For years, Michelle Cuellar exercised five days a week. "But you wouldn't have known it by looking at me," says the 33-year-old mother of two. "I felt fit -- but I was still fat."
No matter what Cuellar did -- run on the treadmill for 30 minutes at a time or attend the occasional spinning class or boot camp, her weight rose. By last summer, she carried 176 pounds on her 5-foot-6 frame.
Then, last fall, for the first time in her life, Cuellar started shrinking. She tried on a pair of pants "that hadn't fit since 1998 -- and they fit!" she says. "In eight weeks, 5 inches came off my butt, 2 inches off my stomach. The weight -- 7, 9, 12 pounds -- just started falling off."
Her breakthrough? "I started doing intervals," says the Centennial, Colo., woman.
Intervals -- short bursts of speed mixed into a running, biking, swimming, elliptical, rowing or other aerobic workout -- are nothing new for organized sports, where they've long been a tried-and-true method to build speed and power. What's new is that high-intensity interval training is being discovered by average people, who like the speed but love the side effects even more: weight loss, muscle toning and reduced workout time.
"Interval training is hot right now and getting hotter," says Joseph Grassadonia, publisher of Santa Cruz-based OnFitness magazine, which is targeted at personal trainers. "It's always been there, but we are writing more and more about it because it's simply the fastest way to get clients fit."
Cuellar says she trimmed 10 minutes from her workout time simply by replacing her old steady-state 30-minute, 6-mph treadmill jog with "Sprint 8," a 20-minute aerobic session peppered with eight 30-second, 8-mph sprints so intense that they left her gasping for breath.
Sprint 8, the centerpiece of the book "Ready Set Go! Synergy Fitness," by Phil Campbell, has a growing list of believers. Gary Green, 45, a Web-based businessman from Tustin, says he halved his workout time and cycled off 25 pounds since switching to the program in August. Internet marketer Robert Burns of San Diego, 43, says he lost 25 pounds since May doing three swimming or running Sprint 8 workouts a week. "I feel younger and get faster and faster every day," he said.
They warn, however, that interval training is not a walk in the park. "At first, I could barely sprint at 5 mph," said 31-year-old Dan Conner, a Sacramento fitness store manager who lost 50 of his 265 pounds and 9 inches off his 45-inch waist since last May. "I was dying. I couldn't breathe. But now my sprints are up to 7 mph -- sometimes 8. They leave me gasping. I know that a lot of people don't want to push like this."
How intervals work
The key to improving one's level of fitness, trainers and sports scientists say, is shocking yourself.
"After a certain period of plodding along, doing the same steady-state jogging and cycling, you don't progress -- your body gets used to what you're doing," says Christopher Drozd, a Santa Monica strength and conditioning coach. "You have to literally shock your body off the plateau. If you push yourself to the limit [with intervals], you're going to get a new limit."
The phenomenon is known as the "stress adaptation response," says Leonard A. Kaminsky, director of the clinical exercise physiology program at the Ball State University human performance lab and editor of the exercise guidelines manual of the American College of Sports Medicine.
"The human body adapts to the stresses placed on it," he says. "Challenge it, and it improves. To effect change, you need to overload your system beyond what it is accustomed to. When you go beyond your aerobic threshold [the point at which you are unable to bring in enough oxygen to support the exercise] -- to where you perceive that you're getting winded -- you initiate a chain of positive events that work for everyone. Even nursing-home populations can improve."
Intervals improve fitness by upgrading the oxygen-processing system with new capillaries and stronger lungs and heart, adding more mitochondria (tiny cellular motors) to muscles, and developing a higher tolerance to the buildup of lactic acid, a waste product associated with going anaerobic (into oxygen-debt). A 2005 study of competitive cyclists at New Zealand's Waikato Institute of Technology even found that intervals can speed up serious athletes in midseason form; eight to 12 sessions gave test subjects power gains of 8.7% for 1 kilometer and 8.1% for 4 kilometers over a control group of non-interval trainers.
But it is the unexpected weight loss, time savings and sense of "feeling younger" that have average exercisers most excited.