In 2009, Danny MacAskill lit up overnight with the YouTube release of his Inspired Bicycles part. At 23, he'd emerged as the face of street trials riding, a blend of BMX and mountain bike in urban environments. It was visionary and it was bone-breaking, and he'd just broken himself pretty badly.

"I never went to anybody to get it diagnosed," MacAskill says. "I just thought I had a sore back."

He was young, and so he did what he'd always done: ride through it. Ride every day. Do crazy backflips off stuff.

But the problems began in 2010. That sore back worked its way down into his left leg, which then blossomed into knee pain, while at the same time weakening it—"a big problem," he says. He ended up at a specialty center in Los Angeles for back surgery and a year's recovery. It was a wake-up call for his health.

"I'm definitely a lot better than I was," he says, "but backs are very complicated things."

​Danny MacAskill's Balls-Out Biking Begins In The Gym

Tricks like the seesaw front flip, from the upcoming Red Bull film "Epecuén," are beautiful when they work, but seldom do they happen first try. You're probably going to eat shit a few times, and the bigger the trick, the bigger the thumping you're going to take and take and take.

"I've learned a lot about rehabilitation and keeping myself running smoothly," MacAskill says. "And also pre-habilitation: making sure that my body is prepared to take that kind of knock in the wrong direction."

MacAskill is 28 now. He's broken his collarbone three times in six months. Surgery. He tore the hell out of his knee's meniscus. Surgery. And his back is still squirrely. To be able to take the pounding, he, like a lot of other extreme athletes, is starting in the gym.

It's not even about weights, he says: "All body-weight, lots of stretching, lots of function, just to make sure that I start strong and stable." When he's home in Edinburgh, Scotland, he'll meet with a trainer four times a week. He now worries about things like hydration. While this might dispel the notion of the biker-bro out late binge drinking, it's the reality for not just Tour de France cyclists but street trials riders as well.

"I've been riding a trials bike for 17 years and you kind of know what you can and can't do," MacAskill says. "You may as well dream big and then try to get that stuff done."

[Watch MacAskill's new Red Bull video "Epecuén," live Tuesday, here.]