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.