J.D. Martinez’s Path From Slap-Hitter To Four-Homer Sensation

In becoming the 18th major-league player to hit four homers in a game (and the second player this season), Arizona Diamondbacks outfielder J.D. Martinez did something rarer than a perfect game. But in accomplishing the feat, Martinez has also become the poster boy of one baseball’s biggest trends: the overnight fly ball-smashing sensation.

Martinez began his career as a slap-hitting prospect for the Houston Astros — and to be charitable, he was not successful at it. Over his first three MLB seasons, Martinez had an adjusted on-base plus slugging that was 12 percent worse than average, accumulated negative 1.3 wins above replacement⁸ and hit for very little power, belting just 24 home runs in 252 games. By 2014, Martinez was struggling so much that Houston released him.

“I’m doing everything the coaches tell me,” Martinez told Travis Sawchik of FanGraphs in March about that period of his career. “I’m swinging down on the ball. In BP, I’m hitting low line drives everywhere. In games, it doesn’t play.”

But the Detroit Tigers saw potential in Martinez, snapping him up a few days after his release. And Martinez repaid their faith by turning his career around, with the help of a new philosophy at the plate. He became one of baseball’s earliest fly-ball revolutionaries, reducing his ratio of ground balls to fly balls from 0.87 in his first three seasons to 0.64 in the seasons since. And the results were striking. Since 1901,⁹ only Hall of Fame second baseman Rogers Hornsby increased his isolated power — which is slugging percentage minus batting average — more from his career average through age 25 to his average between the ages of 26 and 29 (Martinez was coming off his age-25 season when the Astros waived him):

Martinez isn’t the only recent player to go on an out-of-nowhere power spree — Jose Bautista, for instance, went from a light-hitting utility man early in his career to a fearsome, bat-flipping homer machine as he approached his 30s. Nor is Martinez the only exemplar of the fly-ball phenomenon sweeping across the game; from Daniel Murphy to Yonder Alonso, plenty of players have given their careers new life by way of an uppercut swing.

But Martinez might be the best of the bunch.

Because of his newfound affinity for fly balls, Martinez — who landed in Arizona via a midseason trade — has remade himself into one of the game’s most dangerous hitters. Over the past four seasons, Martinez ranks as the eighth-best batter in all of baseball according to Weighted Runs Created Plus, which measures how many runs a player generates per plate appearance.

Early in his career, it would have been a pleasant surprise if Martinez had hit four home runs in a month. But after Martinez modified his approach, Monday’s accomplishment is just the latest signpost along his road to stardom. And with the red-hot Diamondbacks practically assured of making the playoffs, a national audience will have a chance to get acquainted with Martinez’s power stroke this fall.

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