We know exercise is good for us, so why don't more people exercise?

Although science has established that we feel better, sleep better and remember better when we get regular exercise more or less every day, it’s also clear that most people don’t.

Studies suggest that 97 percent of American adults get less than 30 minutes of exercise per day. The spiral away from physical activity has been blamed on everything from lack of time to lack of safe places to work out.

Some guidelines set 150 minutes of workout a week as a minimum goal, while other recent research suggests even 15 minutes a day may be sufficient to improve heath.

One Canadian study of more than 2,300 adults, published earlier this month, found that when it comes to reducing risk for diabetes, heart disease and stroke, the benefit from exercise is about the same whether the 150 minutes comes in short spurts over five to seven days or all in one weekend-warrior gulp.

Even so, motivation is hard to come by. In fact, our genes may work to keep us sedentary.

A recent study of rats at the University of Missouri suggests that certain genetic traits may predispose rodents, and presumably humans, to be motivated to exercise or remain sedentary. Researchers bred rats who liked to run with other rodents similarly inclined -- and also inbred a couch-potato strain -- over 10 generations, then looked at differences in body makeup, cell metabolism and genes in each group.

They focused on more than 17,000 genes in the brain and found 36 that seem to play a role in motivation to exercise. The study was published in April in the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology.

But what if we’re forced to exercise? Another rat study, done by researchers at the University of Colorado in Boulder, found that, at least as far as stress reduction and mental health are concerned, the benefits of exercise apply no matter if the activity is voluntary or imposed.

Over a six-week period, rats either ran on a wheel as they liked or were put on a mechanized wheel that forced them to run for a similar period of time. Later, when they were stressed in the lab and then tested for anxiety, both groups showed they were much less prone to stress and anxiety than a group of rats that had never exercised. The report was published in February in the European Journal of Neuroscience.

Of course, few adults are required to exercise. And it appears relatively few children are, either.

According to a national assessment of physical activity in schools -- the National Association for Sport and Physical Education’s “2012 Shape of the Nation” report -- only Alabama, Florida, Louisiana and New Jersey mandate 150 minutes of physical activity for elementary school students, as recommended by the association. Louisiana, Montana, Utah and West Virginia require 225 minutes for middle schoolers and Montana, Utah and West Virginia mandate the same for high schoolers, again following the association’s recommendations.

Even with the mandates, a 2011 report on recess and PE in more than 1,700 elementary schools in 47 states found that just under 18 percent offered at least 150 minutes a week of PE. But 70 percent did offer at least 20 minutes of recess a day.

The researchers at the University of Illinois in Chicago noted that students were much more likely to get the minimum of 150 minutes of exercise in states that had a law requiring PE than in states that only recommended or did not address physical education.

Physical education requirements are also scarce at four-year colleges and universities, a new study by researchers at Oregon State University shows. Brad Cardinal, a professor of exercise and sport science and colleagues, randomly selected 354 colleges and looked at their physical education requirements going back to 1920.

That year, 97 percent of the schools required students to take PE classes; today, just 39 percent require any exercise courses. Cardinal notes that other studies show even though many colleges offer recreation centers and fitness facilities, they tend to be used mostly by students who are already among the most physically fit.

The study was published in June in the Research Quarterly for Exercise and Sport.

(Contact Scripps health and science writer Lee Bowman at bowmanl@shns.com. Distributed by Scripps Howard News Service, www.shns.com.)