J.D. Martinez always hits the ball hard. It's basically his brand at this point. Since extensive work on his swing in 2014 turned him from Astros castoff into a superstar for the Tigers, D-backs and now Red Sox, no qualified hitter has a slugging percentage higher than his .583. Only six have more homers than his 157. This is why Martinez earned $110 million from Boston last offseason. It's because he crushes baseballs like few others.
Even with all of that as a backstory, 2018 has been something else entirely. Martinez is hitting .321/.386/.629, the best line of his career (and the fourth-best line in the Major Leagues). He's hitting the ball harder than before, he's hitting it hard more often and he's doing it at a rate that puts him in company with the game's elite.
Major League hard-hit rate leaders in 2018 (minimum 150 batted balls)
57.4 percent -- Kendrys Morales, Blue Jays
56.9 percent -- Aaron Judge, Yankees
55.9 percent -- Martinez, Red Sox
54.8 percent -- Matt Olson, A's
52.4 percent -- Christian Yelich, Brewers
51.5 percent -- Nelson Cruz, Mariners
50.7 percent -- Mookie Betts, Red Sox
50.4 percent -- Giancarlo Stanton, Yankees
49.7 percent -- Joey Gallo, Rangers
49.1 percent -- Jackie Bradley Jr., Red Sox
Major League average -- 36.5 percent
You probably don't need to be told that it's good to hit the ball hard; just look at those names. The bottom of the list is comprised of light-hitting speedsters Dee Gordon (14.3 percent), Delino DeShields (12.6) and Billy Hamilton (9.2). The ability to actually make contact is a pretty important component of this, obviously. But when you do make contact, you want to hit it hard. Obviously.
We can't really overemphasize that point. We define a "hard-hit" ball as one that is struck with at least 95 mph of exit velocity, and that's not a number that's chosen by accident. It's because that's where how hard you hit the ball actually starts to matter, with the idea being that it really doesn't matter all that much if you hit the ball 40 mph or 60 mph or 80 mph -- they're all varieties of softly-hit balls unlikely to cause damage, other than to your hitting line.

That's part of the reason that "average exit velocity" has its flaws, as we investigated with Didi Gregoriusearlier this year, and the massive difference in production on hard-hit balls is shown extremely clearly in the production you'll see on balls hit 95 mph and those that are not.
MLB hard-hit balls (95-plus mph) in 2018
.521 batting average, 1.036 slugging percentage
MLB balls that are hit below 95 mph in 2018
.218 batting average, .256 slugging percentage
Nearly 97 percent of home runs and over 80 percent of extra-base hits this year qualify as "hard-hit." Again: Of course you want to hit it hard. This isn't exactly a new idea, nor should anyone think it is. It's just that now, we have better tools to measure these things, and we know that it's not a skill that every player has.

It's not like this is new for Martinez, either. From 2015-17, his 48.1 percent hard-hit rate was top 10, among the 397 players with 300 balls in play. But what is new is that in Martinez's first year in Boston, he's managed to outdo even his own past great performance. After three years of relatively steady (and elite performance), his hard-hit rate has gone up.
Martinez's hard-hit rate from 2015-18
2015 -- 48.7 percent
2016 -- 46.9 percent
2017 -- 48.7 percent
2018 -- 55.9 percent
At the same time, his strikeout rate has gone down.
Martinez's strikeout rate from 2015-18
2015 -- 27.1 percent
2016 -- 24.8 percent
2017 -- 26.2 percent
2018 -- 23.5 percent
So Martinez is putting more balls in play, and he's hitting them harder. This isn't really rocket science; it's what every hitter would like to do.
Beyond that, Martinez is also hitting them hard in the right direction, which is to say that while every ball hit 95 mph or harder counts as being "hard-hit," they don't all lead to the same amount of production. A hard-hit ball on the ground is possibly still an out. A hard-hit ball in the air can lead to a massive amount of production.

It's for that reason (among others) that Martinez is outproducing the only man atop him on the hard-hit list, Toronto's Morales. While Morales has hit 41 percent of his hard-hit balls on the ground (and hit only .209 on them, in part due to his lack of speed), Martinez has hit only 34 percent of his hard-hit balls on the ground -- and hit .510 on them.
Because Martinez hits it hard, and hits it hard in the air, he leads the Majors with 51 Barrels, a Statcast™ metric that is intended to show the perfect combination of exit velocity and launch angle. Only 6 percent of batted balls this year have been Barrels, which show how rare they are, and the Majors hit .757 with a 2.546 slugging percentage on them, which shows you how valuable they are. The more you can compile, the better. No one has more than Martinez, by a gap of eight over teammate Betts.
It all comes back to hitting the ball hard. It's not a new idea. It's easy to understand. It's just something we can quantify now, and it's something Martinez is proving he excels in. Perhaps the 31 homers or the outstanding slugging percentage tell you that by themselves, but now there's science behind the numbers. Martinez is one of baseball's elite sluggers. The hard-hit rate tells the story.