Arm slot data from Baseball Savant

Original post link: https://yagongso.com/?p=22658

A new graph combining pitcher's arm slot and the movement profile of each type of pitch has recently appeared on the baseball savant. Let's take a closer look at this.


Arm slot?

What is the arm slot, and how is it calculated? The arm slot is the angle of the line from the shoulder of the pitcher along which the ball is released. It is parallel to the ground at 0° and perpendicular at 90°.

<The red line is an arm slot>


Movement by type of pitches

Movement of pitch is expressed in the form of IVB(Induced Vertical Break). IVB means the movement of a pitched ball, excluding the gravity received as it flies

<Pitch movement profile of Ryan Pepiot>

The picture above is a graph of the movement profile found in the baseball savant. The horizontal and vertical movements of the pitch type are represented. The difference between the league's average movement and the pitcher's pitch movement for pitch type can also be seen intuitively. Pitcher's arm slot is expressed in the movement profile, too.

How can I use this newly added arm slot data?


Relationship between arm angle and movement

The arm slot affects the movement of the pitch. Even with the same four-seam, the ball thrown by the sidearm pitcher and the ball thrown by the overhand pitcher draws different trajectories. Let's find out exactly what kind of pitch and movement the arm angle affects.

I checked the relationship between 8 pitch types (four-seam, sinker, cutter, slider, sweeper, curve, change-up, and splitter) and horizontal & vertical movements (IVB). I used data from pitchers who pitched more than qualified at-bats from 2020 to 2024. X-axis for the pitcher's arm slot, Y-axis for the pitch movement.

The first is the fastball-type pitches. Four-seam, sinker, and cutter fall under this category.


We can see some graphs seemed to be correlated. The table below shows each graph's correlation coefficient.


FF and SI have a stronger correlation in vertical movement than horizontal movement. Lower the arm slot,  the ball moves towards arm side more and falls more. This is because the effect of side spin gets bigger than back spin. FC shows a low correlation only in vertical movement.

Next is the breaking ball-type pitches. Curve, Slider, and Sweeper fall under this category.



And the correlation coefficient.


No significant correlation was found between movement and breaking ball-type pitches. Only a weak correlation could be confirmed that the higher arm slot on CU, the less glove side movement and more fall below.

Finally, off-speed pitches. Change-Up and Splitter fall under this category.




Both pitch types have weak correlations in horizontal movements and slightly stronger correlations in vertical movements. Both the change-up and the splitter fell less as the arm angle increased.

In summary, pitch types with high spin efficiency, such as four-seam, sinker, and change-up, had a high correlation between the vertical movement and the arm angle. The higher the arm angle, the less the ball falls when throwing a ball with high spin efficiency. 

On the other hand, there was no significant correlation between the horizontal movement and the arm slot. Only the four-seam had a correlation coefficient of 0.5 or higher.

The arm angle affects the movement of the pitch type. Therefore, it can also be used to explain players' changes. Until now, there have been many cases where the reason for the improvement of a player's performance is due to an increase in pitch velocity, command(control), and movement change through grip modification. Arm angle can also be one of those conditions.

Lowering arm slot

Simeon Woods Richardson is one of the pitchers whose arm slot changed a lot between last year and this year. His arm slot, which was 67° in 2023, went down to 47° in 2024. He explained that he put his arm down to pitch from the position a little more naturally.

<Left: 2023, Right: 2024>

The biggest change for Simeon Woods Richardson between last year and this year is the rise in pitch velocity. Four-seam velocity have risen significantly from 90.5 mph in 2023 to 93.1 mph in 2024. Slider velocity has increased from an average of 83.1 mph in 2023 to 86.4 mph in 2024.

There have been several changes in his movement. His curve, which used to fall vertically before, has a horizontal movement that moves towards the glove side. The horizontal movement of the four-seam has also changed from the glove side to the arm side.

The result of the change was success. This year, in his first full-time season as a starter. He pitched 133.2 innings in 28 games with ERA of 4.17 and fWAR of 1.8.

 
Raising the arm's angle

In some cases, the arm slot is raised intentionally. Take the case of Kyle Muller of the Oakland Athletics(Now he transferred to Chunich Dragons). Kyle Muller's arm slot has increased from 36° in 2023 to 53° in 2024.
<Left: 2023, Right: 2024>

He explained that he made the change to move from 12 to 6 rather than moving from 9 to 3. As he intended, his four-seam this year has seen less horizontal movement and less fall below than last year. Is that the result? His four-seam wOBA improved from 0.481 in 2023 to 0.302 in 2024.

His changes have been somewhat successful. After a career-low season in 2023, he has shown improvement in all indicators related to pitching this year.

Nevertheless, he is still the lowest-ranking pitcher in the league. Among the four-seam, curveball, change-up, and slider that Muller throws, the only one whose movement has changed by increasing the arm angle is the four-seam. He can consider adding another type of pitch that can take advantage of the increased arm slot or re-establishing the location of the type of pitch.

Conclusion

Baseball Savant continues to release new data. Before the arm slot data was released, the bat tracking data had been released. It is used with indicators such as wOBA and barrel percent and has functioned as one of the tools to see batter from more diverse angles.

The same goes for arm slot data. Savant already has various tracking and analysis indicators related to pitchers, such as movement, wOBA, extension, and release points. Arm slot data is also one of them.

Every time new data emerges, there are lots of articles related to the new analyses learned from it. As a baseball fan, it is welcome.

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