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Moreover, the early hours of the day are better for workouts involving balance, accuracy, and fine motor control. If evening better serves the runner and swimmer, morning aids the surgeon and the archer, and also the neophyte: Late morning is the best time to learn new motor skills and to remember complex coaching instructions.

Perhaps this explains a humiliating late-afternoon training session I once experienced with an expert archer. I should have known better.

"Face up, sight on your target, release." Allison Duck planted her feet on the narrow strip of grass behind the gymnasium, nocked an arrow on the bowstring of her powerful recurve bow, drew the string back to the point of highest tension, and smoothly let loose the arrow, which neatly pierced the blue ringing the bull's-eye.

She made it look effortless. Allison fell in love with archery in her native South Carolina when she was nine. Now six feet tall and powerfully built, with upper-body muscles that show the weightlifting she does to develop her draw strength, she has been shooting nearly every day for more than a decade and regularly gives lessons.

This was my first go at archery, so I studied Allison's example closely. Nearly any bodily movement—tying a shoelace, kneading dough, doing the Macarena—is best learned by watching the action of a model or teacher. The human brain is built for imitation. Even very young infants show some rudimentary imitative ability; within an hour or two, babies can mimic frowns, smiles, and other facial expressions. New research suggests that our brains possess imitative "mirror" systems, networks of special neurons that fire both when we perform an action and when we watch someone else performing the same action. When I see Allison draw her bow, my mirror neurons automatically simulate the action in my own mind. This helps me understand her motion and her intentions—what she plans to do next. These helpful neurons are found in many areas of the brain, including the premotor cortex and the rear part of the parietal lobe. During imitation, the network of mirror neurons in the premotor area is often more active during the viewing of the movement than the actual performance of it.

At this lesson, I did my best to replicate Allison's stance, positioning my feet so that my left side was facing the target and turning my face, chest, and hips slightly toward it, back straight. So far so good. But then a problem surfaced.

Archery has been described as a contest of the archer with himself. Calm immobility is vital. In fact, so essential is a steady hand that archers are said to try to release the arrow in the lull between heartbeats.

I'll admit here that stillness is not my strong suit. When I was training for the theater as a college student, one exercise always eluded me: the conscious effort to quiet every muscle in my body, in thighs, arms, neck, cheeks. I could not quell the tense, fidgety feeling. Certainly I failed when it came to relaxing my tongue. I hold with Pascal: "Our nature lies in movement; complete calm is death." After all, only 10 percent of the body's mass is meant for quiet contemplation and judgment; the remaining 90 percent is for action.

I nocked the arrow, trying, as Allison advised, to keep my shoulder locked in a down position, head upright, fingers relaxed on the grip of the bow. But my body began to twitch, and my arms to quiver. I yearned for the singular "catch" muscle of the clam, which can contract and then lock itself in that state, easing the clam's discomfort in the awkward posture of holding open its valves to catch prey.

No such molluscan luck. My muscles stiffened; my shoulders wobbled. "Fire away," urged Allison. At the moment of release, I swayed slightly to the right, sending the arrow straight into the wall of the gymnasium, where it lodged firmly in the corrugated metal.

"Well," Allison remarked wryly, "
that's
never happened before."

Would my balance and fine-motor control, my ability to imitate Allison and absorb her coaching instructions, have been substantially better at an earlier hour? I'll never know. In any case, I plan to stick with running. As one sixteenth-century physician wrote, "Every meuving is not an exercise, only that whiche is vehement." In my book, archery doesn't qualify. So what does? Some modern researchers argue that only sustained, vigorous activity such as running, swimming, or working out at a gym for an hour is sufficient to provide the full health benefits of exercise, especially in staving off heart disease. Others believe that more moderate activity once a day is enough to deliver at least some benefits and offers a more realistic goal for most people. The U.S. government recommends at least thirty minutes a day of moderate activity, preferably more.

But what's moderate? A slow run? An energetic walk? Pumping iron three times a week? How about vacuuming or waxing the car? Though common household and garden tasks would hardly seem to fall into the category of a vehement workout, some research suggests that such tasks may count as exercise, depending on how they're performed.

Not long ago, Australian scientists persuaded a dozen men and a dozen women to wear a head harness, nose clip, and respirator valve to measure the energy expended while they swept, mowed the lawn, cleaned windows, and vacuumed. It turned out that some of the more vigorous activities qualified as exercise of moderate intensity if performed for adequate duration and frequency—say, a half hour of brisk leaf raking or lawn mowing a few times a week.

Climbing stairs counts too, as scientists in Singapore discovered. The team enlisted more than a hundred men and women to ascend and descend twenty-two short flights of stairs (the average number in a typical Singaporese high-rise apartment building, where most of the city's residents live). The results were impressive. Descending the stairs was equivalent to an energetic walk at 2.6 miles per hour, and ascending was much like running at a pace of 6 miles per hour. Going up and down once expended almost 30 calories. Climbing stairs is an ideal activity for the masses, the team concluded: convenient, private, requiring no special equipment, and cheap.

Unfortunately, only a quarter of all American adults climb enough stairs or rake enough leaves or walk far enough to get even the bare minimum of recommended daily exercise, and close to a third are completely sedentary. Just 2.7 percent of us walk to work; less than half a percent ride a bike. Such a sluggish existence is a radical departure from our ancestors' aerobic way of life. Evidence suggests that early hunter-gatherers walked up to twelve miles a day and no doubt ran long distances as well.

Pascal was right: We are made for physical activity, not for sloth. Without a workout of some kind—walking, climbing steps, rowing, hunting—our bones thin and our muscles atrophy. Loss of muscle and bone from lack of exercise tends to start in our late thirties and early forties. By age fifty, the sedentary among us may have lost as much as 7 percent of our muscle mass; by age eighty, about 40 percent.

The good news is this: It's never too late to defy the decline.

Say you start your workout by lifting free weights. Stay with this routine and you'll witness the remarkable plasticity of muscle and bone.

As you lift, your arm muscles work in pairs, one raising the arm, the other lowering it by pulling the same bone in the opposite direction. As the one muscle contracts and exerts a force, its counterpart relaxes and stretches. The force of muscle against bone fosters the activity of bone-building cells called osteoblasts. The more powerful the pull of your muscle, the greater the stimulus for bone growth.

You may not be able to perceive your growing bone strength, but you will notice the transformation in your muscles. Resistance training works its muscle magic through two mechanisms: adaptations in the muscle fibers themselves and changes in the neural signals that fire them.

We're born with all the muscles we'll ever have, more than 650 of them. The proteins that make up muscle fibers are breaking down all the time, and new ones are being assembled. Whether your biceps grow, atrophy, or remain the same depends on the balance between the rate of buildup and demolition. To tip the balance, you must load the muscle. Hard training can double a muscle's size; even light effort can boost strength. In one study, subjects asked to grip an object with maximum force for only a second a day gained an average of 33 percent in grip strength within five weeks.

At rest, muscle fibers are soft and flabby. But every fiber is innervated by a neuron from the spinal cord. When stimulated by a nerve impulse, the fibers shorten, shifting from loose rubber to springy steel. Exercising builds muscle strength by reinforcing these nerve signals and synchronizing them.

This may help to explain why just
thinking
about an exercise can boost strength. Scientists have learned that putting people on a regimen of mental gymnastics—instructing them to think about bending a finger or an elbow, or flexing an arm muscle—actually strengthens the muscles involved in the action. A team of scientists at the Cleveland Clinic Foundation in Ohio asked a group of volunteers to perform "mental contractions" of finger and elbow for fifteen minutes a day, five days a week, for twelve weeks. The mental workout did not affect muscle size, the scientists found, but did significantly increase muscle strength, by 35 percent in the hand and 13 percent in the elbow—most likely because it served to reinforce the brain's nerve signals to the muscle. This may be the science behind the visualization technique that many athletes use to improve their performance, mentally rehearsing the motions of an athletic task before attempting it. However, the researchers emphasize that such mental exercise can never replace a regimen of daily vigorous exercise, including strength training.

To keep your muscle and bone mass throughout life, say the experts, you should load them by lifting weights two or three times a week. The new research on the gain from this kind of resistance exercise—making muscles exert near-maximal force for brief periods—is irrefutable.

There is a catch, though: Not everyone will benefit to the same degree. In 2005, a team from the University of Massachusetts, Amherst, looked at changes in the strength and size of the biceps muscles in 585 men and women after twelve weeks of pumping weights twice weekly. Men showed more increase in size compared with women, while women outpaced men in relative gains in strength. However, both men and women varied greatly in response to identical regimens of training. Some subjects gained little in muscle size or power, while others doubled their strength and increased muscle size by inches. At least some of the benefit from exercise, it seems, depends on your genes.

If you've never lifted weights before, or if you engage in a particularly challenging session, you may pay later. Muscle soreness generally peaks twenty-four to forty-eight hours after vigorous exercise. Stretching does not prevent it. My own worst case of this "delayed-onset muscle soreness," as sports scientists call it, came a few days after I climbed a volcano in Guatemala. The hike up the twelve-thousand-foot summit was slow and arduous, but it was the next morning's descent that undid my thighs. Later that week, I wandered in circles through the beautiful colonial town of Antigua, scrupulously avoiding street curbs. So painful was it for my quadriceps to execute the downward motion that I could negotiate a curb only by swinging my legs awkwardly out to the side to step down.

Muscle soreness results from so-called eccentric contractions—the lengthening of contracting muscles that occurs with downward motion (for example, lowering a weight or descending a steep slope). To teach this lesson to his students at the University of Aberdeen, exercise physiologist Henning Wackerhage asks them to step up onto a bench five hundred times with one leg and step down with the other. "Students are often convinced that the upward-pushing leg will be sore in the days after the exercise," says Wackerhage. "But they're in for a surprise. The upward-pushing leg is normally fine, whereas some of the muscles in the downward stepping leg are usually sore." Going uphill or hoisting a heavy load may seem like hard work (and it is for heart and lungs), but going downhill or lowering that load is harder on the muscles.

Delayed muscle soreness is caused by microtears in the muscle, which, after a day or two, become inflamed. White blood cells migrating to the site to help repair the tiny tears release chemicals that trigger pain—a protective device to signal injury and the need for rest.

On the bright side, muscles respond to the damage by growing back stronger and larger. Satellite cells scattered on the surface of the muscle fibers proliferate, migrate to the damaged area, and insert themselves into the muscle tissue. There they lend their protein-building resources to the muscle fibers. With their help, the fibers can pump out more proteins, thus not only repairing the tears, but going beyond to build themselves up. Muscles adapt with training so that they're more resistant to the damage of subsequent exercise and are repaired at a faster rate.

Perhaps you've shifted to running now. Our species is built for this in limb, lung, and heart, says the biological anthropologist Dan Lieberman: "We're capable of running at a wide variety of speeds and altering our breathing patterns to suit them, and we're able to make use of energy stored in our tendons and muscles."

When we run, Lieberman explains, we shift from the "inverted pendulum" mode of walking to a bouncy "pogo stick" mode, using the tendons and ligaments in our legs as elastic springs. Elasticity is that property that makes objects spring back to their original shape after being deformed. When your foot strikes the ground while running, your tendons and ligaments stretch, absorbing elastic energy from the impact, like a bow when bent; when your foot rebounds, the tendons and ligaments contract again, recoil, and release their energy. Through such stretching and recoiling, tendons do much of the work in running, relieving the labor for muscles.

The secret to swiftness, it turns out, is maximizing this bounce. Speed is not a function of how quickly you reposition your legs in the air, but how hard you push off the ground. Using a treadmill with a force plate to analyze runners of varying abilities at top speed, one Harvard research team learned that virtually all runners take the same time to reposition their legs, called swing time. But the fastest runners apply more vertical force to the ground with each stride, which results in greater upward force captured by those elastic tendons and ligaments. The difference between you and Marion Jones, then, is not your speed of limb movement but your "ground force," which determines how far you go with each bounce.