Slow-Twitch vs. Fast-Twitch in Swimming

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.

Skeletal muscles are made up of individual fibers bundled together. The two types of muscle fibers are called slow-twitch and fast-twitch. Slow-twitch fibers contract slowly and use oxygen efficiently. Fast-twitch muscle fibers contract quickly and powerfully but fatigue rapidly. Everybody has their own genetically determined ratio of slow- to fast-twitch muscle fibers. You can train the different muscle fibers through specific endurance and strength activities.

Aerobic means with oxygen. Slow-twitch fibers use oxygen to create energy. Because they create and store energy efficiently, these muscle fibers are slow to fatigue and can work for long periods of time. The opposite of aerobic is anaerobic -- without oxygen. Fast-twitch muscles fibers engage quickly and release energy rapidly through anaerobic metabolism.

In his book, “Swimming Fastest,” Ernest W. Maglischo says it’s not the speed of the movement that determines which muscle fiber type will contract. Slow-twitch fibers don’t contract when you swim slowly and fast-twitch fibers don't contract when you swim quickly. Instead, the different fiber types contract in response to the muscular force required to produce a movement. Slow-twitch fibers contract first and do most of the work when less force is required. With greater resistance, both slow and fast-twitch fibers contract. At maximum force, both fiber types contract.

Interestingly, the percentage of fast- or slow-twitch muscle fibers doesn’t appear to matter as much in swimming as in other sports. In fact, according to Maglischo, there is not much correlation between a swimmer’s predominant muscle fiber type and performance in different race distances. This may be because the difference between sprinting and endurance isn’t as extreme in swimming as it is in other sports. Swimming requires both speed and endurance. And both slow and fast twitch fibers contract when you swim at 70 to 75 percent of your maximum effort. Swimmers with a high percentage of fast-twitch fibers aren’t always the fastest sprint swimmers and swimmers with more slow-twitch fibers aren’t always the best distance swimmers.