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The evolution of Hisashi Iwakuma

Hisashi Iwakuma changed his pitch repertoire last year. Did it help?

Seattle Mariners v Oakland Athletics Photo by Jason O. Watson/Getty Images

Evolution is a core mechanic in the Pokemon series of games. When most Pokemon reach a certain level after accumulating experience, they evolve into a stronger, more powerful version of themselves. Just like Wartortle evolves into Blastoise at level 36, Hisashi Iwakuma is also evolving at age 36. Unfortunately, Iwakuma isn’t getting stronger or more powerful.

Last week, Evan Davis of Beyond the Box Score wrote a particularly pessimistic evaluation of Iwakuma. Beyond the questionable speculation about a potential injury, Davis’s analysis of Iwakuma’s repertoire is incomplete at best. I don’t want to pick on Davis too much—I had this article in mind after looking at Drew Smyly’s batted ball mix—but I do want to dig into Iwakuma’s numbers a bit deeper because I think there’s something interesting there.

First, let’s just get a feel for how Iwakuma’s pitch repertoire has changed over the years:

Last year saw a huge jump in the number of cutters thrown, at the expense of Kuma’s slider and splitter. His slider has always been an average pitch, never generating too many swinging strikes, and almost exclusively used against right-handed hitters. But in 2015, that pitch was demolished by opposing batters; he allowed 5 home runs and a .286 ISO off his slider that year. To address these problems, Iwakuma started incorporating his rarely used cutter more often in 2016. In this interview in September, he briefly describes the changes he made to his slider and the three different grips he uses.

So did adding a cutter to his repertoire help him mitigate some of the damage he allowed in 2015? Sort of. Below is a table of pitch results for his slider and cutter from 2015-16:

Year - Pitch Type Count Whiff/Swing FB+IFB/BIP BABIP HR ISO
Year - Pitch Type Count Whiff/Swing FB+IFB/BIP BABIP HR ISO
2015 - Slider 313 25.85% 30.30% 0.271 5 0.286
2016 - Slider 282 26.56% 40.81% 0.340 1 0.121
2016 - Cutter 516 13.96% 32.50% 0.266 6 0.173

As you can see, Iwakuma’s slider was much more effective in 2016. He was able to generate just as many whiffs with the pitch while greatly lowering the amount of damage suffered off the pitch. However, the cutter that he added to his repertoire was not an effective pitch. The whiff rate on that pitch was well below average and it also contributed to a huge jump in fly ball rate last season. Those additional fly balls were the result of a loss of control. Observe:

Above are two heat maps showing where opposing batters made contact with Iwakuma’s slider and cutter in 2015 (left) and 2016 (right). No matter where he threw those pitches last season, batters were able to make contact with regularity. Before, he was able to bury his slider low and away against right-handed batters, leading to a healthy amount of whiffs. But he wasn’t able to do the same thing with his new cutter in 2016.

In his article, Davis keys in on Iwakuma’s splitter as the source of his troubles last year. That’s objectively wrong. He was generating just as many swinging strikes with the pitch and it was still inducing a groundball more than half the time opposing batters made contact with it. Rather than the deterioration of his best pitch, it was a change in repertoire and a loss of command that led to much of Iwakuma’s struggles last season. For 2017, Iwakuma will need to lean on his splitter more than ever since it’s still an excellent pitch. But the loss of control is more concerning, and with his age affecting his entire arsenal more and more, he’ll need to regain that command to make the most of his limited repertoire.