Recently, however, scientists have discovered that it is not simply a case of the muscles relaxing after they have been fired off, but that the body produces chemicals which serve as natural tranquilizers. These chemicals, called endorphins, are released into your blood stream after a good workout and provide a real, chemically-induced state of tranquility, e.g., the Runner's High that many joggers regularly experience.
Endorphins are a group of substances formed within the body that naturally relieve pain.
They have a similar chemical structure to morphine. In addition to their analgesic affect, endorphins are thought to be involved in controlling the body's response to stress, regulating contractions of the intestinal wall, and determining mood. The relaxing effects of a good workout can last for up to 24 hours, but are most pronounced in the first few hours after the exercise. Consequently, you can use exercise as a way to reduce the anxiety which giving a speech may cause you. Obviously, you should not go overboard, or do a type or amount of exercise which will cause you pain or injury.