BTS, the biggest band in K-pop, have begun the US leg of a world tour covering at least 11 months, 80 concerts and 34 cities.

The physical demands will be intense. In addition to performing high-energy dance routines onstage, they will also devote hours each day to rehearsals, fittings, hair and make-up sessions, photo shoots and media appearances.

All that typically happens on as little as four hours of sleep, and much of the preparation happens in the gym before the tour begins, the group’s former trainer, Kim Jin-woo, said in an exclusive interview with The New York Times.

Kim, 42, who was BTS’ conditioning coach through their first four tours, compared the members to professional athletes.

But unlike athletes, he said, K-pop stars must train to develop stamina and prevent injuries while also maintaining the specific physique their industry demands.

“It’s a job that puts much more strain on the body than most people realise,” he said at his fitness club in Seoul. A song from the new BTS album, Arirang, was playing in the sleek, all-black weight room.

Kim began working with BTS when the members – Jin, Suga, J-Hope, RM, Jimin, V and Jungkook – were in their early 20s. (Now, most of them are in their 30s; Jungkook is 28.)

Kim can be seen in Burn The Stage, a 2018 documentary about the septet’s 2017 tour, helping Jin stretch on a mat in a hotel room. In another scene, he tends to a member’s injured ankle while Jungkook and J-Hope look on.

“BTS were just like little kids back then,” Kim said, recalling their 2017 performance at the American Music Awards. “They looked incredibly nervous. I actually choked up, thinking, ‘Will my kids be able to do well out there?’”