As a doctor, I can give you a lot of useful advice about how to get healthy and stay that way, but one thing you don't need me to tell you is that exercise is good for you. By this point, it's not news to anyone that staying active can benefit the heart, the waistline, even the mind. Still, there's a real disconnect between what we know and what we do. More than 60% of American adults do not exercise regularly, and many are content to admit they don't exercise at all. More than 72 million are obese, and almost every one of them would like to shed the extra pounds. So if exercise is such a good idea, why don't more people do it?

The most paradoxical part of our sedentary nature is that we don't start out that way. Even as I write this, I am watching my 2-year-old run around in circles. In the last paragraph alone, she has made six circumnavigations of the house. Kids seem to be born in constant motion, but along the way that behavior gets hijacked.

According to kinesiologist Steven Bray at McMaster University in Ontario, the slowdown occurs for many of us at around the time we start college. Bray followed 127 subjects and found that on the whole, first-year college students participate in significantly less exercise than they did just one year before. Academic demands and lack of organized sports are certainly part of the problem. A bigger part may be a curious human tendency to look at life changes as an occasion to blow up the old rules and not create new ones in their place. This is especially so when it comes to staying fit. "College is the first big transition in life," Bray says. "And it becomes an excuse not to exercise."

That's a pattern we repeat over and over. The demands of a new job usually mean less time at the gym or on the jogging track. How about a new marriage? How many times have we seen newlyweds looking a lot plumper in first-anniversary photos than they did in the wedding pictures? And whatever exercise resolve that married couples have left can be wiped out when a new baby comes along. "A lot of people don't like to exercise," says Bray, "so it's the first thing to go when you get the opportunity to rearrange your schedule."

In a recent issue of Observer, the magazine of the Association for Psychological Science, Ian Herbert, a journalist and triathlete, reported on numerous other studies that explain why we fall off the exercise wagon. Research by psychologist Roy Baumeister at Florida State University, for example, suggests that self-control is like a psychological muscle — one that can simply become exhausted. Spend your day trying to maintain your composure with a willful toddler or a demanding boss, and you may not have enough discipline left later to stick to your fitness routine. If that routine involves a diet, things can get even more complicated, as the effort you make to resist having a Snickers in the afternoon depletes your resolve to work out in the evening. "The more you use the self-control muscle," Herbert says, "the more tired it gets."

Not having a clearly defined exercise plan can hurt too. Investigators at Berlin's Free University found that people who set general goals, like "I will exercise in my free time," did a far worse job of sticking to that plan than did people who made a firm commitment, like "I will walk to my friend's house and back every Monday, Wednesday and Friday."

Even something that ought to help — having a personal trainer — can hurt over time. Research at the University of Saskatchewan shows that while a trainer may ensure that we stick to a fitness program, our resolve melts away once the training sessions end. In these cases, we become so dependent on someone else to monitor our progress that we never develop what psychologists call the self-efficacy to follow a plan on our own.

The good news is, there are solutions to all of these problems. Baumeister thinks the self-control muscle may be strengthened and trained — sometimes beginning with exercises as simple as remembering to sit straighter or drink enough water. Specific workout plans, like scheduling a gym visit with friends, can turn a general desire to exercise into a firm commitment. Trying to do without a trainer, or at least tapering off slowly when you quit, can help you learn to be accountable only to you. We may never again have the stamina of a 2-year-old, but recapturing even a little of our early-life energy can make our later lives a whole lot healthier.

— With reporting by A. Chris Gajilan / New York