Work/rest intervals ratchet up workouts

A man running in the desert near Reno, Nevada, in May 2010.

NEW YORK (Reuters Life!) - If your cardio routine is in the doldrums, try mixing a little tortoise in with that hare.

Experts say interval training, alternating bursts of intensity activity with less intense periods, can burn more calories, boost stamina, and stave off that burning pain that stops any workout dead in its tracks.

"Interval training burns calories quicker. If you're doing interval training for 20 minutes it can burn as many calories you would in 40 minutes," said Kerri O'Brien of Life Fitness, which designs and manufactures exercise equipment.

"It depends on level of intensity, the weight of your body, the intensity of what you're doing," she added.

And the principle holds whether you're an elite athlete, a weekend warrior or a couch potato.

"We can do different things based on your goals and fitness level," she said. "We can work with low intensities that require walking, then stop and do jumping jacks."

O'Brien said interval training increases endurance by clearing lactic acid, the byproduct of exercise.

"That burning pain is lactic acid building up," said O'Brien. "Whether you're strength training, or walking, or climbing stairs, it's lactic acid that tells your body to stop. Interval training is literally training your body to clear that lactic acid more efficiently"

There is also the psychological boost.

"Interval training reduces training time, and the less time you have to commit to exercising, the more likely you are to stay exercising. It reduces boredom and increases variety and challenge," O'Brien added.

Andrea Metcalf, a Chicago-based group fitness instructor, said interval training burns more calories and increases fitness lung volume faster than steady state work.

"The best intervals are those with a two-to-one rest/work interval: sprint for one minute and mid- pace recovery for two minutes," said Metcalf, a fitness expert for more than 20 years.

Julia Valentour, of the American Council on Exercise, said interval training works because when you alternate bouts of high and low intensity your overall intensity level rises.

She said a 2005 study found that cyclists who did four to seven bouts of 30-second all-out cycling doubled their endurance in only two weeks.

"You wouldn't want to do it every day," she cautioned. "You could over train. I like to warm up, then increase speed, run fast for a minute, then slow down for a minute, then run fast for a minute," she said.

Fartlek is an unstructured method of interval training. The word means speed play in Swedish.

"With Fartlek you say, 'I'm going to jog to that telephone pole.' Interval training can be very structured. Fartlek can be tailored to how a person feels that day," she said.

O'Brien said while the Fartlek method is used mostly for long-distance running, it can apply equally to bikers and swimmers.

The key to any interval training, she said, is a really good warm-up to prepare your body for what's to come.

Integrating interval training into your routine can be as easy as changing the program on your treadmill or elliptical machine.

"The interval button is a wonderful one to push," O'Brien said. "Avoid the manual button. I call it the complacency button. Hit it and you're just going to read your magazine."