We hear it all the time: "I don't have time to exercise because I am too busy at work."

But what if we told you that exercise during the workday actually improves productivity and efficiency, along with mood and energy levels? In other words, what if we said that taking the time to exercise makes up for any productivity lost while you're away from your desk?

You'd say it was a crock.

But today we have research to back it up: A British study conducted at the University of Bristol showed that workers were consistently and significantly more productive and better able to manage time demands -- and got along better with colleagues -- on days when they exercised during their lunch breaks than on days when they didn't.

The six-week study, of 130 women and 80 men aged 23 to 57, had participants work out (mainly aerobics, but some yoga and stretching) on some days and refrain from working out on others. All participants worked for employers that offered on-site exercise programs.

Participants exercised for 31 to 60 minutes during each session. They were instructed to exercise at least one day per week, but "most reported that they exercised two or more times per week at work," said lead investigator Jim McKenna, a professor of physical activity and health at Leeds Metropolitan University. "Two in three people reported improvements" in at least one measure of work performance, he added.

A few caveats: The workers in the study, most of whom had sedentary jobs, were all involved in voluntary workplace exercise programs before the study started and reported feeling confident in their job performance. But prior research has shown mood and energy boosts in sedentary people who begin similar exercise programs. Further, the assessments of productivity are based on self-reports, a notoriously unreliable method.

Still, McKenna called the results "striking," noting that he expected mood improvement from exercise but not the clear productivity gains, which averaged 15 percent.

So let's do the math. One hour is 12.5 percent of an eight-hour work day. If you're 15 percent more productive during the rest of your work time, the exercise more than makes up for itself, with a few minutes to spare! Exercise: the cold fusion of employment, yielding more energy than it consumes!

(Note that this assumes you're reducing your time working at your desk by an hour a day. If you keep your work hours steady and exercise during your lunch break, your boss is getting 15 percent more out of you than before! Are union leaders aware of this scandal?)

Mark Occhipinti, president of American Fitness Professionals Associates, a fitness education company in Ship Bottom, N.J., noted that when Johnson & Johnson, Xerox and other U.S. firms put fitness facilities on their corporate campuses in the 1980s, "rates of sick days, employee down time and suicide dropped -- basically, exercise improved every bad thing you could think of." In fact, J&J reported that its employees took 13 percent fewer sick days in their first year after joining an organized exercise program.

So wave this column in your boss's face and explain why he or she can't afford to tolerate for one more day the reduced productivity of a wan and flaccid workforce. And why you need an in-house fitness facility or a subsidy for a nearby gym. Hey, we're willing to put your job on the line over this.

(But not ours.)

And remember: New Moving Crew online chat day is Tuesday, starting next week (July 12). E-mail is move@washpost.com.

-- John Briley