Bonnie Falbo is into leg lifts.

As a full-time aerobics instructor, she teaches 15 classes around the city: The Body Bank, Suissa Fitness Studio, the Department of Justice, the International Monetary Fund, Capitol Hill Squash & Fitness Club, and Riggs Bank's corporate headquarters.

Falbo, 39, figures she's exercising about 25 hours week. But sometimes even that isn't enough.

"I see a clear blue sky and white puffy clouds and I may have just taught three classes but I want to go for a run. I don't think that's sick," she says. "What's neurotic for one person is a positive goal for another. It depends on the motivation."

But what is the motivation?

For a lot of people, the catalyst is not just health concerns but compulsive or addictive behavior in the guise of participating in an activity that's "acceptable" by society's standards because it's perceived as "good for you."

Of the 168 hours in a week, most of us are awake for 108. Out of that, at least 40 hours are consumed by a full-time job, diminishing leisure time to 68 hours. Many Americans are spending that time working out -- to the exclusion of socializing, maintaining their relationships with girlfriends or boyfriends, children or spouses, or even dealing with their problems. In effect, working out, like other addictions, can be a way of avoiding reality.

It started off as a joke. The guy explained to his new girlfriend that if he didn't work out, his face would break out into a sweat and his limbs would ache.

Being athletic herself, she indulged her boyfriend's passion for exercising until it became clear that he would work out to the exclusion of all other activities, including sex.

"He was so tired by the time he'd appear on my doorstep," she says, "the only thing he could do with any finesse was fall asleep."

After two years of dating, she says, each time a dinner party was planned, a debate would erupt over what time to invite the guests. He demanded that they arrive after his workout (around 8:45 p.m.). This also meant that he wouldn't be around to help prepare. Other times, when he agreed to skip his daily workout, sweat would start pouring down his face in mid-conversation.

"It was clearly psychosomatic," she says.

Finally, she gave up. "I envisioned spending the rest of our lives squabbling over whether he'd be home in time to have dinner with the kids."

The two still are good friends. Other than the problems in their relationship that developed as a result of his exercise fanaticism, they got along well. "He calls me from time to time while on an exercise bike, as he's about to jump into a pool, or just after he finishes an aerobics class. Now it just makes me laugh."

Ian Hersey is a full-time linguist at IBM in Bethesda. He works 50 hours a week, leaving 58 hours of free time of which he spends a little less than half -- during the height of triathlon season -- working out.

"I do it several times during the day," he says, "at lunch, in the evenings, and on Saturday and Sunday I'll go for a three- or four-hour bike ride."

While Falbo and Hersey deny that they are "workoutaholics," they admit that there's a fine line separating them from many people around them -- in Falbo's aerobics classes and triathletes that Hersey meets -- who are fanatics about exercise.

According to Gabe Mirkin, author of "The Sportsmedicine Book": "Too many people become so infected with their desire to exercise that they suffer the over-use syndrome. The basic principle of training is that every time you exercise, your muscles are injured. If you exercise again before they've recovered, they tear. You're not supposed to stress the same muscles more than every 48 hours, no matter who you are -- even Olympics athletes know that. So -- very basically -- people overtrain because they are under the mistaken impression that more is better." Studies show that muscles actually recover faster by doing nothing, says Mirkin, than by exercising at a relaxed pace, and that injuries increase dramatically after four workouts a week.

That's because, "When you exercise more than three times a week, you start avoiding the 48-hour recovery rule," says Mirkin.

Indications that you're working out too much, says Mirkin, include frequent colds, frequent injuries and not looking forward to working out.

So, by incorporating a few basic concepts into your exercise regime, you can avoid becoming a workoutaholic, inflicting injuries on yourself and boring your friends: The easiest cure is to exercise every other day or alternate sports -- shifting the stress to different parts of the body. Ideally a combination of both options is the best tactic.

Besides injuring yourself and being antisocial, there are other dangers of becoming a workoutaholic:

Hersey, who tries to cut back to working out just 15 hours a week during the winter, says that disinterest in sex is one of them.

"I don't think you'll find many guys readily admitting to this, but I get all the exercise magazines and several say that excessive workouts decrease your sex drive -- particularly among high-mileage runners. Some of it is just plain exhaustion but, whatever the specific reasons, your sexual interest is diminished."

Hersey says he tries not to let his exercise routine interfere with his social life. "I would never refuse to go to a dinner party. I'll cancel my training most of time because of social obligations. What happens more often than not is, I go to a race under-trained," he says.

Falbo agrees. "Exercising can become a selfish pursuit," she says. "You have to realize the effect of what you're doing on the other people in your life."

It used to be exercising was primarily a "male-bonding thing" but, according to a survey this year by the Melpomone Institute in St. Paul, Minn., which studies females and exercise, 62 percent of women over age 18 exercise regularly. Women now make up more than half the participants in the eight most popular sports in this country, including 95 percent of the 15 million people who do aerobics.

"Exercise is perfect for women who are obsessed with control," says aerobics instructor Falbo. "I've seen exercise abused as a way of lowering body weight by women who clearly already have a problem. Exercise actually feeds their obsessiveness."

Falbo says that working out between 30-40 minutes five times a week is an acceptable criterion. "If you're working out more than that," she says, "you're doing it for other reasons besides just fitness."

This, she says, is where you can figure out whether your behavior is neurotic or normal. "It's not the exercising that's problematic but what's underlying it," she says. "If you are going to miss a workout and the thing that crosses your mind is: 'Oh no. Now I'm falling off, I'm losing control, I'm not disciplined and I'm going to get fat,' you've got to start thinking about your own personal happiness and why it is you feel you have to work out. Is it for fitness's sake or is it an escape from other problems?"

In Hersey's case, he's motivated by the specific goal of a triathlon and would not train as much if he weren't racing.

"I don't work out just to work out," he says. "There are plenty of people who work out, never race and feel badly if they don't work out. There's no goal of improved time to spur them on. I'm not sure what kind of addiction you'd call it but, for some people, instead of smoking or drinking, they have an addiction to exercise."

Some people would argue that Hersey is obsessed. He's never actually won a triathlon and he says he doesn't think he ever will. And what about the future? Right now, Hersey is young and relatively unfettered but what about when he has responsibilities to a family?

"It's a good question: Do I not want kids because it would interfere with my training? It's hard to answer."

Falbo, as Hersey, would like to keep exercising forever. But she also recognizes when she's overtraining. "I've learned that I need recovery time," she says. "So I just accept that X amount of time of rest will actually help improve or maintain my performance."

Besides worrying about overtraining, Falbo is sensitive to the liabilities of her full-time career. "If I never wanted to do anything else, it would make me one-dimensional and boring, even if it didn't make me neurotic," she says. "It's like any other addiction: You have to take control and not let it control you. Get some perspective and tell yourself that you don't want to be obsessed with exercising, that you do want to be a Renaissance person: Healthy, happy and well-rounded."