MY husband and I were riding our bikes not long ago, and when we were about a mile from home, we did our usual thing. We call it the sprint to the finish: ride as hard and as fast as we can until we reach our driveway, racing to see who could get there first.
We pulled up, slammed on our brakes and hopped off our bikes. A neighbor was walking by and said, "How did you do that?"
"I just put on my brakes," I told him. No, he said, he meant how could we just stop like that without cooling down?
Strange as it might seem, that had never occurred to me. But the cool-down is enshrined in training lore. It's in physiology textbooks, personal trainers often insist on it, fitness magazines tell you that you must do it — and some exercise equipment at gyms automatically includes it. You punch in the time you want to work out on the machine and when your time is up, the machine automatically reduces the workload and continues for five minutes so you can cool down.
The problem, says Hirofumi Tanaka, an exercise physiologist at the University of Texas, Austin, is that there is pretty much no science behind the cool-down advice.
The cool-down, Dr. Tanaka said, "is an understudied topic."
"Everyone thinks it's an established fact," he added, "so they don't study it."
It's not even clear what a cool-down is supposed to be. Some say you just have to keep moving for a few minutes — walking to your car after you finish a run rather than stopping abruptly and standing there. Others say you have to spend 5 to 10 minutes doing the same exercise, only slowly. Jog after your run, then transition into a walk. Still others say that a cool-down should include stretching.
And it's not clear what the cool-down is supposed to do. Some say it alleviates muscle soreness. Others say it prevents muscle tightness or relieves strain on the heart.
Exercise researchers say there is only one agreed-on fact about the possible risk of suddenly stopping intense exercise. When you exercise hard, the blood vessels in your legs are expanded to send more blood to your legs and feet. And your heart is pumping fast. If you suddenly stop, your heart slows down, your blood is pooled in your legs and feet, and you can feel dizzy, even pass out.
The best athletes are most vulnerable, said Dr. Paul Thompson, a cardiologist and marathon runner who is an exercise researcher at Hartford Hospital in Connecticut.
"If you are well trained, your heart rate is slow already, and it slows down even faster with exercise," he said. "Also, there are bigger veins with a large capacity to pool blood in your legs."
That effect can also be deleterious for someone with heart disease, said Carl Foster, an exercise physiologist at the University of Wisconsin-La Crosse, because blood vessels leading to the heart are already narrowed, making it hard for blood to get in. "That's always a concern," Dr. Foster said. "But to my knowledge there is not a wealth of experimental data."
But does it matter for the ordinary, average athlete? "Probably not a great deal," Dr. Thompson said. And, anyway, most people don't just stand there, stock still, when their workout is over. They walk to the locker room or to their house or car, getting the cool-down benefit without officially "cooling down."
The idea of the cool-down seems to have originated with a popular theory — now known to be wrong — that muscles become sore after exercise because they accumulate lactic acid. In fact, lactic acid is a fuel. It's good to generate lactic acid, it's a normal part of exercise, and it has nothing to do with muscle soreness. But the lactic acid theory led to the notion that by slowly reducing the intensity of your workout you can give lactic acid a chance to dissipate.
Yet, Dr. Foster said, even though scientists know the lactic acid theory is wrong, it remains entrenched in the public's mind.
"It's an idea we can't get rid of," he said.
In fact, Dr. Tanaka said, one study of cyclists concluded that because lactic acid is good, it is better not to cool down after intense exercise. Lactic acid was turned back into glycogen, a muscle fuel, when cyclists simply stopped. When they cooled down, it was wasted, used up to fuel their muscles.
As far as muscle soreness goes, cooling down doesn't do anything to alleviate it, Dr. Tanaka said. And there is no physiological reason why it should.
That's also the conclusion of a study of muscle soreness by South African researchers who asked 52 healthy adults to walk backward downhill on a treadmill for 30 minutes — an exercise that can cause sore leg muscles. The participants were randomly assigned to cool down by walking slowly uphill for 10 minutes or simply to stop exercising. The result, the researchers reported, was that cooling down did nothing to prevent sore muscles.
And muscle tightness?
"In a different generation we would have called it an old wives' tale," Dr. Foster said. "Now I guess I'd call it an old physiologists' tale. There are no data to support the idea that a cool-down helps." But, he added, once again, "it's an idea we can't get rid of."
Exercise researchers say they act on their own advice.
Dr. Thompson says if he is doing a really hard track workout he will jog for a short distance when he finishes to avoid becoming dizzy. If he runs a half marathon, he will "start shuffling forward," after he crosses the finish line, for the same reason.
As for Dr. Tanaka, he does not cool down at all. He's a soccer player and, he says, he sees no particular reason to do anything after exercising other than just stop.