The development of Nathan Karns' changeup

After the Nationals selected Nathan Karns in the twelfth round of the 2009 MLB draft, some post-selection analysis suggested that Karns had a few good pitches, but poor mechanics would likely cap his potential. During the 2011 and 2012 seasons, Karns' performance began to prove draft skeptics wrong. In an extended stay in high-A in 2012, he posted a 2.26 ERA and struck out nearly 11 batters per nine innings. He debuted in the major leagues in 2013, and was traded to the Rays during the following offseason.

In the minors, Karns relied on a 93 mph fastball and a devastating curveball. In his preseason review of the Rays' prospects, Kiley McDaniel labeled Karns' curveball as "above average, and flashed plus in some starts", grading it at 50/55. However, McDaniel also noted that many scouts believed that the lack of a consistent third option would push Karns to the bullpen.

Usage and Results

However, Karns worked on his changeup before this season, and so far the results have been encouraging. He has posted a league-average whiff rate, causing batters to miss on 29% of their swings. Furthermore, he has used it to generate groundballs at a 53.66% clip, which is above the league average rate of 47.8%. This has given him a viable weapon against left-handed hitters, and he has used it as such, throwing his changeup 17% of the time to them in 2015.

A heatmap of his changeup usage helps explain how he has generated whiffs with the pitch. Because he needed to improve the pitch to help get lefties out, it'll be more meaningful to look at pitch location trends against them than right handers, who see few changeups from Karns.

The image above shows that he tends to throw it on the outside of the plate, or even off the plate against left handed hitters. The movement of the pitch causes it to run away from lefties, and often leaves them flailing as it breaks down and away from their bat.

According to FanGraphs' PITCHf/x data, Karns has drawn swings on 31.4% of the changeups he has thrown out of the strike zone, and hitters have made contact on only half of those swings. This has helped Karns post a solid whiff rate, and is a crucial part to the pitch's effectiveness. Karns' changeup has helped him hold lefties to a .292 wOBA, and combat platoon advantages that lefties would have.

In addition to getting good results on the pitch, Karns has made improvements to its shape as well.

Changes in Shape

Last week, Eno Sarris labeled Karns' changeup as the most improved pitch from the first half of the season. In his article, Sarris points out that his changeup has been improving even as the season progresses, as Karns' changeup's vertical movement has decreased by more than an inch since the start of the season.

Because of it is negatively correlated with groundball rate,  having less vertical movement on a changeup would increase the expected groundball rate. But, groundball rate is determined by more aspects than vertical movement, like velocity and velocity difference, so we can't say that he should generate more groundballs without considering these other aspects. Using the xGB% formula, we can see how Karns' xGB% has changed from his changes in pitch shape.

While vertical movement has been decreasing, changes in the other components of xGB%, velocity and velocity difference, have counteracted this progress. Karns' changeup velocity has been decreasing since May, causing the velocity difference to increase. Since velocity is positively correlated (more velocity = more theoretical groundballs), and velocity difference is negatively correlated (larger velocity difference = fewer theoretical groundballs), these two factors are holding back some potential growth of Karns' changeup, at least as far as groundballs are concerned.

Even though the increase in vertical movement has been mitigated by other components, the xGB% has increased from the start of the season, moving from 51.2% to 52.8%, so it has developed into a more effective pitch as the season has progressed.

So what does this all mean?

Overall, Karns' changeup has taken major steps forward this season. His third pitch was once a liability and a potential cause to move him to the bullpen, but it has developed into an effective option. While he has added vertical movement to the pitch over the course of the season, other aspects of his changeup have changed, mitigating some of the potential improvement. But, that's not to say that the change in vertical movement has gone to waste - it has likely increased the number of whiffs he gets with pitch, as vertical movement is negatively correlated with whiff percentage, too.

Karns tends to throw this pitch out of the strike zone, with 59.6% of his changeups being out of the zone. While Karns has been getting hitters to chase these pitches, he runs the risk of having hitters start to lay off of his changeup. If they do stop chasing, he may start walking more batters and be a less effective pitcher.

While his control is still a little rocky, and his fastball doesn't touch 97 as much as it did in the minor leagues, the improvement of his changeup makes Karns a viable rotation piece for the next few years.

Statistics are from FanGraphs, PITCHf/x data from BrooksBaseball.net.