In the aerobic '80s, legions of Americans learned to lace up their walking and running shoes to exercise that most important of muscles, the heart.

Today, fitness professionals are recognizing that the body's other muscles deserve equal time. Without strength training exercises to tone up muscles and keep in shape, most people face a midlife slide into flabbiness.

"Even the most dedicated aerobic exerciser will lose muscle mass with age," says exercise physiologist Wayne Westcott, fitness director of the South Shore YMCA in Quincy, Mass. and strength training consultant to the YMCA of the USA. "After age 20, adults who do not perform some type of strength training lose about one pound of muscle every two years."

That means a sedentary 40-year-old woman who weighs the same as she did in college has simply replaced 10 pounds of muscle with 10 pounds of fat. Although body weight may remain the same, he says, body composition changes markedly with age. So the newest campaign in America's war on the couch potato is powering up -- which means performing exercises aimed at strengthening muscles, be it lifting weights, working out on machines such as Nautilus or doing calisthenics.

Muscling up can improve more than just your looks. People with strong muscles have a lower risk of osteoporosis, are less prone to back or joint injury and are better able to perform the many physical demands of life, from carrying groceries to climbing stairs to opening jars.

What's more, as athletes in virtually all sports have discovered, pumping iron can improve sports performance. Over the past 10 years, endurance athletes have debunked the myth that strength training bulks you up and slows you down. Inspired by the Greek-god physiques of East-bloc competitors -- who proved that strength can boost speed -- U.S. athletes have powered out their wimpy upper bodies and boosted their game.

But this new Iron Age is not for jocks only. Frail elderly people in their eighties and nineties are reclaiming weakened muscles and walking unassisted again after working out with weights in research sponsored by the National Institutes of Health. Midlife executives pumping iron with personal trainers are banishing back pain and regaining the youthful spring in their step. And countless others in physically demanding professions -- from dancers to doctors to parents -- are flocking to gyms for the physical and psychological benefits of building muscles.

This spring, the American College of Sports Medicine ended what some considered an era of "aerobic snobbery" by revising its guidelines for adult fitness to include twice weekly strength-training sessions. The previous guidelines, issued in 1978, recommended performing an aerobic activity three to five times per week. The new guidelines advise adding eight to 10 strength-training exercises involving the major muscle groups, to be done at least two times per week.

"Training aerobically doesn't necessarily give you fitness muscularly," says exercise physiologist Michael L. Pollock, who chaired the committee that revised the ACSM guidelines. "We decided to recommend strength training, because people need a well-rounded program that exercises all the major muscle groups."

Maximum Muscles

To build muscle strength and endurance, you must move your muscles against resistance. The resistance can be your own body -- in calisthenics such as a push-up -- or it can be a weight in the form of a machine such as Nautilus or in free weights, such as barbells or dumbbells. Some other popular devices -- such as heavy elastic tubing, weighted balls and strap-on wrist and ankle weights -- can also be used.

Greek mythology holds that Milo of Crotona used a bull as his resistance. He set about becoming the strongest man in the world by lifting the animal each day from its birth to maturity. While times have changed, the physiology of progressive resistance remains the same. Working a muscle to its maximum, then gradually adding weight, will make it as strong as nature will allow.

But before lifting any bull, or pumping any iron, it is essential to seek expert help.

"Technique is critical in strength training," says health educator Robert Mulligan, owner and director of Fitness Success Communications in Golden, Colo. "If you do it wrong, you can hurt yourself."

Check with reputable gyms, university or hospital-affiliated exercise programs to find an instructor who has a college degree in physical education and/or is certified by a professional group such as the National Strength and Conditioning Association, the American College of Sports Medicine or IDEA: The Association of Fitness Professionals. Before you touch a weight, your instructor should take a complete health history and help you target your fitness goals.

It's also wise to consult your doctor before beginning a strength-training program, particularly if you are over age 40 or if you have any risk factors for heart disease or if you are overweight, have diabetes, joint or bone problems. Hypertensives need to be monitored by a medical professional since lifting heavy weights can result in an exaggerated increase in blood pressure.

But even people in poor physical condition can strength train and reap significant benefits when working with a qualified professional.

"The elderly people in our study had an average of four to six diseases and took five to eight medications per day," says Maria Fiatarone, a physician specializing in geriatrics at the Harvard Medical School and the Human Nutrition Research Center at Tufts. "They were not selected for their health. They were selected for their frailty."

Fiatarone's ongoing study of nursing-home residents ages 85 to 100 shows that working out with weights that are 80 percent of the maximum a person can lift can improve muscle strength, gait speed, mental status and functioning. "Their balance is better, and they feel more confident," she says. "There's a mindset change inherent here from thinking that aerobic exercise is the most important kind of exercise."

While aerobic exercise strengthens the heart, she notes, "what very old people die of is osteoporosis, hip fractures and immobility. Inability to get out of a chair, not inability to run across the street, is limiting their quality of life."

Reversing Middle-Age Spread

Aerobic exercise can help prolong life by reducing the risk of heart disease and other life-threatening illnesses, but strength training can improve the quality of life by making physical work easier and by helping to slow or reverse a phenomenon known as "creeping obesity."

This common battle-of-the-bulge occurs because people tend to become sedentary as they get older, but they eat the same amount as they did when they were younger and more active. The result is less muscle and more fat -- even though their weight may stay the same.

This is why the scale alone isn't a good indicator of fitness. Instead, try on a uniform or dress that you haven't worn in 20 years to see how your shape has changed. Since a pound of fat takes up more space than a pound of muscle, you may not be able to fasten the buttons-even if your weight hasn't changed.

The key to reshaping your body is to get moving. "Strength training, performed in combination with aerobics and a low-fat, reduced-calorie diet, can reduce fat and reshape your body faster than any other type of fitness program," says the YMCA's Westcott. The only way to lose weight is to expend more calories than you consume, and aerobic exercise is the best calorie burner. But strength training can boost weight loss because muscular people tend to have higher metabolism rates and burn more calories while they are at rest. This is because muscles are the "engine of the body" and are more metabolically active than fat tissue.

"Every pound of muscle we lose lowers our metabolic rate by about 50 calories per day, and every pound of muscle we gain raises our metabolic rate by about 50 calories per day," Westcott says. "Research shows that basal metabolism slows down at the rate of 3 to 5 percent per decade. This may be more closely related to our muscular conditioning than to our chronological age.

"Take two people who eat 2,000 calories per day. Ten years later, one person has done no strength training, has lost five pounds of muscle and requires 250 fewer calories per day. Unless he is considerably more active, he can no longer eat 2,000 calories per day without gaining fat. Ten years later, the other person has done regular strength training, has actually gained two pounds of muscle and requires 100 more calories per day. Other things being equal, she can eat more than 2,000 calories per day without adding fat."

Busy people can combine strength training with their aerobic workout and perform a time-efficient, 30-minute "circuit training." This involves following a sequence of exercises, usually on a series of weight machines, within a set amount of time. Moving quickly keeps your heart rate up and makes the workout aerobic, with the added dimension of strength training. Or you can add a 15-minute strength-training session onto a 30-minute aerobic workout. Performing this 45-minute exercise routine three times a week will help you develop and maintain a well-rounded, high level of fitness.

"My studies show that people who do strength training and aerobic exercise, rather than just aerobic exercise, lost more fat and gained more muscle although their diets were the same," says Westcott. "The aerobic-exercise-only group lost an average of 3.2 pounds. The group that did strength training, too, lost an average of 10 pounds of body fat."

Too Much, Too Fast

But be wary, when beginning a program, of the biggest mistake most people make when embarking on a new physical activity: trying to do too much, too fast.

"You can't make up for 15 years of non-activity in one month," says sports physiologist Steve Fleck of the United States Olympic Training Center in Colorado. "It takes time to see results."

Stick with weights, rather than calisthenics, if you're interested in maximum efficiency, he says. "In calisthenics, you're limited to lifting your own body weight," he says. "Weights are much more versatile."

Whether you choose strength machines or free weights is largely a matter of personal preference. "Exercise machines have the advantage of being easier to use and safer, since you're usually strapped in and can only move certain muscle groups," he says. "They offer some amount of motivation, too."

Experienced athletes often prefer the flexibility and variety of free weights, he notes. "But they require a certain amount of skill. Basically, your muscle tissue doesn't care if it's working against a machine or a barbell. As far as the results for general fitness, there's virtually no difference."

A good beginning strength-training program should consist of eight to 10 exercises that work all the major muscle groups of the body: arms, shoulders, back, chest, abdomen, buttocks, thighs and and calves. Get expert advice when starting out to learn proper technique to maximize benefits and avoid injury.

"You may want to start without a weight at all," says exercise physiologist Sue B. Thompson, general manager of the Women's Club in Rolling Meadows, Ill. "Once you've got the technique down and the movement is easy, then you can add weights."

Pick a weight that you can lift eight to 12 times. "Twelve times should be about your limit," she says. "By the time you finish that last rep {repetition}, you should be glad it's over. I'm not talking about pain, I'm talking about fatigue."

When you can do one set of 12 with ease, take a one-minute rest and try a second set. When you can do three sets of 12, with a one-minute rest in between sets, increase the weight by 2.5 percent to 5 percent.

But don't just automatically put the machine up to the next notch. Some weight machines go up in 20-pound increments, which might be too great a jump. Look for smaller add-on weights to make sure you don't increase by more than 5 percent.

Don't rush your workout, cautions Thompson. "Most people go way too fast, and they're cheating themselves and their muscles. Think slow and rhythmic. Lift up for two counts -- 1,001, 1,002 -- have a slight hesitation at the top, and slowly let the weight down for two to four counts. Remember that the lowering is as important as the lifting.

"Be sure to exhale on exertion and inhale on release. And keep breathing; never hold your breath."

Before hitting the weights, warm up with five minutes of easy activity, such as pedaling a stationary bicycle or doing simple calisthenics. Stretch lightly before your weight workout, and stretch more vigorously after.

Make sure to take at least one day of rest between weight workouts so your muscles can recover. "Bodybuilders may do split sets -- working the arms one day and the legs the next," Thompson says. "But I wouldn't recommend that for the average person."

When you reach a plateau, she says, "change exercises. It's important to vary your routine to challenge your muscles and avoid boredom. You have to push yourself to improve. But that shouldn't mean pain. If you experience pain, stop and seek professional help. Pain is a signal that something is wrong."

Don't be surprised, too, if you feel sore the morning after a new workout. Delayed onset muscle soreness typically occurs 12 to 72 hours after doing new kind of exercise or after doing a familiar activity at a new intensity or in a new way.

"All this means is that your muscles are waking up," says athletic trainer Gary Shankman, a strength and conditioning specialist at the Sports Medicine Foundation of America in Atlanta. "In time, that old soreness will be replaced by new strength."