BOSTON (AP) — Teammates have long stopped laughing at J.D. Martinez and his iPad.

Boston's All-Star slugger is serious about every swing he takes, even in batting practice, and he has the tape to prove it. Get to the park early enough for Red Sox BP, and you'll see a pair of iPads attached to tripods, one on each side of the batting cage. When the 30-year-old Martinez steps up, video coordinator Billy Broadbent will point a third iPad at the batter's box — Martinez's personal device, which he's used to record his practice cuts since 2015.

After batting practice, Martinez retreats to the clubhouse to review the footage, making sure every piece of his carefully crafted swing is in the right place.

"It makes me who I am," Martinez told The Associated Press. "I always tell everybody that I'm not a natural hitter. I'm a trained hitter."

His meticulous preparation amazes even the hardest-working major leaguers, and it's paid off. He signed a $110 million, five-year deal with the Red Sox in the offseason, a long way from his early career struggles with Houston, and he was named an All-Star for the third time this season while leading the majors with 29 homers and 80 RBIs.

After posting a meager .650 OPS in 86 games in 2013, Martinez went to the drawing board on his swing. He jokes that he took roughly 10 million flawed swings before overhauling his approach prior to a breakout 2014, and he's nervous those old habits might creep back in if he's not diligent.

"I had to teach myself the proper swing and the mechanics that I have to do," he said. "I have to stay on top of it because my body will find a way to let 10 million swings I've taken when I was a kid slowly come back in. So, I have to grind away to get out of the bad habits."

Thus the daily video reviews, though it wasn't always so easy to have his cuts captured.

Martinez was released by Houston during spring training in 2014 and landed with Detroit. He experimented with the iPad that season but was razzed and discouraged about it by Tigers veterans. He didn't make the iPad a staple of his routine until he stuck with the major league club the next season.

"In '14 I was too scared to do it with Detroit because I was too young, and it was an older team," he said. "In '15 I had more confidence."

So what does Martinez look for day after day when he's going over BP footage?

"Similarities," he said. "It's like the game; ever play the game matchup and what's different in this picture? You have two pictures side by side. That's pretty much what I do.

"Usually when I'm swinging well, feeling well and know that I'm in a good place, I just save them," he added. "Ones that when I'm feeling crappy, I just delete them. Then I have a reference point for the future."

Martinez hit .302 with 28 homers and 65 RBIs in 62 games with Arizona last season after a midyear trade from Detroit. His short stay made a major impression with the Diamondbacks.

"I learned a lot from him just in the two months we were together," said Arizona slugger Paul Goldschmidt, also known around the league for his thorough preparation. "He's just so dedicated; as prepared as anyone is out there on his own swing, on the way pitchers are going to pitch, and just really committed."

Arizona manager Torey Lovullo said the club still borrows from Martinez's routine.

"J.D., his work habits were really unbelievable," he said. "Once he got to the batting cage, it was very specific with one particular detail he was working on. That's what we've inherited this year with all of our hitters."

Fellow Boston All-Star Mookie Betts has also taken a keen interest in Martinez's habits. The pair regularly talk hitting around the batting cage, with Betts eager to figure out how Martinez generates so much opposite-field power.

"That's why he's one of a kind," Betts said. "But he can hit it out of any part of the park, too."

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Freelancer Jose M. Romero in Arizona contributed to this story.

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