Training for US Navy SEALs, the special operations force, follows a warrior tradition that harkens back to the samurai, but fitness experts say the tough regime is gaining popularity with entrepreneurs, corporate executives, lawyers and elite athletes.
The workout, geared toward mental as well as physical transformation, is so demanding that the casual gym-goer looking to shed some flab before swimsuit season need not apply.
"We look at training as being as important to our life as eating and sleeping," said retired Navy SEAL commander and fitness instructor Mark Divine, the author of 8 Weeks to SEALFIT: a Navy Seal's Guide to Unconventional Training for Physical and Mental Toughness.
SEALFIT draws on the varied, high-intensity interval training of CrossFit, Olympic weightlifting, plyometrics, powerlifting, gymnastics, calisthenics, strongman exercises, yoga, and martial arts.
"CrossFit is baked into the SEALFIT model," said Divine, "but our workouts are much longer: two hours if you go through the whole thing."
Divine believes if you lean into hard work it becomes enjoyable, even transformational, although he admits the rigorous type of training has become rare in modern society.
Along with first responders, extreme athletes and special ops candidates, Divine's training site outside San Diego, California, attracts entrepreneurs and executives. About 20 per cent to 30 per cent of his clients are women.
Breathing exercises, concentration drills and visualisation exercises are as crucial as physical prowess to Divine, who is trained in Ashtanga, a rigorous form of yoga, and in martial arts.
Working in as well as working out, he said, cultivates the warrior spirit, or kokoro, a Japanese word he defines as the merging of heart and mind in action.
Danielle Gordon, a 35-year-old sales representative and an endurance cyclist for many years, admits to finding the SEALFIT training initially intimidating, even as she was drawn to the community of local surfers, triathletes, military people and professionals.
"I'm a strong woman, but this was a step out of my comfort zone," said Gordon, "It wasn't just stepping in and out of the gym."
She said the training, often for hours a day, was transformative for body and mind: "My strength has changed; my speed has changed. I can pedal harder, cycle harder. I might not be fastest (or) strongest but I know I can do anything."
Not for everyone
Neal Pire, a sports conditioning expert with the American College of Sports Medicine, said high-intensity, performance-oriented training can be powerfully motivating, but people need to be mentally ready to be able to do it, and to do it faithfully.
He cautioned that the "go faster, go harder, go home mentality" can put the un-coached and unprepared at risk for injury.
"The data out about CrossFit does show that an inordinate number of people have injured themselves. I equate it to people that have no business performing like an athlete, actually performing like an athlete," he said.
Divine writes in his book that the SEALs have a saying: "We do today what others won't, so tomorrow we'll do what others can't."
Meanwhile, Pire says it's all about goals. "Most people are not going to be thrown into battle," he said. "If your goal is to lower your blood pressure, fit into that little black dress or look good for a girl, do you need this?"