What summer vacation?

The transformation began in mid-June, when Corey Gatewood arrived in Palo Alto, Calif., to get a jump on his freshman year at Stanford University.

Sure, Gatewood -- a 19-year-old Roslindale native and three-sport star at the private Belmont Hill School -- was already a fine-tuned athlete and student who had earned all kinds of accolades on the football field and, according to coaches, the academic résumé that gave him the choice of any school in the country.

The changes, physical and mental, are already obvious. Gatewood has gained 10 pounds of muscle as he builds the body of a Division 1 Pacific-10 Conference cornerback. He has also been adjusting to campus life while studying calculus, English, and new Stanford coach Jim Harbaugh's defensive playbook.

And since the official start of training camp two weeks ago, football has been nearly a full-time job. With Gatewood still finishing two classes, his days have been packed solid, full of workouts, meetings with special teams and defensive coaches, and sometimes two practices per day.

"It's a little different than high school. Everything's faster, everyone's stronger," said Gatewood, who called training camp "cutthroat. . . . Everything is hard, even in the stretching lines and warming up."

But Gatewood was as prepared as he could be, after taking part in Stanford's optional summer program for freshmen, working out four days a week in June and July with strength coaches and teammates.

"There's a tremendous difference between coming to campus and competing with your teammates and being coached by me than there is staying at home working out with your buddies," said Shannon Turley, Stanford football's strength and conditioning coach. "If you want to come in and compete for an opportunity to play as a freshman, you need every advantage, because you're playing catch-up."

That's the predominant theory among Division 1 coaches, many of whom have implemented or modified extensive, sport-specific summer fitness plans for their recruits in recent years, on and off campus.

At Stanford, where Turley arrived last winter along with Harbaugh, a former NFL quarterback, the staff quickly compiled a DVD that it sent to recruits after they signed national letters of intent in February, recommending workout routines and nutritional habits.

"Over the last 10 years, I think the whole strength and conditioning paradigm has really taken precedence, not just in collegiate, but professional and even some youth programs now," said Joe Colotti, a strength and conditioning specialist who works out of Peabody.

Gatewood hopes to put all that preparation to work when the Stanford Cardinal squad opens the season Sept. 1 against UCLA. The path to Palo Alto wasn't quite so clear, however. In February 2006 he had committed to Boston College, as did a Belmont Hill teammate and friend, Dan Williams, a Cambridge native.

But when BC head coach Tom O'Brien left for North Carolina State, Gatewood switched directions, as he does so deftly on the field.

"I felt real comfortable with the previous coaching staff," he said, "and I wasn't really sure how the program was going to change."

His mother, Faye Gatewood, said, "I was never really in on the decision with Boston College. I felt like he should weigh all his options. . . . As a child, he always said, 'Well, I'm going to go to Stanford.' "

Gatewood committed there in January -- Williams stayed with BC -- and though he got the DVD in February, he didn't get started. That's because Gatewood also played basketball and ran track for Belmont Hill, and Stanford's coaches encourage athletes to stick with their in-season regimens.

That changed when he switched coasts in June -- a decision that isn't mandatory so early, but recommended by Stanford and embraced by Gatewood.

"Corey's always prepared himself for what's next," his mother said. "You can call on him, and he will be ready."

As a first step, Turley put Gatewood through what's called a "functional movement screen," a process designed by Virginia-based physical therapist Gray Cook to detect any physical imbalances that could lead to injury.

Since Gatewood had a history of hamstring injuries, Turley corrected his workout techniques and then got him started on a position-specific training program. Though Gatewood was recruited by many schools as a wide receiver, he's slated to be a defensive back at Stanford; as a result, he has been working on backpedaling, starting and stopping quickly, and changing direction, repeating those movements and stressing those specific muscle groups in the weight room.

"It's pretty detailed and intricate, and everything we do is very specific," said Turley, who can spend just eight hours per week with his players, per NCAA regulations, before training camp starts. That time includes everything from lifting to sprinting drills to reactive agility drills, where offensive players compete against the defense.

Gatewood -- who has bulked up to 185 pounds on his 5-foot-11-inch frame -- was no stranger to the workout room during his high school days.

"Corey realized after sophomore year that to play at the level that he wanted to play at, he had to make a commitment in the weight room," said Belmont Hill coach Kevin Fleming, who deflected credit to Gatewood and the school's trainer, Ed Doherty. "He was as ready as he could be. He knew what he had to do physically."

Still, Stanford is a long way from home. Gatewood was born in Roslindale, and Faye Gatewood -- who was up late one night last week fielding a call from her son and looking up flights to the Bay Area -- said his athletic prowess was apparent when he was just 8 years old, playing Pop Warner football. At the time, he attended Agassiz Elementary School.

He spent sixth grade at the Academy of the Pacific Rim in Hyde Park, where he was a top student. But the charter school had no athletic programs, so his mother urged him to look elsewhere. After she convinced him that going to an all-boys school wouldn't be so bad, he enrolled, with full financial aid, at Belmont Hill.

"She was right, obviously," he said last year.

He said Belmont Hill prepared him for both the academic rigors and the on-field challenges this summer. And though redshirting -- which limits a player to practicing with the team but saves a year of eligibility -- is a strong possibility for most freshmen, Gatewood has one thing on his mind: playing football.

"He's definitely a player who has acclimated himself quickly," Turley said. "He's shown signs immediately in camp that he's a guy who is competing for playing time."

"The most important thing for me is just getting on the field," Gatewood said. "It doesn't matter where I am, as long as I'm playing."

Odds are, the transformed Corey Gatewood is ready.

Mike Lipka can be reached at mlipka@globe.com.