Last year was a career year for Michael Brantley. Over 676 plate appearances, the most of his career, Brantley amassed 6.2 fWAR on the strength of a .327/.385/.506 batting line for a 153 wRC+. His .178 ISO was by far the highest of his career, minor leagues or major leagues. His .333 BABIP, while a bit high, did not suggest something completely unsustainable.
Eno Sarris went on a business intelligence-like deep dive into Brantley's breakout about two weeks ago. Comparing Brantley's 2014-2015 with previous years, he discovered that Brantley was being more aggressive on first-pitch fastballs.
However, lumping together 2014 and 2015 loses a bit of nuance. Brantley's ISO has declined to .152 this year. This is not surprising; when a guy whose previous three ISOs were between .110 and .120 suddenly has a power breakout, we just expect regression in the future. Sarris' deep dive gets into how the breakout occurred. I'll try to detail the regression.
The first part is easy to spot. Looking over his batted ball profile, his stuff is pretty stable. Year over year, there is not a ton of variation. Last year had the greatest variation in a power-related stat - HR/FB. His home run per fly ball rate of 12.7 percent was the highest of his career by nearly two times his previous career high. His rate this year is 7.8 percent, only slightly above his career value of 7.0 percent.
Diving deeper into fly balls on FanGraphs reveals a stark difference from last year. His .292 BA / .779 SLG last year was pretty high, and it seems to be related to the rate at which he pulls fly balls. His career-high pull rate of 28 percent last year led to a career-high 43.3 percent hard-hit rate. His pull rate and hard-hit rate this year have both declined back to career norms, which has led to a .198 BA / .514 SLG on fly balls.
Those are the results. The process is that Brantley has toned down the aggressiveness in 2015. That's not necessarily due to a change in how he is being pitched. Here are two heat maps from Baseball Savant that show his pitch distribution. Guess which image belongs to which year.
The first one is 2015. The second one is 2014. Does it matter? You could maybe say that pitchers have shifted slightly away from him (he's a lefty), and his career-high 10.5 percent walk rate would agree with you. In reality, can you say for sure that there is a difference?
In addition to being pitched similarly in terms of location, Brantley hasn't exactly seen different pitch types. He's seen a few more hard pitches (Brooks Baseball) than offspeed or breaking, but the difference is small. He has swung more at offspeed pitches (48.3% this year vs. 44.2% last year), but he has a .255 ISO on changeups this year. I'm not sure offspeed stuff is involved in Brantley's power decline.
The biggest difference, and likely the cause of the power decline, is Brantley's swing decisions. In short, he's decided to swing less on inside pitches, including the hard stuff (four seamer and sinker). Many of those swings have found themselves on outside pitches instead of inside pitches.
2014 swing distribution against four seamers and sinkers
2015 swing distribution against four seamers and sinkers
Brantley's extra swings on outside pitches and low pitches almost make it seem like he is adjusting to the strike zone distribution for left-handers, which includes outside pitches, and the general lowering of the strike zone. His walk rate is up, and his power is down. He's laying off the pitches that he could pull (inside pitches), and those pitches are more likely to be called balls against lefties. He's swinging more at pitches outside and low, and those are more likely to be called strikes these days.
Despite the lower power, he's retained a lot of his increased overall offensive output. As I mentioned earlier, his walk rate is up. His line of .316/.388/.468 and .330 BABIP for a 138 wRC+ is still pretty darn good. For a guy whose previous career high wRC+ was 105, a little power decrease in exchange for more walks is no big deal.
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