A bout of dry heaves on her first day may not be the greatest endorsement for a new exercise routine, but Kimberly Miyashiro insists that she loves what the workout is doing to her body.

The 15-minute session of squats, lunges and running packed such a wallop that she was still sore after her second workout two days later. And the day after that, too.

"I still can't sit," said Miyashiro, a 32-year-old from Manoa. "I am in a lot of pain. It's a good pain. It makes me know there are some muscles there, but I've been pretty sore."

Count her among the newest disciples of CrossFit, an exercise program that routinely pounds its afficionados into sweaty exhaustion with workouts designed to leave them gasping for air.

"It was horrible, but a good horrible," Miyashiro said. "I wanted to throw up afterward. I was lightheaded. Your bearings are all off."

There are no routines in CrossFit; the only real guiding principle is an adherence to hard work, said Kimo Kockelman, owner of CrossFit Hawaii, the gym where Miyashiro works out.

The idea is to combine strength and conditioning exercises that involve multiple muscle groups and to complete various combinations of them as quickly as possible, he said.

The combinations change every day and can be finished in about 20 minutes — far shorter than the hour-long routines often found in mainstream gyms.

"The difference between this and an hour workout is intensity," Kockelman said. "It's extremely intense."

The exercises can include Olympic-style lifting, lunges, push-ups, pull-ups, sit-ups, tugging on a rowing ergometer and short runs. Jumping up and down from a 20-inch high box is a staple at CrossFit gyms, but some have exchanged the box for a huge tractor tire.

CrossFit trainers dislike machines and weight-lifting exercises because they focus on individual muscles. By combining muscle groups, they believe CrossFit prepares a person for real life. For that reason, the program has become popular among law enforcement officers, firefighters and the military, as well as anyone who might struggle to put a suitcase in an overhead compartment.

"This kind of workout translates to so many other things in your life that improve your quality of life," said Sean Simon, a 35-year-old firefighter who runs CrossFit Molokai out of a community fitness center. "It protects you from daily living. Whatever you do translates to what you are doing."

But Simon, who used the program to get back in shape after a serious knee injury last year, said the intensity of the workout may not be for everyone.

"Most people bodybuild or jog," Simon said. "We are kind of more in the sprinting stage. It is a little more strenuous than the average person wants. How many people recreationally sprint?"

Like many CrossFit enthusiasts, Kockelman and Simon initially found the program online, where it first gained popularity as an online-based workout group in 2001. Workouts would be posted and participants would submit their results in a friendly competition that still continues.

But it has grown wildly in the last six years as hundreds of outlets opened around the world. There are five in Hawai'i.

Kockelman, a 40-year-old personal trainer, visited a facility last summer in Kalihi and his first workout left him humbled and "severely out of breath," he said. By March, though, he had opened his own facility in Kaka'ako.

The setting is spartan: An exposed garage front, no air conditioning or mirrors and pull-up bars fabricated from galvanized pipe. And yes, he has a really big tractor tire out front.

But even though the facility boasts fresh paint Kockelman applied himself, the room fits the philosophy.

"In this room, we never talk about aesthetics," he said. "Everything is performance driven."

Kockelman believes that anyone can handle the workouts, although it will take two weeks for them to adapt. His members include doctors, housewives and teenagers, as well as FBI agents and Honolulu police officers.

"They redefine what is hard," he said.

Robyn Fuqua, a 33-year-old public relations executive from Kailua, enrolled at CrossFit Hawaii when it opened. The workouts added leg strength, lowered her blood pressure and now she can see her abs, Fuqua said.

And her stamina increased without a lot of extra running, in part because running longer hurts less than her typical CrossFit routine.

"What surprised me was how exhausted you could be after only 10 or 12 minutes," she said. "It's not just exercising. It is a survival experience."