Final answer:
Athletes who continue to play despite being injured experience a reduction in pain sensitivity due to increased production of endorphins, which are natural painkillers. Endorphins are released during vigorous aerobic exercise and contribute to the runner's high. Regular intense exercise can cause the brain to downregulate endorphin production during rest periods.
Step-by-step explanation:
Athletes who continue to play, despite being injured, experience a reduction in pain sensitivity because of increased production of endorphins. Endorphins are natural painkillers produced by the body during vigorous aerobic exercise. They can contribute to the phenomenon known as a "runner's high" and help reduce anxiety and depression. Regular intense exercise can cause the brain to downregulate endorphin production during rest periods, leading athletes to push themselves harder to experience the same feeling.