Research indicates that releases painkilling chemicals in the brain

Running releases painkilling chemicals in the brain called endorphins. Studies indicate that the level of endorphins in the blood increases during aerobic exercise causing a state called "runner's high" and reducing pain. Interestingly enough, drugs like morphine, heroin and opium have some similar characteristics to natural human endorphins, Running produces these chemicals and they bind with opiod receptors in the nervious system to produce pain relief and mild euphoria during exercise. Running has been associated with increase endurance, injury resistance and the capacity to endure pain based on these characteristics.