The fitness world has come a long way since the late 1980s, when Jane Fonda and Richard Simmons dominated the popular home-video market and StairMasters and Nautilus machines were newfangled attractions in burgeoning gym chains. Fat was irredeemably evil, the surgeon general had only recently issued a report on the dangers of secondhand smoke, and a certain guy named Tabata was still a decade away from inventing HIIT. In 1988, personal training was such a wildly new concept that the The New York Times actually launched an investigation into the curious rise of “highly disciplined, one-on-one workouts, not just for professional athletes and celebrities but for ordinary people who can afford them.” Meanwhile, the undisputed king of professional sports was Larry Bird, a pasty basketball player from Indiana who had a blond mullet, chewed tobacco, crushed cans of Bud—and, by 1986, had never seriously lifted a free weight in his life. At the time it didn’t matter. He not only guided his Boston Celtics to the NBA championship that June but also capped the season by winning his third consecutive MVP award. Then, the last month of ’86, something ominous occurred that affected his life and career:

He turned 30.

It’s an age that feels so inconsequential today. LeBron James is over 30. So are soccer star Cristiano Ronaldo and NFL quarterback Aaron Rodgers. Remember when the New England Patriots’ 38-year-old quarterback Tom Brady turned 30? Neither does he. Since then he’s won two league MVP awards and a Super Bowl and thrown 281 touchdown passes, which is more than Hall of Famer Joe Montana threw in his entire career. Forty-year-old San Antonio Spurs big man Tim Duncan turned 30 seven All-Star Game appearances and two NBA championships ago. And since the season after Florida Panthers 44-year-old (not a typo) winger Jaromir Jagr celebrated his 30th birthday, he’s played for seven NHL teams, scored 279 goals, and logged 14,774 minutes of ice time. (And that doesn’t even include the four and a half seasons he spent playing in Russia and the Czech Republic.)

But not long after Bird turned 30, he started missing games. His Achilles tendons became painfully tender. His back acted up. His body began to fall apart in front of sports fans’ eyes. On the court he grimaced with every twist and twinge; on the sideline he lay on his stomach to relieve the pain. In the locker room, he crammed his torso into a quarter-inch-thick fiberglass body brace. He tried to fight back from his injuries with immobilization and bed rest. (Both bad ideas, as we now know.) Before Bird’s final season, surgeons extracted a disk from his spinal column to relieve the pain but the operation didn’t help much. In 1992, at the age of 35, he announced his retirement at the Boston Garden.

“My back problems cost me at least three or four years [of my career],” Bird told reporters in 2012. “After I hurt it, I always played with spasms. I kept playing because I always wanted to play for one more championship. That was just me.”

To be sure, Bird wasn’t an outlier in his era. Michael Jordan retired—for the second (though not final) time—at age 35. Troy Aikman left the NFL at 34. Baseball star Don Mattingly bowed out at that age, too. The reality is that their retirements didn’t seem unusual—they were “old.” Yet if those guys were playing today, the timing of their exits would no doubt feel premature.

So what, specifically, has changed?

Any dedicated Men’s Fitness reader can give at least a partial answer: Today’s athletes are eating better, resting better, and training smarter than ever before. But that hardly explains why Vince Carter, just shy of 40, can dunk over guys 20 years younger than he is, or how Jagr keeps the team’s strength and conditioning coach at the gym past midnight.
 So what are the specific training techniques that athletes have adopted that have prolonged so many careers?
 With that question and more in mind, we launched a formal investigation into longevity training, reaching out to team doctors and sports scientists, academic physiologists and elite strength coaches—and, of course, the age-defying athletes themselves. Some of theses experts, like Marcus Elliott, M.D., director of the “applied sports science” firm Peak Performance Project, are at the bleeding-edge of forward-looking fitness technology. Others, like legendary NFL strength coach Johnny Parker—who, in the ’80s, tracked down Soviet coaches so he could pick their brains about plyometrics and periodization—are decidedly more old-school, but no less inquisitive.

New-school, old-school—they still agreed on the fundamentals. So we compiled their wisdom into six lessons, crucial techniques, and habits that have helped prolong the careers of aging stars.

Best of all, the lessons apply to you, too. Whether you’re a lifter, a runner, or an all-around sports nut, you’ll find something here to keep you performing at the very highest level. Because age, science and real-world results are now telling us, is just another number.