitcher seasons from 2004 through 2011 where the pitcher was in the same role in consecutive years (meaning, either a starter or reliever in year 1 and year 2). 214 pitchers experienced a velocity drop in April compared to the previous April, while 661 did not. Using Jeff Zimmerman’s injury data, I then calculated the injury rate of each group, focused here only on arm injuries.

Pitchers that were down at least 1 mph in April had an arm injury rate of 11%. Compared to 4% for non-velocity decliners, that’s an increased likelihood of 2.6. The amount of time missed is also quite similar. When an arm injury was sustained after an April velocity decline, pitchers lost about 22 days on average to the injury. When the arm injury was not preceded by a velocity loss? 20 days on average. The only difference was in the number of trips to the disabled list that lasted at least 30 days. For April velocity decliners, there were four trips to the DL longer than 30 days. For non-velocity decliners there was only one such trip.

So, yes, a pitcher who is throwing softer in April compared to the previous April does have an increased chance of experiencing an arm injury, but the over rate of arm injury is quite low (11%). Compared to how well April decline predicts full season velocity loss (38% and an increased likelihood of over 4), velocity decline this early in the year is certainly no slam dunk indicator.