It's true that exercise is an excellent stress reduction tool, and this is true for several reasons:
- Exercise helps release built-up tension in the body.
- Exercise can give you a venue for releasing emotional tension as well.
- Exercise releases endorphins and other "happy hormones" in your body, promoting a feeling of wellbeing.
- Exercise helps promote overall health and wellbeing, which can also lessen your experience of stress.
- Some forms of exercise allow you to be social, which can also be great for stress reduction. Other forms of exercise can allow you to get into a meditative state. Exercise can also raise feelings of self-esteem and bring other benefits that improve life quality.
http://stress.about.com/od/programsandpractices/f/exercise.htm