Americans are gung-ho on the idea of getting in shape. Seventy percent say they're interested in taking proactive steps to improve their health, according to a new study from UnitedHealthcare.

Those steps usually stop at the gym door. More than six in ten workers don't take advantage of subsidized gym memberships and other wellness benefits, even though nearly three-quarters of employers offer them. This lack of physical activity takes a toll on worker well-being -- and drives up health costs.

Employers may think they can't force their workers to exercise. But indeed they can -- by subtly integrating more physical activity into the nine-to-five. Office spaces that force -- or rather, "nudge" -- employees to move around are increasingly proving that they can provide a hefty boost to workers' health and productivity.

Most Americans are sedentary. The typical person sits 13 hours a day. Only one in five people currently exercises enough, according to the Centers for Disease Control.

All this sitting makes people sick. Inactivity increases the risk of costly chronic diseases like diabetes, obesity, heart disease, depression, and stroke. These diseases devastate workers and their families. They also harm businesses' bottom lines. Lost productivity due to poor worker health costs the economy $84 billion a year.