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 replacement 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, 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):

Late-blooming power hitters Largest difference in isolated power (ISO) between appearances through age 25 and age 26-29, 1901-2017 THROUGH AGE 25 AGE 26-29 PLAYER ISO WRC+ ISO WRC+ ISO DIFF. Rogers Hornsby .162 162.6 .300 203.5 +.138 J.D. Martinez .136 87.3 .265 145.8 +.129 Todd Hundley .136 67.0 .251 128.1 +.115 Jeff Bagwell .170 138.4 .284 169.1 +.115 Kirby Puckett .071 86.0 .182 138.3 +.111 George Foster .148 98.0 .258 151.3 +.110 David Ortiz .182 101.5 .288 143.9 +.106 Duke Snider .203 124.0 .306 163.8 +.103 Gorman Thomas .162 81.8 .264 129.5 +.101 Albert Belle .222 118.0 .323 159.9 +.100 Gil Hodges .147 96.4 .244 135.9 +.098 Damion Easley .092 72.9 .190 105.5 +.098 George Sisler .096 136.7 .192 160.0 +.096 Joe Adcock .152 97.5 .246 138.5 +.094 Tony Armas .127 73.6 .219 105.1 +.092 Harry Heilmann .125 118.3 .216 165.4 +.092 Sammy Sosa .183 96.7 .273 123.5 +.090 Andy Pafko .114 111.7 .203 132.2 +.089 Ken Griffey Jr. .234 141.9 .321 146.0 +.087 Ivan Rodriguez .149 97.8 .235 127.7 +.086 For players with at least 750 plate appearances through age 25 and at least 1,500 from age 26 to 29. Sources: FanGraphs, Baseball-Reference.com

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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