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While we may be built for running, it's still strenuous and provides an aerobic workout, raising the amount of oxygen we extract from the air we breathe. How much aerobic activity we get in large part determines our fitness. Older people who engage in regular aerobic activity are more fit than their younger sedentary counterparts. With this kind of exercise, heart rate accelerates, shooting up to as much as triple its usual pace, and with it, the output—the so-called stroke volume of each beat. Blood also circulates faster, thanks in good measure to the tortuous architecture of the heart.

The importance of the heart's sinuous and loopy curvature was recently revealed through magnetic resonance imaging. British scientists showed that blood flows through the asymmetrically curved cavities of the heart with swirling movements that redirect the inflow and send it around, slingshot style, to the outlet of each cavity. When your heart rate steps up during exertion, the asymmetrical cavities pull vigorously back and forth, each helping to fill the other, and then sending the blood rocketing through the vessels, so that the average time it takes for a blood cell to travel the body's whole circuit is reduced from one minute to about fifteen seconds.

At the same time, your body shifts its priorities for where your blood goes. At rest, about 20 percent of the blood that leaves your heart goes to muscle, 24 percent to the digestive system, 19 percent to the kidneys, and about 34 percent to the brain and various other organs. But when you work strenuously—run, bike, swim—the amount going to muscle jumps to 88 percent, with the flow to stomach and kidneys reduced to a total of just 2 percent (which may help explain the potential for stomach cramps when you exercise hard after a meal).

Scientists have known for some time that aerobic activity has the net effect of making the heart pump more efficiently, reducing blood pressure and boosting blood volume and rate of flow. But only lately have they come to understand how this protects against heart problems. It turns out that the risk of heart attack is increased by inflammation, which can trigger the rupture of plaque and other events in the coronary arteries. Researchers have found that the drag of faster blood flow during exercise activates anti-inflammatory mechanisms in our blood vessels, potentially reducing the risk of both heart attack and stroke. Even lower-intensity exercise can diminish the chances of developing heart disease by raising the level in the bloodstream of "good" cholesterol and also by ridding the body of visceral fat cells, which release hormones that tend to inflame parts of the cardiovascular system.

If you want to get the most bang for your exercise buck, says the cardiologist Michael Miller, you should watch your favorite comedy while on the treadmill or swap jokes with your running partner: The benefit of laughter to blood vessel health is nearly equal to that of aerobic activity. In 2005, Miller used clips from the movie
Kingpin
to make twenty volunteers giggle and roar while he measured the dilation of their arteries and blood flow. The laughter provoked by the funny stuff seemed to make the endothelium—the protective inner lining of the blood vessels—dilate or expand, increasing blood flow by 22 percent. (Disturbing, stressful scenes from
Saving Private Ryan,
on the other hand, constricted arteries and reduced blood flow by 35 percent.) This suggests to Miller that laughter may be good for the heart, offsetting the negative impact of stress on the body's blood vessels. Miller does not recommend replacing jogging with jokes, but he does suggest a daily dose of fifteen minutes of hearty hilarity to supplement your aerobic exercise.

When do you stop your workout? When your thirty minutes are up? When you've finished your running route? When your body says "no more"? Most of us don't push ourselves long enough or hard enough to hit what endurance athletes call "the wall," that physical and psychological barrier that makes bikers wobble and runners buckle. Still, even the minor weariness we feel is real. Where does it originate, in the muscles or the mind?

On my longer training runs, I go about seven or eight miles before I get really tired. My friend Francesca Conte goes four times that distance. One of the best in the sport of ultramarathons, Francesca routinely runs races of fifty and one hundred miles, mostly on rough, single-track trails through the woods—and wins them. To do so, she trains intensely, running in the daytime and at night, in the winter, on rock trails slick with ice, and in the summer, when she sweats so profusely she loses as much as 7 or 8 percent of her body weight. Sometimes she runs so long and so hard, she says, that her brain shuts down and she can't navigate her route, calculate how far she has run, or even remember her own last name.

Trained as a scientist, Francesca is smart, conscientious, methodical—hardly a fanatic—but she tells stories about her own workouts that give one pause. Once, to keep ahead of a challenger late in a hundred-mile race, she ran seven consecutive seven-minute miles downhill, suffering extreme pain in her thighs with each stride. She won the race, but the next day her quadriceps were swollen and bruised purple, and she could hardly stand.

Another time, she set her mind on preparing for a big fall race by running the length of the Appalachian Trail in Great Smoky Mountains National Park, a seventy-one-mile stretch of rugged terrain. Despite the forecast for strong winds and heavy snow at higher elevations, she and four fellow runners drove all day to arrive at the trailhead at 7
P.M.
and began running up the mountain in the dark.

Francesca loves to run at night. "It's like scuba diving," she says, "everything calm and quiet." This night was no exception, with a sky full of stars and the moon showing through the clouds. But ten miles into the climb, the wind rose, and the sleet started, followed by a torrential freezing rain. The trail was soon covered in thick, slippery slush, and Francesca's clothing was soaked through. "We couldn't stop for more than a few seconds because the wind would cause us to shake uncontrollably," she says. "This made it impossible to eat or drink. As time went by, I was getting weaker and colder, and it was getting harder and harder to move fast enough to stay warm." Tired, hungry, at serious risk for hypothermia, she pressed on. Ten hours later, she made it—but, she says, she feels she only "just survived."

Francesca and I have different definitions of tired. That she can push herself to such extremes and keep running well beyond the point of exhaustion begs the question of the nature of fatigue.

"It's the brain, not the body," Francesca told me. "The hardest stretches are the ones your mind is not prepared for, that you don't expect. When you hit bad weather, for instance, or when you're ten miles from the end of your hundred-mile race, and you see a steep hill you had forgotten was part of the course. Suddenly you feel completely exhausted. But it's not in your muscles; it's in your mind."

Science is beginning to back her up.

Hippocrates held that muscle fatigue from exercise resulted from a melting away of flesh. For the past century or so, physiologists have assumed that the sensation arises when muscles reach their physical limit—when they run out of oxygen or the body fuel known as glycogen, or when they produce excessive amounts of poisons, such as lactate.

However, certain puzzles have plagued this hypothesis. For one thing, fatigue is not always accompanied by a shortage of energy or oxygen. In fact, according to Timothy Noakes, an exercise physiologist at the University of Cape Town, South Africa—and an ultramarathon runner himself—muscles do not run out of anything during exercise. They do not use all their fuel stores, and they rely on only about 30 percent of their fibers for even the most demanding tasks. "There's no evidence that we use all of the work capacity of our skeletal muscles, even when we exercise to exhaustion," says Noakes. Moreover, athletes such as Francesca often seem to have a little something extra left at the end of a race, which allows them to pick up their speed—to run those final seven-minute miles, for instance. If muscles were somehow depleted or poisoned by their own byproducts, how would you account for this ability to rev up in the last miles of a race?

"No study has yet clearly established a direct relation between any single physiological variable and the perception of effort or fatigue," says Noakes. Like Francesca, Noakes believes that fatigue begins in the brain. To demonstrate the mental component of exhaustion, he and his colleagues put sixteen well-trained runners on treadmills and periodically asked them to rate their own perceived fatigue. At the start of the experiment, Noakes's team told the athletes they were going to run at full speed for ten minutes, when in fact they would have to run for twenty minutes. Between the tenth and eleventh minutes, when the runners were informed that they would have to run an additional ten minutes, their reported feelings of fatigue skyrocketed.

According to Noakes's theory, the brain has a kind of "central governor," which sets the level of perceived fatigue based on the expectations associated with a task and establishes a subconscious pacing strategy to shield the body from exhaustion and damage. It does so by monitoring a blend of cues. These include physiological signals from the muscles about their working rate and stores of energy and oxygen, as well as signals from the brain's center for temperature regulation. In Noakes's view, the central governor is what made those runners feel tired between the tenth and eleventh minutes, before it adjusted to the new information. When the brain senses that the body is reaching its limits, says Noakes, it responds through feedback loops to the muscles, triggering the sensations of fatigue. It uses conscious cues, as well, to set a pacing strategy, postponing fatigue until the expected end of a race and creating the sensation of overwhelming exhaustion only when it's time to quit. In this way, the brain protects itself and the rest of the body from catastrophic collapse.

Just what sort of signals serve the central governor, flashing between brain and muscles to regulate fatigue, remains largely a mystery. One possibility is a molecule called interleukin-6 (IL-6). After protracted exercise, blood levels of the molecule jump from sixty to one hundred times the normal. Giving IL-6 to trained male runners makes them feel tired, slows them down, and impairs their performance. It may be that endurance athletes like Francesca have IL-6 receptors that are less sensitive than yours or mine, say some scientists, so fatigue, for them, really is a different beast.

Noakes's theory is still controversial, but I like it for its elegant explanation of common experiences: the weariness Francesca feels at the prospect of an unexpected incline late in one of her races. Or the reverse: the feeling experienced by many of us amateurs that the first mile of a ten-mile race is somehow easier than the first mile of a four-miler, though there is no objective difference. In the longer run, our central governor tells us not to feel tired yet; it's still too early in the race.

You've finished your workout. Consider its benefits. "Exercise invigorates and enlivens all the faculties," said John Adams. "It spreads a gladness and satisfaction over our mind and qualifies us for every sort of business, and every sort of pleasure."

It's true. Moderate exercise actually makes us feel
less
tired because it builds both strength and stamina. It boosts mood. It buttresses muscle and bone and improves cardiovascular health. A 2006 study showed that it reduces the incidence of colds in postmenopausal women, perhaps by increasing the number of white blood cells known as leukocytes, which fight infection. It elevates sensitivity to insulin, thereby diminishing the risk of type 2 diabetes. And it controls weight.

Exercise may help to reduce caloric intake by making some foods seem too sweet to take in large doses. In 2004, a team of Japanese researchers reported that in athletes, at least, a good workout heightened sensitivity to sweetness. But by far the most powerful impact of physical activity on weight comes from its effect on the body's energy balance. An hour of strenuous exercise can burn off about a quarter of a day's energy intake and also raise metabolic rate. Even after a workout, we tend to burn more calories than we did before, and the effect may last for hours. New studies show that this stepped-up metabolism arises in part from the boosted blood circulation and body temperature, as well as the body's efforts to replenish its oxygen stores and remove lactate.

The Amish people of Pennsylvania beautifully demonstrate this phenomenon. Though they eat quantities of calorie-rich foods—pies, cakes, eggs, ham—their rates of obesity are extremely low, less than one-seventh the U.S. average. The key to their leanness lies in their active lifestyle. Exercise physiologists have found that Amish men walk about nine miles a day; women, about seven. In addition, the men engage in some 10 hours a week of energetic farm work (the women, about 3.5 hours), and 43 hours a week of more moderate activity, such as gardening (women, 39 hours).

Anyone worrying about weight control might take a hint from this. Researchers have calculated that decreasing energy intake or increasing energy expenditure by only fifty to one hundred calories a day can offset weight gain in about 90 percent of people. For most of us, this extra hundred could be easily sheared away by a little of the Amish lifestyle: gardening for twenty minutes, walking a mile, bicycling for a quarter of an hour.

Lately has come news that exercise raises not just metabolism but brain power. Here's the exercise benefit that takes my breath away: Working out triggers changes in the brain that enhance learning and memory and protect against dementia.

Some years ago, the brain researcher Henriette van Praag gave a group of mice free access to a running wheel and withheld it from another group. She found that the mice that ran regularly learned new tasks faster than the ones that didn't—and their brains made more new cells. Mice that ran five kilometers (3.1 miles) a day learned to navigate a water maze more quickly than their sedentary colleagues. When van Praag and her team examined the brains of the mice, they discovered that the runners made two and a half times more new cells in the hippocampus, that part of the brain central to learning and memory.