Rory McIlroy has the swing and the temperament of a champion.

Now he has the body to go with it.

As McIlroy rose in the past 4 years from skinny teenager (below) to U.S. Open champ to the world No. 1 ranking -- thanks to Sunday’s heart-pounding victory over Tiger Woods at the Honda Classic -- his body and bearing changed as well (right).

His secret: bringing modern science to bear on the ancient game, and working on his legs and core muscles so he wouldn’t mess too much with a classical swing that Jack Nicklaus calls “the most natural motion in the game today.”

That, plus some inspiration from his girlfriend, No. 2 ranked tennis star Caroline Wozniacki. McIlroy found the way professional tennis players practice almost humbling.

“They work so hard,” says McIlroy, who good-naturedly joined Caroline and Maria Sharapova on the court at Madison Square Garden during an exhibition match Monday night. “That sort of made me realize that I could probably work harder, and gave me a little bit more motivation to go in the gym and hit more balls. It’s definitely paid off.”

After more than 15 years of domination by the hard-sculpted, hard-swinging Woods, the golf world is accustomed to fit golfers. McIlroy, by comparison, presents a slightly more approachable image: 5-foot-10, 160 pounds -- but with muscles that only began appearing in early 2011.

His posture and physique changed under the direction of Steve McGregor, Ph.D., a British trainer who previously worked with the burly-yet-trim Lee Westwood, no. 3 in the world. (Speaking of great physiques, make sure to check out the 12 Sexiest Female Golfers.)

McIlroy made it clear in an interview with Men’s Health that his new body is the foundation for his success. For visual proof, look no further than the recent photos of McIlroy in the gym in this article.

"I feel a lot more stable in my golf swing,” McIlroy says. “There’s a lot less moving parts.”

His newfound strength allows him to swing powerfully yet stay balanced, with a finish that evokes the swings of Snead and Hogan.

In 2010, McIlroy was that familiar sight: a skinny-soft golfer, with obvious natural ability and flexibility -- but with a tiny bit of paunch and jiggle. To make the jump to superstar, he turned to McGregor. McIlroy’s body was wired up to sensors to measure muscle activation during his swing.

"That gave us a good idea of what muscles were working during the swing, what weren’t, what needed to fire more,” he says.

McIlroy wasn't interested in extra length -- he's been topping 300 yards for years. It's just that now he can do it without coming out of his shoes.

"I feel like I can hit it harder without losing balance," he told MH. “The length of my driver hasn’t increased that much but I just feel like I don’t have to go after it as much to get the length.”

He used to thrust his hips more at impact, “and I didn’t have a strong enough core or lower back and glutes to stabilize my pelvis."

McGregor’s first step was to balance the discrepancies between McIlroy’s strong right side and relatively weak left side. Workouts included lots of single-arm and single-leg work, including dumbbell presses and split squats. Much focus was put on the lower body -- the true source of power. (Want to get in peak shape yourself? Try The Ultimate Boot Camp Workout from Men's Health.

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"The lower-body work stabilized all the power that I had generated on the way down," he says. "I could generate the power but I needed the stability to hold onto it."

The data gathered by McGregor's work showed that "the maximum velocity of my clubhead was happening 12 inches before the ball, and then it would start to slow down," McIlroy says. "Whereas if I could hold onto that power the whole way through until impact, I'd hit the ball further."

By the time he reached Congressional Country Club last June and won the U.S. Open by a stunning 8 strokes, the change was evident. Combined with healthy eating -- lots of chicken and broccoli -- McIlroy dropped his body fat from 22 percent to 16 percent. (Burn fat anywhere with this Flab-Torching Kettlebell Exercise.)

“I’m the same weight as I was when I started, but I’ve gained muscle, I’ve gained mass,” he says.

McIlroy, still a fun-loving kid whose mates back in Holywood, Northern Ireland needle him when he stumbles, shows mature resolve. “To be a top-class athlete, you have to train hard, you have to eat right, you have to get enough rest,” he says. "I feel the way golf is going nowadays, you have to treat yourself as an athlete. Even though maybe golf isn’t one of the more physically demanding sports in the world, you’ve still got to do your bit and train like it is."

McIlroy admits lifting didn’t feel right at first. "I used to not really like going to the gym when I was playing tournaments because I’d be sore and stiff,” he says. "But the more you keep doing it, the less soreness you have. And you actually start to enjoy it."

He was walking tall and feeling strong. "People start to compliment you -- 'Oh, you're looking good.” Which is obviously good for your self-esteem."

As is holding off Tiger Woods on a Sunday and ascending to the top spot in the world rankings. McIlroy plays this week in the Cadillac Championship at Doral in Miami, then takes three weeks off to rest and prepare for the Masters, the one that got away from him last year.

But that was a different McIlroy.

Credit: Photographs courtesy of Oakley.

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