“Use it or lose it.” I’m sure you’re familiar with this advice. And I hope you’ve been following it. I certainly thought I was. I usually do two physical activities a day, alternating among walking, cycling and swimming. I do floor exercises for my back daily, walk up and down many stairs and tackle myriad physical tasks in and around my home.

My young friends at the Y say I’m in great shape, and I suppose I am compared to most 77-year-old women in America today. But I’ve noticed in recent years that I’m not as strong as I used to be. Loads I once carried rather easily are now difficult, and some are impossible.

Thanks to an admonition from a savvy physical therapist, Marilyn Moffat, a professor at New York University, I now know why. I, like many people past 50, have a condition called sarcopenia — a decline in skeletal muscle with age. It begins as early as age 40 and, without intervention, gets increasingly worse, with as much as half of muscle mass lost by age 70. (If you’re wondering, it’s replaced by fat and fibrous tissue, making muscles resemble a well-marbled steak.)

“Sarcopenia can be considered for muscle what osteoporosis is to bone,” Dr. John E. Morley, geriatrician at Saint Louis University School of Medicine, wrote in the journal Family Practice. He pointed out that up to 13 percent of people in their 60s and as many as half of those in their 80s have sarcopenia.

As Dr. Jeremy D. Walston, geriatrician at Johns Hopkins University School of Medicine, put it, “Sarcopenia is one of the most important causes of functional decline and loss of independence in older adults.”

Yet few practicing physicians alert their older patients to this condition and tell them how to slow or reverse what is otherwise an inevitable decline that can seriously impair their physical and emotional well-being and ability to carry out the tasks of daily life. Sarcopenia is also associated with a number of chronic diseases, increasingly worse insulin resistance, fatigue, falls and, alas, death.

A decline in physical activity, common among older people, is only one reason sarcopenia happens. Other contributing factors include hormonal changes, chronic illness, body-wide inflammation and poor nutrition.

But — and this is a critically important “but” — no matter how old or out of shape you are, you can restore much of the strength you already lost. Dr. Moffat noted that research documenting the ability to reverse the losses of sarcopenia — even among nursing home residents in their 90s — has been in the medical literature for 30 years, and the time is long overdue to act on it.

In 1988, Walter R. Frontera and colleagues at the Department of Agriculture Human Nutrition Research Center on Aging at Tufts University demonstrated that 12 previously sedentary men aged 60 to 72 significantly increased their leg strength and muscle mass with a 12-week strength-training program three times a week.

Two years later in JAMA, Dr. Maria A. Fiatarone and colleagues at the Tufts research center reported that eight weeks of “high-intensity resistance training” significantly enhanced the physical abilities of nine frail nursing home residents aged 90 and older. Strength gains averaged 174 percent, mid-thigh muscle mass increased 9 percent and walking speed improved 48 percent.

So, what are you waiting for? If you’re currently sedentary or have a serious chronic illness, check first with your doctor. But as soon as you get the go-ahead, start a strength-training program using free weights, resistance bands or machines, preferably after taking a few lessons from a physical therapist or certified trainer.

Proper technique is critical to getting the desired results without incurring an injury. It’s very important to start at the appropriate level of resistance. Whether using free weights, machines, bands or tubes, Dr. Moffat offers these guidelines:

“Start with two repetitions and, using correct form through the full range of motion, lift slowly and lower slowly. Stop and ask yourself how hard you think you are working: ‘fairly light,’ ‘somewhat hard’ or ‘hard.’ If you respond ‘fairly light,’ increase the weight slightly, repeat the two reps and ask yourself the same question. If you respond ‘hard,’ lower the weight slightly and do two reps again, asking the question again.

“If you respond truthfully ‘somewhat hard,’ you are at the correct weight or machine setting to be exercising at a level that most people can do safely and effectively to strengthen muscles. Continue exercising with that weight or machine setting and you should fatigue after eight to 12 reps.”

Of course, as the weight levels you’re working at become easier, you should increase them gradually or increase the number of repetitions until you fatigue. Strength-training will not only make you stronger, it may also enhance bone density.

The fact that you may regularly run, walk, play tennis or ride a bike is not adequate to prevent an incremental loss of muscle mass and strength even in the muscles you’re using as well as those not adequately stressed by your usual activity. Strengthening all your skeletal muscles, not just the neglected ones, just may keep you from landing in the emergency room or nursing home after a fall.

Dr. Morley, among others, points out that adding and maintaining muscle mass also requires adequate nutrients, especially protein, the main constituent of healthy muscle tissue.

Protein needs are based on a person’s ideal body weight, so if you’re overweight or underweight, subtract or add pounds to determine how much protein you should eat each day. To enhance muscle mass, Dr. Morley said that older people, who absorb protein less effectively, require at least 0.54 grams of protein per pound of ideal body weight, an amount well above what older people typically consume.

Thus, if you are a sedentary aging adult who should weigh 150 pounds, you may need to eat as much as 81 grams (0.54 x 150) of protein daily. To give you an idea of how this translates into food, 2 tablespoons of peanut butter has 8 grams of protein; 1 cup of nonfat milk, 8.8 grams; 2 medium eggs, 11.4 grams; one chicken drumstick, 12.2 grams; a half-cup of cottage cheese, 15 grams; and 3 ounces of flounder, 25.5 grams. Or if you prefer turkey to fish, 3 ounces has 26.8 grams of protein.

“Protein acts synergistically with exercise to increase muscle mass,” Dr. Morley wrote, adding that protein foods naturally rich in the amino acid leucine — milk, cheese, beef, tuna, chicken, peanuts, soybeans and eggs — are most effective.

Do You Have Sarcopenia?

To help doctors screen patients for serious muscle loss, Dr. John E. Morley and Theodore K. Malmstrom devised a simple questionnaire that anyone can use. It asks how difficult it is for you to lift and carry 10 pounds, walk across a room, transfer from a chair or bed or climb a flight of 10 stairs. It also asks how often you have fallen in the past year. The more challenging these tasks and more often you’ve fallen, the more likely you have sarcopenia.

This is the first of two columns on countering muscle loss.