Best of all, the benefits become evident in a matter of weeks.  “There’s definitely renewed interest in interval training,” said Ed Coyle, the director of the human performance laboratory at the University of Texas at Austin.  A 2005 study published in the Journal of Applied Physiology found that after just two weeks of interval training, six of the eight college-age men and women doubled their endurance, or the amount of time they could ride a bicycle at moderate intensity before exhaustion.  Eight volunteers in a control group, who did not do any interval training, showed no improvement in endurance.  Researchers at McMaster University in Hamilton, Ontario, had the exercisers sprint for 30 seconds, then either stop or pedal gently for four minutes.  Such a stark improvement in endurance after 15 minutes of intense cycling spread over two weeks was all the more surprising because the volunteers were already reasonably fit. They jogged, biked or did aerobic exercise two to three times a week.  Doing bursts of hard exercise not only improves cardiovascular fitness but also the body’s ability to burn fat, even during low- or moderate-intensity workouts, according to a study published this month, also in the Journal of Applied Physiology. Eight women in their early 20s cycled for 10 sets of four minutes of hard riding, followed by two minutes of rest. Over two weeks, they completed seven interval workouts.  After interval training, the amount of fat burned in an hour of continuous moderate cycling increased by 36 percent, said Jason L. Talanian, the lead author of the study and an exercise scientist at the University of Guelph in Ontario. Cardiovascular fitness — the ability of the heart and lungs to supply oxygen to working muscles — improved by 13 percent.  It didn’t matter how fit the subjects were before. Borderline sedentary subjects and the college athletes had similar increases in fitness and fat burning. “Even when interval training was added on top of other exercise they were doing, they still saw a significant improvement,” Mr. Talanian said.  That said, this was a small study that lacked a control group, so more research would be needed to confirm that interval training was responsible.  Interval training isn’t for everyone. “Pushing your heart rate up very high with intensive interval training can put a strain on the cardiovascular system, provoking a heart attack or stroke in people at risk,” said Walter R. Thompson, professor of exercise science at Georgia State University in Atlanta.  For anyone with heart disease or high blood pressure — or who has joint problems such as arthritis or is older than 60 — experts say to consult a doctor before starting interval training.  Still, anyone in good health might consider doing interval training once or twice a week. Joggers can alternate walking and sprints. Swimmers can complete a couple of fast laps, then four more slowly.  There is no single accepted formula for the ratio between hard work and a moderate pace or resting. In fact, many coaches recommend varying the duration of activity and rest.  But some guidelines apply. The high-intensity phase should be long and strenuous enough that a person is out of breath — typically one to four minutes of exercise at 80 to 85 percent of their maximum heart rate. Recovery periods should not last long enough for their pulse to return to its resting rate.  Also people should remember to adequately warm up before the first interval. Coaches advise that, ideally, people should not do interval work on consecutive days. More than 24 hours between such taxing sessions will allow the body to recover and help them avoid burnout.  What is so special about interval training? One advantage is that it allows exercisers to spend more time doing high-intensity activity than they could in a single sustained effort. “The rest period in interval training gives the body time to remove some of the waste products of working muscles,” said Barry A. Franklin, the director of the cardiac rehabilitation and exercise laboratories at the William Beaumont Hospital in Royal Oak, Mich.  To go hard, the body must use new muscle fibers. Once these recent recruits are trained, they are available to burn fuel even during easy-does-it workouts. “Any form of exercise that recruits new muscle fibers is going to enhance the body’s ability to metabolize carbohydrates and fat,” Dr. Coyle said.  Interval training also stimulates change in mitochondria, where fuel is converted to energy, causing them to burn fat first — even during low- and moderate-intensity workouts, Mr. Talanian said.  Improved fat burning means endurance athletes can go further before tapping into carbohydrate stores. It is also welcome news to anyone trying to lose weight or avoid gaining it.  Unfortunately, many people aren’t active enough to keep muscles healthy. At the sedentary extreme, one result can be what Dr. Coyle calls “metabolic stalling” — carbohydrates in the form of blood glucose and fat particles in the form of triglycerides sit in the blood. That, he suspects, could be a contributing factor to metabolic syndrome, the combination of obesity, insulin resistance, high cholesterol and elevated triglycerides that increases the risk of heart disease and diabetes.  By recruiting new muscle fibers and increasing the body’s ability to use fuel, interval training could potentially lower the risk of metabolic syndrome.  Interval training does amount to hard work, but the sessions can be short. Best of all, a workout that combines tortoise and hare leaves little time for boredom.
New findings suggest that for at least one workout a week it pays to alternate between short bursts of high-intensity exercise with easy-does-it recovery.