MLB: Expect the Home Run Surge to Continue in 2017

Major League Baseball has seen a significant home run surge over the last year-and-a-half, and spring training stats suggest it will continue this season.

It’s been almost 20 years since the great home run race of 1998. That was the year both Mark McGwire and Sammy Sosa shattered the single-season home run record set by Roger Maris in 1961. McGwire hit 70 and Sosa had 66, both easily eclipsing the 61 home runs that Roger Maris hit 37 years before. Over in the American League, Ken Griffey, Jr. hit 56 big flies.

McGwire and Sosa were at it again in 1999, hitting 65 and 63 home runs, respectively. MLB hitters combined for 5,528 long balls that year, setting the record for homers in a season that had been set the previous year. The record fell once again a year later, when MLB hitters connected for 5,692 home runs, still the most in major league history.

That 2000 season was the apex of this home run era. Over the next dozen-plus years, home run totals would gradually decline. They would bottom out in 2014, when MLB had its lowest home run total since 1995.

Then something strange happened in 2015. The first half was uneventful as far as home runs were concerned. Using the metric “Home Runs Per Balls In Play” to account for the increasing rate of strikeouts over the last decade, the first half of the 2015 season had a similar rate of home runs per balls in play as before the low-homer 2014 season. Home runs were up from the previous year, but it was nothing unusual compared to 2010, 2011, or 2013.

The second half of the 2015 season was a different story. Suddenly, balls were flying out of the park at rates not seen since the turn of the millennium, back when Barry Bonds hit a single-season record 73 no-doubters in 2001. Something strange happened that summer.

Consider the numbers: In 2013, MLB hitters had 17.8 home runs per 500 balls in play. In the low-homer 2014 season, this dropped to 16.2 home runs per 500 balls in play. The first half of the 2015 season saw a return to the 17.8 home runs per 500 balls in play that we had seen in the 2013 season. It was business as usual. Then, for reasons as yet unexplained, the second half of the 2015 season had a surge to 20.4 home runs per 500 balls in play. Home runs had not been hit at a rate that high since the peak of the home run era (2000).

The big blasts continued last year. While hitters fell short of setting the all-time record for home runs hit in a season, they did set the record for home runs per balls in play because the strikeout rate is so much higher than it was 16 years ago. In the 2000 season, hitters slammed 5,692 long balls and struck out 16.5 percent of the time. Last year, they hit 5,610 home runs, but struck out 21.1 percent of the time. That slight difference in home runs hit and the big difference in strikeout rate meant that the rate of balls in play that landed over the fence was the highest in baseball history.

There have been numerous attempts to explain this recent surge in home runs. As usual, MLB insists nothing has changed with the ball. In a New York Times article published last July, Mike Teevan, a spokesman for MLB, said the league does “extensive reviews of the performance of the baseball and there have been no differences” to explain the increase in homers.

In that same article, Chicago Cubs pitcher Jon Lester pointed out that hitting coaches are advocating for more players to hit the ball in the air. He said, “There’s no slug on the ground. Guys are willing to take their punch-outs to hit the ball in the air, and swing hard in case they hit it.” Strikeouts have definitely increased. In fact, the whiff rate has increased every year since 2005.

The idea that a change in approach by hitters is fueling the surge in home runs has become more popular as players like Josh Donaldson and Kris Bryant advocate hitting the ball in the air as much as possible, but it’s by no means an airtight explanation. For one thing, while the fly ball rate increased from 33.8 percent in 2015 to 34.6 percent in 2016, it was 34.4 percent in the low-homer year of 2014. It’s not so much the number of fly balls being hit, it’s the number of fly balls that go for home runs that has increased. That’s why I used the metric “home runs per 500 balls in play” earlier.

An article at last summer hinted at changes to the ball being the culprit, but without definitively laying the blame there. They did interview a number of pitchers who were suspicious, including Orioles reliever Zach Britton, who said, “I know MLB wanted to get more offense in the game, so can you do that without changing a strike zone or something in general? You can somehow change the cork maybe.” Naturally, the hitters they quoted did not agree that the ball has been altered. They prefer to think that it’s the result of a change in approach. Color me skeptical of this claim.

Something definitely changed in the middle of the 2015 season that resulted in an increase in home runs on batted balls. We may never know exactly what happened in the past, but we can wonder what will happen in the future. As we look toward the 2017 season, it’s natural to question if this home run surge will continue.

Since 2007, the rate of home runs per balls in play has bounced around a bit. It increased from 2008 to 2009, dropped in 2010 and again in 2011, increased in 2012, then dropped for two straight years to the low point of 2014. It’s increased in each of the last two seasons. That’s a bit of a roller-coaster ride. Where do we go from here?

Most of the time in baseball it’s important to keep in mind the concept of regression to the mean. If a player or team has a very good year, they are more likely to regress back to the pack the following year. If a player or team has a very bad year, they are likely to be better the next year.

For example, Mark Trumbo hit a major league-leading 47 home runs last year. His previous career high was 34, set in 2013. Mark Trumbo is highly unlikely to hit 47 or more home runs in 2017. He will regress back to the pack. He’s projected to hit 32 home runs this year, which is reasonable considering his pre-2016 career.

Normally, we would expect similar regression in the league-wide home run rate. There were 382 more home runs hit in 2012 than 2011, then 273 fewer home runs hit the next year. Home runs went up and back down. Home runs dropped again in 2014 but rebounded in 2015. Down and back up. After two straight years with an increase in home runs, it normally wouldn’t be surprising to see a decrease in 2017.

Except, there’s another piece of information to take into consideration that suggests we could see another record-setting year in home runs per balls in play. I tracked the spring training rate of home runs per 500 balls in play going back to 2006 and compared it to the regular season rate of home runs per 500 balls in play (I used 500 balls in play so we don’t have to deal with too many decimal places). They match up fairly well (0.73 correlation). In every season but one, there were slightly more home runs per 500 balls in play during the regular season than during spring training. When the spring training rate goes up significantly, the regular season rate goes up significantly. I tweeted about it here:

Over the last two seasons, there has been a difference between spring training home runs per balls in play and regular season home runs per balls in play of a little more than 2 percent. Over the last 11 years, the average difference is 1.6 percent. As the chart shows, the spring training rate is up again this season, which suggests that the regular season rate will rise or at least stay close to the same level as last year. Based on what we’ve seen in spring training, the recent home run surge will continue, so take your glove with you when you go to a ballgame because there will be plenty of souvenirs landing in the bleachers again this year.

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