Kelly Starrett looks more like an Olympic weightlifter than your typical physical therapist. At 6'2" and 227 pounds, he’s thick, imposing, and covered in a hodgepodge of tattoos that stream down his left arm and cover his bulging calves. So it’s fitting that when this doctor of physical therapy launches into a sermon about athletic performance, he skips the anatomy lesson and uses race cars to make his point.

You can drive a Beemer around a racetrack in second gear at 60 miles per hour, but you can’t do it forever, he says. Not only are you undermining performance, but you’re also priming the car for catastrophe. Starrett says this is what we do with our bodies on a daily basis: We sit all day, shortening our hip flexors, hamstrings, and calf muscles. Then we hit the gym with dysregulated tissue and complain when we get knee pain. It’s like we’re driving around with the emergency brake on, and eventually we blow out a knee or a hip. “So we need to fix our basic spinal position before we go after poor movement in the hips or shoulders,” he says. “You’ll never fix the big engine if the chassis’s broken.”

His race car analogy isn’t a groundbreaking observation, to be sure, but simple nuggets of wisdom like these, delivered in his trademark upbeat and accessible way, have made Starrett one of the most successful and sought-after trainers in all of fitness. He’s a celebrity thriving within the world of CrossFit, the hardcore mishmash of training styles that now encompasses nearly 10,000 gyms across the world. Few amateur gymgoers, one could argue, need a stronger chassis than CrossFitters.

If there’s any criticism continually aimed at CrossFit and its high-intensity workouts (like, say, multiple rounds of wall-ball shots, sumo deadlifts, box jumps, push presses, and reps at a rowing machine—WOD name “Fight Gone Bad") it’s the notion that they’re not for the average person—at least until that person is properly conditioned. In April, the University of Rochester School of Medicine and Dentistry published a study that found that 19% of a group of 486 CrossFitters sustained an injury over a four-month period, mostly in the back, shoulders, or knees.

“With CrossFit, you’re training and working out really hard,” says Zach Even-Esh, a respected strength and performance coach and the founder of New Jersey’s Underground Strength Gym. “And when you push yourself—whether in CrossFit or something else—you get bumps and bruises. But people get injured from sitting too much and eating crappy as well. Once you get injured, you go to a doctor, you get painkillers, and you’re told to stop working out. Kelly’s become the guy who does the opposite. He fixes people.”

Starrett doesn’t deny that people get injured doing CrossFit. But he argues that the injuries pale in comparison with the ones sustained in other daily activities. “If you take a holistic view of sports, CrossFit is safe,” he says. “Eighty percent of runners get injured every year—and there are 30 million runners in America.” He attributes any rise in CrossFit injuries to the greater numbers of people doing CrossFit. “What I teach is proper positioning. Once people have good biomechanics and good motor control, then we increase intensity. The problem is, just as in any other sport, moving at high intensity without proper biomechanics is a recipe for disaster.”

CrossFitters aren’t the only ones flocking to Starrett for instruction. His client list spans the entire big-name pro-athlete spectrum: Olympians, English Premier League soccer stars, guys in the NBA, MLB, and NFL, as well as world-class cyclists, endurance athletes, and classical ballerinas. He’s even trained breakdancers. They all come to Starrett to get his distinctive take on the fundamentals of mobility—he’ll teach a world-class sprinter the proper way to stand, for instance—and to master his singular methods for eradicating physical pain.

Lately, it’s been harder and harder for Starrett himself to float under the radar. He’s a bestselling author, and his website, mobilitywod.com, has had more than 5 million unique users since launching in 2011. “People in CrossFit like to suffer together—it’s like in military units,” says Even-Esh. “There’s something about Kelly’s aura that connects with a lot of people, and inspires them. It’s a blend of intensity, enthusiasm, and wisdom. The CrossFit community has created superstars. It’s like big-time music fans who went crazy when they saw Michael Jackson. It’s that way with Kelly.”

Last May he visited Seoul, South Korea, to visit local CrossFit boxes and to lecture at the Reebok headquarters there (Reebok is CrossFit’s official sponsor) about devising a new kind of office environment that’s better for the body. While there, he estimates, he took nearly 2,000 selfies with local CrossFitters and coaches. It’s all happened so fast.

“It’s so cool to see this performance revolution capture the imagination of the world,” says Starrett. “Four years ago, I posted my first YouTube video. I didn’t even tell my wife. And it turned into all this.”

You’d never guess it from Starrett’s California accent, but he grew up in Garmisch, Germany, in the Bavarian Alps, where he fell in love with Alpine ski racing and kayaking.

“That was before athletes started specializing in sports,” he says. “Everyone we looked up to did everything—skied, played soccer, kayaked.” Starrett’s mother, however, was an American professor, and moved the family back to the States in time for Starrett to attend high school here.

He went on to study at the University of Colorado, where he became obsessed with extreme kayaking and whitewater rafting. When you hear him talk about injury treatments and he says something like, “Grab an ice cube out of your gin and tonic—gin is paleo, right?—and apply it to your elbow,” it’s easy to imagine his previous life on the river. Starrett competed around the globe, paddling in more than a hundred class-V rapids, and won two national championships on the U.S. team. He also spent a lot of time guiding tourists on dangerous runs. There, he says, he honed his CrossFit instructor skills. Both roles required him to be funny and personable one second and turn drill-sergeant tough the next.

During his last few whitewater seasons, Starrett experienced two things that would forever change his life: He met his future wife, Juliet, a world champion paddler, and he developed a repetitive-use injury.

For years, Starrett had spent nearly every day paddling on the right side of a canoe. Eventually his right hand went numb. He couldn’t turn his head more than five degrees. He tried to fix the pain with massages, cortisone shots, and prednisone treatments, but nothing worked.

CrossFit has created superstars. It's like music fans who went crazy when they saw Michael Jackson. It's that way with Kelly Starrett.

“I went down the rabbit hole,” he says. “I asked the doctors why this had happened, and they were like, ‘This always happens.’ I was like, ‘What the fuck? You knew my hand was going to go numb?’”

After Starrett got injured while on the national canoe/kayak team, he became a rep for Necky Kayaks. His region was Montana to Texas, and he lived out of his truck with his dog. Being a river rat was acceptable when he was in his 20s, but the time was fast approaching when having no health insurance and being oversunned would provide diminishing returns. Juliet started law school in Oakland in 2000, and Starrett joined her there and attended Samuel Merritt College, where he earned his doctorate in physical therapy.

Over time, he meticulously fought the pain and numbness in his right arm. Then he became interested in Olympic lifting and moving better. When he learned about a new workout that strength coach Greg Glassman was developing—which was the fledgling CrossFit—he got hooked. Soon after, he and Juliet opened one of the nation’s first CrossFit boxes.

“We took our interest in risk taking and applied it to starting a business,” he says. “Taking risks is what I’ve been doing since I started kayaking at age 12, and I’ve been fucking scared—a lot.” During this period Starrett started shaping his views about strength and conditioning. “I started physio school and CrossFit simultaneously. I saw that when people were put in positions to work out more efficiently, these were also the positions that were the most sound for injury prevention,” he says.

In light of his experience regaining the mobility in his arm, Starrett believes the regular athlete shouldn’t even jog or ride a bike without proper form, that you need to fix your range of motion before you destroy yourself. “Practice doesn’t make perfect,” he says, “practice makes permanent. [Former cyclist] Floyd Landis rode until he rode his hip out of the socket—what the fuck?” he says, incredulous.

Starrett worked with Team RadioShack and their performance coach Allen Lim in 2010 to prepare them for the Tour de France.

Starrett believes that CrossFit will be the first sport to not blow out athletes like old cars. “Your tissue is designed to last 110 years,” he says, and trainers need to understand this. But rather than studying proper form, they focus too much on sport-specific training or building show-off muscles. Instead, Starrett argues, it’s important to emphasize movement patterns that heal the body. He teaches clients to diagnose themselves, so when they’re at home hey can release the tension built up in their bodies rom sitting all day. “Everyone should be able to perform their own basic maintenance,” he says.

When Starrett started writing articles for the CrossFit Journal, the sport’s website of record, he realized people were simply not knowledgeable about their own bodies. “You don’t know where your psoas is?” he asks. “Well, that’s the fucking problem.” (For the record, it's a hip flexor that starts at the bottom of the rib cage and extends down to the inner pelvis—“the filet mignon of a human being,” Starrett says.) He focused on teaching people the basics and started improving their posture and gait before he even let them look at a barbell. He believes there are many more similarities between a Cy Young winner and a stockbroker than there are differences.

“One percent of orthopedic problems are related to acute injuries from sport, another one percent are from diseases like bone cancer, and the rest is lifestyle,” Starrett says. “When we started going after pain, people thought it was a Ponzi scheme. They thought when you’re 55, you’re supposed to be in pain. That’s crazy. Pain is a lagging indicator of injury. Ninety-eight percent of orthopedic surgeries are preventable.”

In 2010, Starrett came up with the idea of filming a short video focused on mobility every day for a year.

“The thought is, if you spend 10 minutes a day on yourself, you can improve things,” he says. “We know people don’t have the time to do this for an hour. We just wanted to send them in the right direction.”

Starrett’s first video instructed CrossFitters in the proper mechanics of the squat. He explained that, in countries where people still squat regularly to eat, talk, or, he says bluntly, shit, there are few hip and lower-back problems. So he simply asked people to squat for a full 10 minutes. “I was like, ‘Pretend you’re at a campfire, that you’re having dinner in Thailand, and just be human,’” he says. “People were blown away.”

Just as CrossFitters use the acronym WOD for “Workout of the Day,” Starrett decided to call his lessons MobilityWODs. It wasn’t the flashiest name, but it stuck. By 2014, his site had upward of 5 million unique users. “I thought we’d do it for a year, and we’d cover everything and be done,” he says. “That was four years ago.”

Last year he published his first book, Becoming a Supple Leopard, based on the premise that at any moment a leopard is ready to pounce and fight for its life. It doesn’t need to stretch its hamstrings or warm up. Humans, he argues, used to be the same. Most CrossFit boxes keep a copy on hand as a sort of handbook or bible of movement. “What we’re trying to say is: Are you a skilled human? Yes or no?” Starrett says. The book—again, a $60, four-pound physiology tome on joint mechanics—is a New York Times best seller.

Many CrossFitters wear a visit by Starrett to their CrossFit box like a badge of honor. In June, 60 Minutes took notice, and profiled him for their sports spin-off on Showtime. The news crew followed Starrett to England to watch him work with the Arsenal soccer club and to New Orleans to help the NFL’s Saints. Starrett’s fans argue his work resonates with them since what he’s saying is universal. And his fundamental belief is a rosy one. Simply put: It’s not natural to be in a state of pain.

According to Starrett, everyone should master the basics of human movement: standing, squatting, and doing pushups, dips, pullups, and deadlifts. You might think that standing, for example, is pretty simple; but according to Starrett, it’s not.

“Standing,” he says, “is a skill.” You should “screw your feet into the ground” to create a stable hip position. To accomplish that, he directs you to stand with your toes pointing directly forward, then slightly spin your feet outward as though you’re literally screwing them into the ground.

Starrett also has proper positions for computer use and even texting. Both involve externally rotated shoulders, an erect spine, and a neutral head position. Speaking of your upper body: There’s a term he uses for what you get when you sit too long and hunch—“DB shoulders,” which stands for douchebag. In one memorable MobilityWOD, Starrett slaps the phone out of the hand of an athlete who happens to be texting with DB shoulders. “I don’t care if you’re the Twitter god,” he scolded. “You’re disrespecting yourself, son.”

Starrett’s motto: “Test, retest, and share.” And if you’ve ever been told to roll around on a lacrosse ball to loosen up stiff tissue in your back or hips, you’ve been part of his DIY treatment movement. Starrett started promoting the use of this simple device years ago to release trigger points where tissue gets bound up and causes restriction.

“If you lay on a lacrosse ball anywhere on your body and it hurts, that’s not normal,” Starrett says. Rolling it over your muscles with pressure from head to toe—back, legs, feet, chest—will reveal how tight and compressed your musculature has become. “If you lay on your stomach and put the ball into your quad and you stop breathing,” he adds, “you’re over-restricted. You should be able to smash the crap out of yourself and be pain-free.”

Most people’s pain, he says, is this myofascial pain, which can be relieved with 10–15 minutes of soft tissue release each day. Find a spot where it hurts and keep manipulating until—much like the way people use foam rollers—you feel the tissue decompress.

Another mandatory lesson you learn in Starrett 101 is rooted in diaphragm mechanics. “We spend a lot of time teaching people how to breathe,” he says. For this he uses a squishy softball to relax the tissue of the diaphragm. “People are so shocked to learn how packed down their abs are and how stiff their diaphragm is,” he says. Starrett believes that doing 10 minutes of gut smashing, literally rolling over a softball with your abdomen, before bed helps A-types sleep.

“Oftentimes the nervous system is the limiting factor in human performance and in sleep and recovery,” he says. “We see people with wretched sleep patterns and we have them do this gut smashing, and it fixes their sleep cycle.” As with the lacrosse ball, you lay on the softball and move around as you feel the muscles of your stomach relax. “Anyone who’s ever had a massage can relate to this,” he says. “How do you feel after a massage? Sleepy.”

Starrett recently launched his own line of mobility devices with Rogue Fitness, a Detroit barbell manufacturer extremely popular with CrossFit. Together they’ve launched a device called the Gemini—a high-end version of a lacrosse ball—and a bigger roller called the Supernova, which Starrett recommends for loosening up your diaphragm to improve breathing and sleep.

Starrett will also release his next book, Ready to Run, next month. The title is a play on Christopher McDougall’s runaway New York Times best seller Born to Run, which argues that humans are designed to run on their forefeet, and that cushiony sports sneakers with large, brace-like heels are wreaking havoc on foot health. Starrett agrees.

“There’s no question that athletes perform better when they’re wearing shoes that allow their feet to move naturally,” he says. He believes in flatter, more stable shoes that promote a more natural foot function. “I don’t think you’re a normal human being unless you can blaze a 5K,” he says. “Running is the motion that unites all sports.”

Not that everything he preaches is new, Starrett acknowledges. “Look, this is what the yogis talked about—a stable shoulder position; and martial artists have been talking about it for thousands of years,” he says. “Joseph Pilates [yes, the father of your girlfriend’s Pilates training] worked on this, but finally we have the tools to fix it.” And by tools, Starrett is referring to the technology that made him famous: the Internet, with its blogs and message boards and myriad other ways athletes learn and trade information.

Toward the end of his Korea trip last year, Starrett passed a roadside stand where a woman had a pile of animal bones and horns shaped to scrape the skin to alleviate pain and tightness, and to boost the immune system. The practice, which the Chinese call Gua Sha, has been around for thousands of years.

“This is the same thing we’re doing with modern techniques,” Starrett says. “It’s not like we haven’t done it before, but now, with the Internet, we’re filling in the dots.

“If we don’t figure it out this time—test, retest, and share—then shame on us.”