According to Gatorade Sports Science Institute, muscles hurt when they tire due to the accumulation of lactic acid. Muscles produce energy by converting glycogen into adenosine triphosphate with the release of acid and carbon dioxide. Muscles tire quickly during heavy exercises causing a demand for more energy. When the amount of oxygen supply to exercising muscles is limited, aerobic respiration takes place, resulting in an acidic environment within muscle fibers.
Gatorade Sports Science Institute reveals that wearing tight clothing during exercise constricts blood vessels to muscle fibers, which contributes to muscle fatigue. Muscles achieve optimum performance when they receive adequate supply of nutrients. Resting between exercises allows muscles to restore their energy. The acidic environment within tired muscles is known as acidosis. An increase in lean muscle mass causes muscles to tire less often. Muscle growth is achieved through regular exercising, taking an adequate protein diet and having enough rest. Athletes prevent over-training and muscle strain by engaging in progressive exercise sessions. Long training allows muscles to get used to low levels of oxygen, making it possible for long-distance athletes to run for a long duration and distance without getting easily tired.
WebMD notes that the pain in muscles often disappears after a few days. Muscle pain that does not result from exercising requires immediate medical attention.