And it has nothing to do with his 100 mph fastball.
''A year ago you'd just see a kid bounding around here. It's amazing the transformation,'' St. Louis manager Mike Matheny said Monday. ''In general there's a whole different demeanor to him.''
The Cardinals are raving about Martinez's maturity this spring. He's focused on improving, and he's taking the little things seriously.
Matheny compared Martinez's maturation to that of fellow pitcher Lance Lynn a couple years ago.
''We just watched that transformation.'' Matheny said. ''It just takes time. It takes some struggles.''
Martinez arrived in Jupiter after pitching in winter ball, giving him a bit of a head start on some others in the Cardinals staff.
He enjoyed success in the Dominican Republic and thus far it's continuing in the spring.
Martinez had his second spring bullpen session Monday. Afterward, pitching coach Derek Lilliquest characterized his pitches as crisp and lively.
''You almost have to hold him back a little bit,'' Matheny said.
Martinez is only 23 and he is determined to break camp as part of the Cardinals' starting rotation.
Last year he arrived in South Florida looking to land the same role. Martinez performed admirably in four starts - a 1.76 ERA - but the Cardinals chose Joe Kelly as their fifth starter, relegating Martinez to Trevor Rosenthal's setup man for most of the season.
Martinez made seven starts among his 57 appearances last season, a year in which he was 2-4 with a 4.03 ERA.
''What I want to let everyone know is that I do have a little more experience, and with experience comes a little maturity,'' he said. ''I hope to build off of that.''
The Cardinals are once again adamant there is an open competition for that fifth starting spot, but the feeling around Jupiter is that this time the job is Martinez's to lose.
''He's going to be given a lot of opportunities to win that spot, but he has to earn it,'' general manager John Mozeliak said.
With Kelly in Boston following a midseason trade last year, Marco Gonzales and Jaime Garcia are Martinez's chief competitors for the spot in the rotation that will follow, in an order yet to be disclosed by Matheny, Adam Wainwright, Michael Wacha, John Lackey and Lynn.
Gonzales pitched well late last season, getting two wins in relief against the Dodgers in the NL Division Series, and could potentially make the team out of spring as a starter or reliever.
Garcia remains the wild card, with his durability being the main concern. He's rebounding from shoulder surgery that limited him to seven games last season.
But he isn't considered a candidate for the bullpen - the ability to work consecutive days is a concern for Matheny and Mozeliak, Garcia simply considers himself a starter.
If Garcia excels this spring, the Cardinals will have to decide whether five (or fewer) spring starts are enough to guarantee Garcia's durability.
''I feel really good,'' Garcia said. ''I'm really excited about 2015. I'm here to compete.''