Got a minute? Fitness that fits your schedule

Cross “too busy” off that list of excuses for not making it to the gym. If you can’t fit a long workout into your day, you may be able to get the same benefits by working out harder for a shorter period of time.

Sprint interval training, which combines repeated, short bursts of intense exercise with intervals of rest, has long been the domain of professional athletes, such as swimmers and track runners. Such athletes use the method to ratchet up their training, taking advantage of short rest periods to avoid overdoing it.

But now, said Dr. Gerald Fletcher, a cardiologist with the Mayo Clinic in Jacksonville, Fla., “interval exercise is something we’re recommending to people more and more.” New research shows it may offer out-of shape people many of the benefits of long, steady aerobic workouts -- in much less time.

The interval concept has been around since the 1930s, but only in the last decade or so has it come under intense scientific scrutiny. Now, researchers at McMaster University in Hamilton, Ontario, for the first time have shown that -- contrary to traditional thinking -- interval training can vastly improve aerobic exercise endurance. The researchers also showed that interval exercise produces some of the same changes in skeletal muscle chemistry as aerobic workouts -- in just two weeks.

In the study, published this month in the Journal of Applied Physiology and led by associate professor Martin J. Gibala, 16 volunteers -- all adults younger than 30 -- were divided into two groups. Half went about their usual activities for two weeks, while the other half added to their routines three sessions a week of high-intensity interval exercise training. The sessions alternated 30 seconds of sprint pedaling (cycling as fast as they could) on a stationary bike with 4 minutes of rest for a total of 20 minutes per session.

Two weeks of the training doubled the amount of time subjects were able to spend doing aerobic activity before becoming exhausted -- a finding that Gibala said “basically stunned” the researchers. Before the study, the volunteers lasted an average of 26 minutes of cycling before petering out. After two weeks, those in the training group increased their endurance to an average of 51 minutes.

Gibala, a muscle physiologist by training, was also interested in looking at the biochemical markers of exercise performance. His team of researchers measured citrate synthase, an enzyme whose levels in muscle reveal the muscle’s ability to use oxygen and do aerobic exercise. In the interval trainers, citrate synthase levels increased by more than a third -- which, said Gibala, is comparable to increases usually seen after several weeks of traditional endurance training.

In other studies, low levels of citrate synthase have been associated with insulin resistance -- a major risk factor for Type 2, or late onset, diabetes.

Dr. Cedric Bryant, chief exercise physiologist with the American Council on Exercise, said that interval exercise should not be equated with cardio workouts. “Improving the performance of your quads [thigh muscles] does not necessarily mean you’ve improved the performance of your cardiovascular system,” he explained. Although the strength developed through interval training can provide the boost an out-of-shape person needs to make it through a long stretch of aerobic exercise.

Interval training has its unpleasant side effects. High-intensity exercise can cause blood to pool in the legs, leading to feelings of dizziness or even nausea. The muscle “burn” is also more intense than it is with low-intensity exercise, Gibala said.