”Muscle soreness is caused by lactic acid”

This idea has been disproved for many years but a lot of people still believe it. Muscle soreness is common after a strenuous workout, particularly with beginners and in previously untrained athletes. Doctors now commonly agree that muscle soreness is the result of inflammation in the muscle tissue, caused by small torn muscle fibers. Lactic acid is a by-product of anaerobically generated energy found in the muscles after particularly intensive workouts. This lactic acid, however, is fairly quickly removed from your muscles and not the cause of more persistent muscle soreness.