Swimming is a competitive sport that requires cardiovascular endurance, strength, correct stroke mechanics and natural ability. In many sports, an important factor determining natural ability is the genetic makeup of your muscles, particularly the percentage of slow-twitch and fast-twitch muscle fibers. In general, people who are good at aerobic endurance activities like marathon running have more slow-twitch fibers. People who excel at powerful, explosive activities such as sprinting or weightlifting often have a greater percentage of fast-twitch fibers. Successful swimmers must be able to compete in both power and endurance.