Paul Laursen, a performance physiologist at the New Zealand Academy of Sport who competes in Ironman triathlons, recently undertook a tough 144-kilometer bicycle ride.

“It was an epic training session with friends, testosterone levels were high, and we were all trying to drop one another on the climbs,” Dr. Laursen wrote in an e-mail.

Afterward, he said, he was hot for a long time. “It was like I had a fever the rest of the day. And even into the night as well. My wife slept with the quilt, but all I wanted was the sheet. My body resembled a furnace.”

Exercise researchers used to say that this was an exercise bonus - that you burn more calories not just when you work out but for hours after you stop, even for the rest of the day.

But then the naysayers appeared, reporting that such an exercise effect is just a myth. It turns out that there is no easy answer.

“One thing we know for sure: your metabolism goes sky-high when you exercise,” said Nisha Charkoudian, an associate professor of physiology at the Mayo Clinic College of Medicine in Rochester, Minnesota. “Then, when you stop, the interesting thing we don’t understand is that your body temperature stays up for about two hours.”

The effect is dependent on how hard you exercise. “If you go out for a walk, your temperature does not go up much,” Dr. Charkoudian said, but if you run hard for an hour or so, you can have what seems like a fever.

It’s an effect that Glenn Kenny, a professor in the School of Human Kinetics, Faculty of Health Sciences at the University of Ottawa, spent years investigating. He built a million-dollar machine - the only one in the world, he says - that can measure minute-by-minute changes in the body’s heat loss.

It looks like a giant can. The subject sits inside and, if exercise is being tested, pedals a recumbent bicycle. The device can detect the amount of heat dissipated by the subject’s body at every moment of exercise and at every moment of post-exercise rest under different conditions - warmer or cooler air temperatures, more or less humidity.

Dr. Kenny learned the reason for the feverlike state that arises when the body’s core temperature is elevated: not because you keep burning calories at the rate you did during exercise, but because the body has a hard time getting rid of the extra heat it generated . Heat dissipation is sharply reduced after exercise: for some reason the body just can’t seem to rid itself of the extra heat that it gained.

Dr. Kenny thinks that the effect is linked to exercise’s impact on the cardiovascular system. But even though you may feel hot, you are not burning more calories, he says, so you are not going to lose more weight.

From other studies, in which he measured metabolic rates, he discounts claims that exercise might also increase the rate at which people burn calories for hours afterward. He found that any effect on metabolism after exercise was so small as to be almost immeasurable, and so fleeting it was gone within five minutes after exercise stops. His subjects, though, were not people like Dr. Laursen.

Joseph LaForgia’s subjects were. Or at least they were experienced athletes. Dr. LaForgia, an exercise physiologist at the University of South Australia, says people who exercise intensely - doing repeated sprints, for example - can experience a prolonged metabolic effect. Their metabolic rates can go up and remain elevated for seven hours after the session is finished.

Even so, the extra calories burned were about 10 percent of the calories burned during the intense exercise. As for people who exercised moderately, like most people do, the small increase in metabolism lasted no more than two hours and added up to only about 5 percent of the amount they burned while exercising.

And since a modest exercise bout does not burn nearly as many calories as an intense one, people who exercised modestly ended up with very few extra calories burned afterward.

That still leaves a question, though. If your metabolic rate increases slightly, why would you feel warmer as much as seven hours after a long, hard workout?

Dr. LaForgia says he has not studied sensations of warmth, and Dr. Kenny says that if someone feels warm that long, it is not an effect of delayed heat dissipation.

Instead, it might be caused by yet another exercise effect - the body’s efforts to repair subtle tissue damage from all that exercise. The immune system can kick in, and so can enzymes that repair muscles and require heat-producing energy. Maybe the heat-generating effects of damage repair are the reason Dr. Laursen kicked off the covers that night after his 144-kilometer ride. I

f so, he probably was not burning many more calories. But then again, that tough ride over the steep hills of New Zealand burned more than enough.


By GINA KOLATA