Why does exercise make us feel good?

--David Graybill, Wilton, Conn.

Jeannine Stamatakis, instructor at several colleges in the San Francisco Bay Area, responds:

THERE IS NO DENYING the high you feel after a run in the park or a swim at the beach. Exercise not only boosts your physical health--as one can easily see by watching a marathon or a boxing match--but it also improves mental health.

According to a recent study, every little bit helps. People who engaged in even a small amount of exercise reported better mental health than others who did none. Another study, from the American College of Sports Medicine, indicated that six weeks of bicycle riding or weight training eased stress and irritability in women who had received an anxiety disorder diagnosis.

To see how much exercise is required to relieve stress, researchers at the National Institute of Mental Health observed how prior exercise changed the interactions between aggressive and reserved mice. When placed in the same cage, stronger mice tend to bully the meeker ones. In this study, the small mice that did not have access to running wheels and other exercise equipment before cohabitating with the aggressive mice were extremely stressed and nervous, cowering in dark corners or freezing when placed in an unfamiliar territory. Yet meek rodents that had a chance to exercise before encountering their bullies exhibited resistance to stress. They were submissive while living with the aggressive mice but bounced back when they were alone. The researchers concluded that even a small amount of exercise gave the meeker mice emotional resilience.

The scientists looked at the brain cells of these so-called stress-resistant mice and found that the rodents exhibited more activity in their medial prefrontal cortex and their amygdala, both of which are involved in processing emotions. The mice that did not exercise before moving in with the aggressive mice showed less activity in these parts of the brain.

Although this study was done in mice, the results likely have implications for humans as well. Exercising regularly, even taking a walk for 20 minutes several times a week, may help you cope with stress. So dig out those running shoes from the back of your closet and get moving.