BTS, the biggest band in K-pop, will on Saturday kick off the U.S. 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 makeup 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 band’s former trainer, Kim Jinwoo, said in an exclusive interview.
Mr. Kim, 42, who was the band’s conditioning coach through their first four tours, compared its 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 that their industry demands.
“It’s a job that puts much more strain on the body than most people realize,” he said, speaking at his fitness club in Seoul. A song from the new BTS record, “Arirang,” was playing in the sleek, all-black weight room.
Mr. Kim began working with BTS when its members — Jin, Suga, J-Hope, RM, Jimin, V and Jung Kook — were in their early 20s. (Now, all but one, Jung Kook, are in their 30s.)
He can be seen in “Burn the Stage,” a documentary about the band’s 2017 tour, helping Jin stretch on a mat in a hotel room. In another scene, he tends to a band member’s injured ankle while Jung Kook and J-Hope look on.
“BTS were just like little kids back then,” Mr. 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?’”
He traveled with the group until the pandemic, when trainers from the band’s management company, Hybe, took charge of their fitness routine. Now he works with other celebrity clients at his club in Seoul’s exclusive Cheongdam neighborhood.
The actors and K-pop stars who make up about half the club’s clientele define their figures by targeting specific body parts, Mr. Kim said. Warming up on a stair machine is preferable to running on a treadmill because it burns calories and builds gluteal muscles without bulking up calves, for example. Another machine with a wide metal platform sends electrical current through the body to stimulate the core muscles, which are key to good posture.
For BTS to maintain visual symmetry over the years, some members had to gain muscle while others had to lose weight, Mr. Kim said.
Fans noticed that some members were more muscular after they returned from their mandatory South Korean military service. Mr. Kim, who keeps in touch with the band, said he tried to talk one of them out of bulking up in the military.
“When these seven members are onstage, if some are very broad and others are slim, the overall balance is off,” he said.
Losing weight also makes performers less susceptible to knee and joint injuries during high-impact dance performances. In a March interview with Bloomberg News, BTS members said they were watching their weight for that reason.
Hybe did not respond to requests for comment about the band’s training regiment for the current world tour. The U.S. leg begins on Saturday with a show in Tampa, Fla.
Han Ji-eun, who coaches K-pop stars like Blackpink’s Jennie and Jeon Somi, said she plans their endurance training a year ahead of a tour, and that female stars often start dieting weeks before music video shoots.
If a star’s best physical features are wide hips or slender arms, Ms. Han said, she works to define them without adding muscle mass.
“It’s a job based on your physical image,” she said.
The K-pop exercise routine is not meant for most people, who should focus on strengthening the muscles they use in daily life and maintaining flexibility in their joints, said Lee Jae Goo, a sports physician at Sahmyook University in Seoul.
K-pop stars tend to have a high base level for cardiovascular and muscle stress because they train from a young age. A four-minute dance number is as physically demanding as a 1,500-meter track race, said Dr. Lee, who has studied the routines of high-level performers and athletes.
As for BTS’s diet, Mr. Kim said that he aimed for a proper balance of macronutrients like protein, fat and carbohydrates to fuel their performances, rather than banning specific foods.
Does the band get to eat Korean fried chicken?
“Yes, of course,” he said with a laugh.
Mr. Kim gave massages and rehabilitation treatments to prevent ankle, knee, hip and shoulder injuries, which are common for dancers. Persuading an injured artist not to perform was difficult, he said.
“When that happens, the ones who are actually the most upset are the members themselves,” he said.
RM, BTS’s leader, injured his ankle in rehearsal two days before the group’s comeback concert in Seoul in March. He spent the hourlong show sitting on a stool onstage, gamely performing the choreography with only his arms.
The comeback show featured noticeably looser staging and choreography compared with the intensely synchronized style of their past performances. Mr. Kim said such adjustments could help manage their stamina, now that the members are older.
Dr. Lee, the sports physician, said that the stresses of grueling K-pop tours tend to be harder on performers as they approach middle age.
But Mr. Kim said that with proper conditioning, a typical K-pop career now hinges more on how long a performer can stay popular rather than how long they can stay physically fit.
Case in point: The boy band 2PM, who were some of his first celebrity clients, reached their prime in the 2010s but are still performing even as the band members approach their 40s. The group has two sold-out shows in Tokyo in May.
“I do think it’s possible to keep doing this even after 40,” Mr. Kim said. “As long as the fans are there, I think they can keep going.”
Lee Jiyoun contributed reporting.
Francesca Regalado is a Times reporter covering breaking news.
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