"Muscle endurance" might make you think of running the Boston Marathon or cycling the Tour de France. But the phrase really should make you think of painting your bathroom or sitting at your desk.

"Muscle endurance is how long you can continue an activity until fatigue," says Emily Fagan, a certified strength and conditioning specialist at Flexibility First Fitness at the Citadel in Scottsdale, Ariz., and champion cyclist.

That means any activity, not just exercise. Sitting at your desk all day requires work from your muscles to hold your head up and your back straight. If they're not in good shape, they become fatigued. Not out-of-breath fatigue, which is cardiovascular endurance, but muscle fatigue. You end up grabbing your aching neck and shoulders.

Cesar Quintana, a chiropractor in Phoenix, treats many people with complaints like that. "The cervical muscles are flexing, and your back and neck muscles are trying to hold the neck in position. The muscles become shortened over a period of time. Endurance comes with training."

STAYING ACTIVE

Inside your muscle cells are mitochondria, tiny organisms that act like batteries. The more you work out, the more mitochondria you have. The heart, a muscle that works 24 hours a day, has the most mitochondria of any muscle in the body. The older we get, the more mitochondria we lose.

Scientists don't know if working out the muscles throughout the life span will reduce the loss of mitochondria and slow aging, but they do know that an inactive lifestyle accelerates aging.

Clair Sutter, 74, stays lean by biking 160 miles a week year-round. He's been a serious cyclist for 10 years.

"It keeps everything in working order, and it gets the heart rate up," says Sutter, who recently went on a 100-mile ride. "Where it really does some good is taking the weight off."

MODEST GOALS

Everyone can feel better by working on their muscle endurance. Maintain a walking or running program and your legs will become toned. But you probably won't win a marathon.

"Everyone has a ratio of fast-twitch and slow-twitch muscles," says Fagan. "Power lifters and sprinters use fast-twitch, and marathon runners and triathletes use slow-twitch. That ratio is genetic and not something you can change."

So drop an Olympic gold medal as your goal and focus on painting the bathroom without your arms turning to spaghetti.

Build the endurance of a muscle by working it continually. Think of legs pushing a bicycle's pedals for 25 miles. Or a rower pulling oars for an hour.

Keep resistance, or weight, low, especially if you're just starting a weight-training program.

Using your own weight is an excellent way to build muscle endurance, such as push-ups and sit-ups, says Ron Watkins, owner of Flexibility First at the Citadel.

"At our gym, we offer 12 sessions so people learn to do this at home," he says. "We don't want them to become gym rats."

Quintana also recommends yoga for building endurance and increasing flexibility.

Activities such as cycling and rowing are great for building muscle endurance, and provide cardiovascular exercise as well.

They're also more fun than 50 sit-ups on your bedroom floor.

"You're talking with the other riders, and it's a social thing," says cyclist Sutter who leads weekend rides. "And then we stop at a bakery and have a muffin."

BUILDING ENDURANCE

Muscle endurance is achieved through repetitive activities.

If you want to lift weights, don't increase the size of the weights, increase the number of repetitions to improve endurance. Work up to 25 or more repetitions. People who want to build up muscle tend to move on to increasingly heavy weights, lifting them eight to 12 times.

Calisthenics are good for building muscle endurance using your own body weight. When you reach 30 push-ups or sit-ups effortlessly, there are ways to increase the difficulty. For example, cross ankles while doing push-ups or lift legs while doing sit-ups.

Another way to increase the intensity of an at-home workout is with an exercise ball. Find more information and some good examples in the new book Exercise Balls for Dummies (Wiley, 2005, $21.99 paperback).

Also think about what you do in your daily life. Desk sitters should work on the core muscles: abdominals, lower back and upper back. Those planning a hike need to work the quads (thighs) and lower back. Gardeners need to work on arm endurance and the core muscles.