Activity matters. (Special Feature).

"If there were a magic potion that could make you feel great and could cut your risk of getting heart disease, cancer, and diabetes in half, people would be willing to spend hundreds, even thousands, of dollars for it," says JoAnn Manson of Brigham and Women's Hospital in Boston. "But it's already available and virtually free: regular physical activity."

Here's a look at some of the latest news on what to do and what you get out of it.

NEVER TOO LATE

In 1966, they were five healthy Texans in their 20s taking part in a landmark study proving that just three weeks of bed rest could cause serious deterioration in cardiovascular fitness and work capacity. In 1996, they were five overweight, out-of-shape men in their 50s proving that it's never too late to exercise.

After decades of inactivity and expanding waistlines, the five men started walking, jogging, or cycling as part of an aerobic exercise program at the University of Texas Southwestern Medical Center in Dallas.

"Remarkably, within six months the men restored their cardiovascular fitness to what it had been 30 years earlier, before they went on bed rest," marvels researcher Darren McGuire. Bonus: the men also lost an average of ten pounds.

But they never reached the level of fitness they achieved in their 20s, after they finished three weeks of bed rest and completed an eight-week training program. "We didn't get them back to their 20-something post-training level or to the level of middle-aged master athletes," says McGuire. "But if they had been more physically active all these years, who knows?"

Here's what it took to get the men back into shape:

* A slow start. The men walked, jogged, or cycled for only 30 minutes during the first week, says McGuire. "Then we increased it by five to ten percent each week." By the end of six months, they were exercising for four hours a week.

* Modest intensity. "The tendency for almost everybody who starts an exercise program is to overdo it early, so they get sore and tired," says McGuire. "Then it's not fun, so they quit."

Instead, people should shoot for a heart rate while they're exercising that's right for their age. "The target heart rate should be about 130 beats per minute for a 50-year-old, 120 for a 60-year-old, and 115 for a 70-year-old," says McGuire.

"I encourage the use of a heart-rate monitor that looks like a wristwatch, so people can watch their heart rate as they're exercising and not overdo it at the beginning," he adds.

Not interested in a monitor? McGuire has a simpler option: "Exercise at an intensity level that leaves you short of breath but still able to carry on a conversation."

* Fun. If you don't pick an activity you like, you won't stick with it. "What kind of exercise you do doesn't really matter, as long as you enjoy it and it gets your heart beating at the target rate," says McGuire.

Circulation 104: 1358, 2001.

STEP IT UP

Step aerobics builds muscle and fitness. But add some strength training and you boost muscle strength, endurance, upper-body power, and muscle mass even further.

In a 12-week study, women who did 40 minutes of step aerobics--stepping on and off a platform--three times a week didn't gain as much strength and endurance as women who combined 25 minutes a day of step aerobics with 20 minutes of strength exercises (like squats or bicep curls) for the upper and lower body.

"Including both aerobics and strength training in an exercise program is the most effective way to improve total body fitness and health," says William Kraemer of the University of Connecticut.

Medicine and Science in Sports and Exercise 33: 259, 2001.

THE BEST MACHINES

People burn the same number of calories on one exercise machine as on another, right?

"No, they don't," says Niall Moyna of Dublin City University in Ireland. He and his colleagues (at the University of Pittsburgh) recorded the calories burned by 19 healthy men and women in their 20s while they exercised on six different kinds of machines at "fairly light," "somewhat hard," or "hard" intensity.

The results: Both men and women burned the most calories on the treadmill or the skiing simulator and the least on the stationary cycle or the Healthrider machine (a cross between a stationary bicycle and a seesaw).

The stair-stepper was in the middle, as was the rowing machine for men. For women, the rower was a top-notch calorie-burner.

And whatever the equipment or intensity, men always burned more calories than women. That's because muscle burns more calories than fat and, pound for pound, men have more muscle. Also, it takes more calories to move a bigger person, and men are usually bigger than women.

"To burn 200 calories, men could run on a treadmill for 14 minutes at a fairly light pace, while women would need 22 minutes," says Moyna. "At a hard pace, men would need 12 minutes and women 17."

Compare that to the stationary cycle. Men would need to cycle 23 minutes--and women 32 minutes--at a "somewhat hard" exertion to burn 200 calories.

Medicine and Science in Sports and Exercise 33: 1404, 2001.

NO PAIN, YOUR GAIN

Aches and pains. No one wants to hear about them. No one wants to have them. And with the right exercises, we might not.

In a study of roughly 100 men and women aged 65 or older, the half who participated in stretching and flexibility exercises four times a week for a year reported fewer aches and pains than when they entered the study. The half who participated in an aerobics and strength-training program improved their endurance and strength, but had no less pain.

"This is one of the first studies to find that stretching and flexibility exercises can help older people cope with aches and pains," says Abby King of the Stanford Center for Research in Disease Prevention in Palo Alto, California. "The study shows that it's important to include flexibility exercises in a workout program, especially for older men, who may neglect this portion of their exercise routine."

Journal of Gerontology: Medical Sciences 55A: M74, 2000.

WEIGHTS LIFT DEPRESSION

Lifting weights may help lift clinical depression, says a new study from the Harvard Medical School and the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston.

Symptoms of depression receded in three-quarters of the men and women aged 60 to 84 who followed a 20-week strength-training program to build up their upper bodies (arms and shoulders) and lower bodies (legs and buttocks). In contrast, depression receded in only a third of the control group (which did no weight-lifting but attended lectures on health).

"The benefit was most pronounced for those with more severe depression," noted researchers Maria Fiatarone Singh and Nalin Singh. "And the benefits persisted despite the fact that after the first ten weeks the exercisers were on their own and no one was directly supervising them."

Journal of Gerontology: Medical Sciences 56A: M497, 2001.

FOR ME ONLY

It doesn't matter if you're young or old. Strength training not only builds muscles, it also boosts your resting metabolic rate (RMR). And that's important, because it means that your muscles burn extra calories all day, whether you're sitting at a desk or lounging on the couch. They do if you're a man, that is. Strength training didn't boost RMR in women in a six-month study at the University of Maryland.

"Strength-training exercise is good for women because it increases their muscular strength and endurance," says researcher Ben Hurley. "But it looks like women can't count on it to help burn off extra calories like men can."

"In our study the women's muscle mass increased only half as much as the men's," adds Hurley. "But we don't know if that explains why the men's RMR went up and the women's didn't."

Medicine and Science in Sports and Exercise 33: 532, 2001.