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.

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Bill works as a consultant by day. In his free time, he writes for The Hardball Times, speaks about baseball research and analytics, consults for a Major League Baseball team and appears on MLB Network's Clubhouse Confidential. Along with Jeff Zimmerman, he won the 2013 SABR Analytics Research Award for Contemporary Analysis. Follow him on Tumblr or Twitter @BillPetti.

13 Responses to “Does an April Drop in Velocity Predict An Arm Injury?”

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  1. Dan says:

    I think FanGraphs has a tendancy to make definitive conclusions a little too fast. For example, in this article it is stated “yes, a pitcher who is throwing softer in April compared to the previous April does have an increased chance of experiencing an arm injury,” Does anyone else think that this study would have to been conducted over multiple season in order to say that so conclusively?

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    • thistakesgumption says:

      > 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).

      What do you mean by multiple seasons that isn’t there already

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    • NS says:

      The author provides the level of certainty for his conclusion as well as the basis for it.

      You couldn’t ask for any better or more explicit qualification of the results.

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    • Denard says:

      In a rigorous sense, I agree with you. The conclusions should address the data set, not the question asked. Instead of “yes, a pitcher…does have”, it should be “this data suggests that a pitcher…”, or “within this data set, a pitcher…”. And throw in a p value for good measure.

      Then again, this is Fangraphs not JAMA.

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      • dan says:

        This is exactly what i was looking for. Maybe baseball just isn’t that serious. Thanks and also to everyone else for the input.

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      • J. Alfred says:

        Well if you didn’t need more evidence of the arrogance and stupidity of doctors. Really? JAMA? I love assigning to my undergrads JAMA articles as it is easy for them to tear apart stupid mistakes. Stick with your junkets.

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        • B N says:

          There, there. NBA JAMA was my favorite basketball medicine journal. While not everyone likes it yet, it’s heating up!

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        • NATS Fan says:

          Yes the Miami Heat in JAMA

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        • BigJ says:

          Better than Psych Science, let me tell you. Though I’m starting to wonder about the AER. Better question, Can I get the model statement here? I use a lot of hazard functions predicting right sided risk, and it seems suited for this kind of thing, but if the model’s sort of the same, then there’s no point retreading.

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  2. Ed says:

    I think there’s a key factor being ignored here, which is especially relevant with CC Sabathia’s case. What if the April velocity loss is due to an injury *before* the season?

    CC had an arm injury last year (bone spur problems in his elbow), and had surgery to fix it after the season. He wasn’t 100% recovered at the start of spring training, and didn’t get into games until a few weeks after the other starters did.

    I think it’s far more likely that CC’s reduced velocity is related to last year’s injury and the shortened spring training than it is due to a possible new injury.

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    • Mikhel says:

      Well, it says “experiencing an injury”, and not “to develop an injury”, because in any case, in the analyzed sample, both scenarios are valid since we don’t know if the injury was before the season began (either during the previous season or during the offseason) or it was sustained during the first month, though it says the data shows an increase in DL trips (with such a few DL trips, it seems like it is not significant).

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  3. nick says:

    I’ve heard Keith Law mention the opposite as well, when pitcher’s velocity suddenly rises it’s a harbinger for an arm injury. I can think of two cases off the top of my head last year: Danny Duffy and Drew Hutchison. Wonder if there is any truth to what Law said?

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  4. evo34 says:

    The surprising thing to me about this study was that the overall arm injury rate (DL trip at any time from May-Oct) was ~6%. Seems very low, but perhaps non-arm injuries to pitchers are more common that I had thought.

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