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The Mind-Body Connectction
Researchers don’t know exactly how exercise reduces stress, but they have a number of possible explanations. “More than likely, the results stem from a combination of physiological and mental
factors,” says Steven Petruzzello, a professor of kinesiology and director of the exercise psychophysiology
laboratory at the University of Illinois, Urbana-Champaign.

One explanation is that exercise itself is a form of stress, and forcing your body to move from its normal state of rest provokes a natural “fight or flight” response that activates endorphins — hormonelike
substances in the brain, chemically similar
to narcotics, that are believed to control our moods and emotions along with our responses to pain. This is a popular theory, but Petruzzello cautions that most of the studies that support it measured endorphins in the blood. Researchers still aren’t sure whether these compounds behave the same way in the nervous system.

Another possible explanation is that exercise raises body temperature. Researchers suspect this plays a role in the relaxed state of mind we experience after a workout, because warming the muscles makes them more flexible and releases tension. Based upon his research, Petruzzello speculates that exercise also increases the temperature of our brains, setting off a series of neurochemical reactions that trigger the activity of compounds such
as serotonin and dopamine, two chemicals that facilitate communication between parts of the brain. Researchers believe both compounds play important roles in mood regulation and are associated with pleasurable feelings.
Petruzzello’s research indicates that physical activity also stimulates activity within parts of the brain and gives us more energy — a claim supported by several other studies in which people who exercised regularly reported having
more “vitality” or “vigor.” Animal studies conducted by William Greenough, one of Petruzzello’s colleagues at the University of Illinois, showed that exercise enhanced the mental agility of chimpanzees, enabling them to master new tasks more quickly.

But there may be a much simpler explanation
why exercise helps reduce stress: Fitting comfortably into our favorite pair of jeans makes us happy. While some studies have failed to find a direct correlation between improved cardiac and respiratory fitness and a positive mood, other research has shown that people who begin exercise programs, especially those who were previously sedentary, feel less tense and anxious as they lose weight.

http://www.motherearthnews.com/uploa.../nh_STRESS.pdf