Not long ago, most people—scientists included—were convinced that the biological indignities of aging were more or less inevitable. Survive past midlife and you'd start losing muscle mass, height, energy, and your car keys. Well, nuts to that. New and inspiring research shows that the supposed physical "certainties" of aging are mostly avoidable. Muscles don't necessarily shrivel. You don't have to shrink or slow down. The key to aging well? One word: Move. Even minimal amounts of exercise can counteract the effects of time.

Myth #1: Your muscles will wither.
Reality: Only if you let them.

Many of us lose muscle mass after age 40. We also develop schoolmarm wattles. This is largely because aging muscle can become riddled with malfunctioning mitochondria, cellular structures that convert food and oxygen into energy. Without sufficiently robust mitochondria, muscle cells waste away and opportunistic fat cells move in. But not everyone suffers this fate—and an important study published last year by the Public Library of Science suggests why. Canadian researchers biopsied muscle from both sedentary and active adults between 53 and 75 years old, and found that the couch potatoes' muscles contained few healthy mitochondria. The active people's muscles, by contrast, teemed with almost as many functioning mitochondria as you'd find in 20-somethings. Best of all, the type of exercise was irrelevant—aerobic and weight-bearing activities have the same effect.

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Myth #2: Your mind will become a...that thing full of holes, you drain pasta in it...
Reality: You can change your mind with a few steps—literally.

A growing body of neuroscience indicates that exercise can remake and strengthen the brain, no matter your age. Elderly mice that are given access to running wheels have been found to develop far more brain cells in their hippocampus—a portion of the brain devoted to memory—than nonrunners. And recently, in a telling experiment at the National Institute on Aging, active and sedentary adult mice were put in a Plexiglas box and encouraged to touch their noses to a spot on the wall in exchange for food. Only the mice that regularly ran on a wheel mastered the task (difficult by mouse standards) easily. They were able to remember and learn; the sedentary mice, on the other hand, struggled with the game.

Humans don't have to run to benefit, though. Even walking seems to have the same effect on the brain: In a study published this year, researchers asked previously inactive older adults to take a 40-minute brisk walk three days a week for a year. At the end of the study, MRI scans showed that the volume of the subjects' hippocampus had increased by 2 percent.

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Myth #3: Your bones will shrink and so will you.
Reality: Not if you stimulate some stem cells.

Osteoporosis, the thinning of bone tissue, is not just debilitating but also demoralizing. Who wants to leave the sofa if you're worried your limbs might snap? Yet inactivity is one of the biggest threats to bone health. In experiments at the University of North Carolina, scientists removed stem cells from the bone marrow of mice; these particular stem cells are designed to turn into either bone cells (which strengthen the skeleton) or fat cells (which do not). Left to sit in a lab medium, the stem cells generally turned into fat. But when stimulated with high-frequency, low-intensity mechanical signals—similar to those generated during walking—they stopped turning into fat cells and retained their ability to become bone. Related studies in mice found that those same light forces led to greater bone density, too.

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Myth #4: If you've never exercised much, it's too late to benefit now.
Reality: It's never too late!

According to a survey of more than 13,500 women from the Nurses' Health Study, being active in middle age—no matter your earlier (and perhaps deplorable) exercise habits—significantly improves your odds of aging without cognitive or physical impairments or developing a chronic disease. Meanwhile, research in the United States and Europe has revealed that almost any kind of activity, at any age, helps preserve your telomeres—the tiny caps on the ends of the chromosomes in each of your body's cells. When cells divide, these telomeres fray and shorten, and when they become too short, the cell stops dividing and dies. But a 2008 study showed that exercise can maintain telomere length—potentially extending a cell's life span. And the ideal amount of exercise seems merely to be some: "Moderate physical activity levels may provide a protective effect," the researchers concluded. It's a finding that's pretty easy to live with—longer and better than ever.

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