Flu-Like Symptoms: Detroit’s Earthworm Preservation Society

Balls hit in the air are one of the big stories of the 2017 season. A record number of them are going over the fence, but the larger narrative has been about how players are seeking to hit more balls in the air—elevate is the term of choice—with improved results. Ryan Zimmerman—nearly stick-a-fork-in-him done last year, MVP contender this year—is the poster child, but greater launch angles have been a theme throughout the game.

Rob Arthur of FiveThirtyEight has pointed out that this hasn’t necessarily benefited hitters, but it is, if nothing else, a thing. How big a thing? Well, through games of Saturday (all 2017 statistics in this article are through Saturday), 45.7 percent of batted balls have been hit on the ground this year, compared to 46.1 percent in 2016, which in turn was lower than 2015’s 47.0 percent. So far this year, batters are hitting ground balls at the lowest rate since 2011.

Now, it’s easy to understand why batters might want to avoid grounders. Per Baseball-Reference, batters are hitting .243 with a .265 slugging percentage on grounders. In contrast, they’re hitting .205 with a .642 slugging percentage on fly balls. That .642 slugging percentage is fueled largely by homers—fly balls are yielding an .087 batting average and .145 slugging percentage on non-homers—but that’s a little like saying wine tastes bland without grapes. More elevation yields more dingers.

This isn’t to say that a player can’t have a successful career hitting a lot of ground balls. Or that some batters might not be more successful if they hit more balls on the ground. But the trend is clear: Batters are hitting more balls in the air.

Pitchers, though, are trying to prevent that. On our player cards and in the BP Annual, we list pitcher ground-ball percentages, not fly-ball or line-drive percentages. And for good reason: So far this year, pitchers in the top 10 percent of ground-ball percentage have a combined 3.66 ERA. Those in the bottom 10 percent are at 5.07. As I pointed out last summer, pitchers who get a lot of ground balls are, generally speaking (there are always exceptions, like the fly ball-generating Max Scherzer), more successful than those pitchers who don’t.

So there are two opposing trends in 2017: Pitchers want batters to hit on the ground, and batters want to hit pitches in the air. Who’s winning?

This chart displays the change in ground-ball rate for all pitching staffs from 2016 to 2017. A positive number indicates pitchers who are getting more grounders than last year; a negative number means that the pitchers are yielding fewer. As you’d expect, given that grounders are down slightly this year, the general trend is negative.

Some teams have changed because of changes in personnel. Over the winter, I wrote that no team changed its offensive approach from 2015 to 2016 more than the Brewers. However, the Brewers also experienced a large change in personnel (only four players had 200 or more plate appearances for the club in both 2015 and 2016), so it’s an open question as to whether their hitters had a greatly decreased swing rates because of a deliberate effort or because of the guys wearing the uniform in 2016 compared to 2015.

Similarly, some of the teams exhibiting big swings in ground-ball rates in 2017 are ones that experienced turnover compared to last season. We shouldn’t be surprised that, for example, the Reds are inducing a lot more grounders in 2017 than 2016, since the team’s had only 10 games started by pitchers who started games for them last year. The pitching staff has changed.

Except the Tigers. If you ever want to run a baseball clinical trial for publication in a scientific journal, you want to choose the Tigers as your control group. The team never changes. The infield’s been James McCann, Miguel Cabrera, Ian Kinsler, Jose Iglesias, and Nick Castellanos for three straight seasons. J.D. Martinez plays right field and Victor Martinez is the designated hitter. There’s been a little more movement in the rotation, though not a lot. The bullpen’s a work in progress every year, it seems, but the five relievers who’ve appeared in the most games so far in 2017 were also with the club last year.

Entering the season, we projected the Tigers to have the least-changed personnel in the majors, by a wide margin. So it’s a little surprising that one of the most stable teams in the majors has yielded easily the biggest change in generating ground balls. Here are the ground-ball percentages for every Tigers pitcher with 10 or more innings this year. (They all pitched for Detroit last year as well.)

As you can see, everybody but Norris, Boyd, and Greene are getting fewer grounders this year than last.

Has it affected performance? Here’s a comparison of ERA.

Although almost as many Tigers pitchers have a lower ERA in 2017 (five) as higher (six), the team’s pitched worse overall. Detroit’s ERA/FIP/DRA in 2016 was 4.24/4.11/4.63 and it’s 4.74/4.50/4.62 this year. With fewer ground balls, Tigers pitchers have surrendered a home run every 28.7 plate appearances this year compared to one every 33.2 a year ago.

Look, ground balls don’t tell the whole story. The Twins are inducing a lot fewer ground balls this year than last (down 3.1 percent), and their pitching’s improved from puerile (5.09/4.54/4.97) all the way to mediocre (4.17/4.58/4.82). The Padres are getting more grounders (up 4.2 percent) and have the majors’ third-worst ERA, sixth-worst FIP, and second-worst DRA to show for it.

But the Tigers' pitchers are getting fewer ground balls, and their performance, in aggregate, has declined. And this being Detroit, it’s not a product of personnel turnover. It’s decreased effectiveness. The club entered Sunday with a record one game over .500, which in this year’s American League, put them just a game back in the Wild Card race. A return to something closer to last year’s ground-ball rates, by the same pitchers as the team had last year, could help bring them closer.