In 2001, I had an article published in the now defunct Boston Sports Journal that described the phenomenon known as the "runner's high," a feeling of ultimate enjoyment and possible euphoria engendered from intense exercise. My claims were mainly based on personal experience and other runners' accounts, however, there was no scientific evidence to demonstrate any fluctuation of brainy chemicals during physical activity. The theory existed with no proof.
Rousing research has recently informed us that what was thought to be valid for so many years really is; that something actually is occurring in the brain. That something (or those somethings) are increases in narcotic-like chemicals in the blood known as endorphins. The exciting finding, made possible via PET scans, is that endorphins were produced in higher quantities in the emotion relegated parts of the brain which provided the runners in the study with the high feelings. It's no longer purely a psychological manifestation or an educated guess that the brain's functions are responsible for the change in an exerciser's mood.
I'd like to see a similar experiment with athletes besides everyday road runners or marathon maniacs. Scan football or hockey players before they go out and create enough chemicals to feel no pain at all (discounting the morning after angst). When potential injury or harm to others is possible, what parts of the brain are activated besides the limbic (emotional) and prefrontal (organizational) areas? If pain receptors are somehow strengthened or manipulated to be more resilient during an extreme stimuli provoking activity, the reaction may suggest a flight or fight response.
More so than a pure adrenaline rush, the brain prepares the body for painful outcomes and also helps to organize the motor processes and decision making of someone in danger. The strong survive because they are able to generate more excitement, increase intensity, and become more resilient to whatever force may strike (whether it's an opponent, the pounding of the pavement, or a ball headed straight for the body).
Athletes who train on a daily basis and who are made to practice hard with contact and more physical force may indeed be training not only bodies, but their brains to achieve the high more often. The reality of the physiological high gives more credence to coaches who insist on training the mind as much as the physique. The ability to bounce back and be mentally tough just may be possible because neuronal chemicals are collaborating and attacking the target in one's head, so that the reaction is psychologically created from physiological mechanisms.