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MLB: This Might Be the REAL Reason for Pitcher Injuries

Updated: Jan 15

It Is NOT the Pitch Clock!

I knew I would have to revisit this topic in the MLB offseason. Everywhere one looks someone blames the pitch clock for the increase of injuries especially for the pitchers.

Folks, it is NOT the pitch clock. It is something far simpler. Here is the problem; I don't think MLB pitchers and pitching coaches want to deal with the real reason for injuries.

If I plotted the number of pitchers getting arm and shoulder injuries every year vs. a simple factor you could understand with this level of correlation (an R-square of 0.97 is an extremely strong correlation), would you believe that THIS was the reason and not the pitch clock? I will say correlation is not proof of causation, but it at least should spark questions.


Do you see that group of seasons at the top right? Basically, the number of pitchers sustaining arm and shoulder injuries has been FLAT for the past three seasons. There was no pitch clock in 2021 and 2022. So, the pitch clock is NOT the cause of pitcher arm and shoulder injuries. What is?

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Now let's continue.

Here is our data set:

injury data

The Green, Red, and Purple columns are different groupings of pitch percentages that I am about to show you graphs for.

The Sliders Group in green is the percent of Sliders, Sweepers and Slurves thrown in that season. This is the Baseball Savant (where the pitch data comes from) definition for the Sliders Group. For example, there were 17.6 percent of slider group pitches thrown in 2019.

The Sliders in red is the percent of sliders thrown in that season. You will see that in 2021-2023 the percent of sliders thrown was essentially a constant 17.6-17.7 percent.

For the Purple group I added the splitters to the sliders group to analyze that as well. For example, there were 18.3 percent of those four pitches thrown in 2018.

The column in blue shows how many pitchers were injured in that season factoring out Covid and undisclosed injuries (which in analyzing the data likely included some Covid cases.) In 2022, 359 pitchers got hurt spending time on the IL.

The column in orange isolated those injuries listed as arm, elbow, biceps or shoulder injuries. These are the injuries the popular narrative reflects are out of control and CAUSED by the pitch clock in 2023. You can see the shoulder and arm injuries are essentially flat in 2021-2023 as well.

The injury data I am using is coming from Spotrac here. I have been careful to make sure I am not double counting and factoring out the Covid cases from this data set because clearly that has nothing to do with the pitch clock.

For the following graphs the blue line is the data in blue above- the total pitchers injured in that season. The orange line is the orange data above and only the arm and shoulder injuries.

Starting with the Slider Group here are graphs that show there is a STRONG correlation of overall pitcher injuries to the percent of slider group pitches thrown and an even stronger correlation to the arm and shoulder injuries.

slider group

The correlations are even stronger for arm and shoulder injuries when we JUST focus on the percent of sliders thrown. The orange line here is what I showed you in first graph without labels. The correlation of sliders thrown to pitcher arm and shoulder injuries is STRONGLY correlated to the percent of sliders being thrown in the MLB over the past nine years. Until 2021, the MLB threw more and more sliders every year and there were more and more arm and shoulder injuries.

Slider injuries

I was curious if the current trend of more split finger fast balls being thrown was also part of the injury correlation. I added it in the purple data above and including it does not help the correlations. Maybe the MLB should try to develop more splitters and less sliders.

Plus Splitter

I considered also the slider velocity being thrown and the spin rate of the sliders being thrown. Neither was additive to modeling injury rates in this period.


First, I do wonder if all of the injury data is actually what fans are considering as injuries. The arm and shoulder injury data are more focused and probably a better data set.

I believe this data does show that as the MLB has thrown more sliders, more pitchers are getting injured. I think the correlation to arm and shoulder injuries is VERY strong.

However, I think it may be too early to make this a definitive position. Injuries can be random events, and I think trying to differentiate this data in any smaller sample size than the entire league becomes problematic.

I do not believe after one season of the MLB pitch clock we have any data here that leads a fair-minded person to conclude the pitch clock is causing this issue. If injuries were related to the pitch clock, they would have increased significantly in 2023. The arm and shoulder injuries didn't, and these are the injuries pontificators are stating are caused by the pitch clock. Again, there is simply no data one can point to in support of the pitch clock as the cause of injuries- at least not yet.

Team Data

The challenge to the slider theory is this. Team data does not support it at all. Here are the slider rates and team injuries in 2023. As you can see there is no correlation in these data sets. I don't think one can study a population this small and hope for results. A team has 20 to 40 pitchers in a season and 7 to 20 of them went to the IL for teams in 2023.

The league wide data had all of the 850 some pitchers in it and 350 to 390 got injured.

The sample size of 850 vs. 40 is the same as a season of plate appearances vs. less than two weeks of plate appearances. I just don't think one can study this kind of data at a more discreet level than a full season league wide.

I did want you to be aware of the data challenge, however.

In short, while I think the MLB data shows that increasing slider usage is FAR more likely a cause of the increasing pitcher injuries than the pitch clock folks are ready to blame immediately. I think the league SHOULD monitor this kind of data to see after a few years if the pitch clock is really hurting MLB pitchers.

If one accepts that slider rates ARE a root cause what is the answer? That is why I think there is a distraction. No one wants to give up throwing sliders. Slider group pitches were hit to a 0.297 xwOBA in 2023. Fastball group pitches were hit to a 0.355 xwOBA. No team is going to want to eliminate a 20 percent pitch that is THAT much better at getting outs than fastballs. I don't know what the answer is. but blaming the pitch clock appears to be a distraction at best currently.

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