Fast Twitch Muscle and Genetics
One of the most important biological and physiological parameters vital to an athlete’s success is the microscopic anatomy of the muscles, according to the Genetic Literacy Project. Your muscle cells, also known as muscle fibers, may be the most important factor that determines how fast you run, how much weight you can press, or if you’re a good candidate for a marathon race.
There are two main types of muscle fibers: fast-twitch and slow-twitch.
Fast-twitch muscle fibers—also known as white or Type II fibers—are larger than slow-twitch fibers. They generate more power, strength and velocity. Fast-twitch fibers mostly use sugar for energy, but aren’t very efficient. They fatigue much faster than slow-twitch fibers and require more time for recovery.
Fast-twitch-dominant athletes excel at sports that require quick, explosive bursts of power, speed and strength. World-class sprinters typically have between 65 to 85 percent fast-twitch muscle fibers, explains Scott Trappe, a professor of exercise science at Ball State University.
There are two types of fast-twitch fibers:
* Type IIa, intermediate fast-twitch: Better for endurance but produce less strength.
* Type IIb, super fast-twitch: The strongest, but yield less endurance.
Football players primarily use both types of fast-twitch fibers, whereas pro weightlifters employ more Type IIb muscle fibers. The Genetic Literacy Project explains that fast-twitch fibers “contain relatively few mitochondria (energy organelles needed particularly for endurance), but a lot of an energy chemical compound called creatine phosphate.” This allows rapid regeneration of ATP–“the universal energy currency in all cells, and the fuel needed immediately and constantly to make the muscle contract and relax.” As athletes train, the fast-twitch fibers grow and enlarge, giving power athletes the “bulked up” appearance.
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Slow-twitch muscle fibers—also known as red or Type I fibers—generate less power and strength than fast-twitch fibers. However, they are far more efficient and can sustain activity for longer. Marathoners and world-class distance runners are slow-twitch-fiber dominant. Slow-twitch fibers have a lot of mitochondria, “which allows them to burn fuel aerobically, using oxygen to get more efficiency from sugar and other body fuels,” explains the Genetic Literacy Project. They also contain a high density of capillaries, which helps deliver blood to the muscles. Although these muscle cells get stronger and grow when you train, they don’t bulk you up like fast-twitch muscles. Instead, your endurance training gives you long, slender and well-defined muscles.
Genetics vs. Training
How much control do you have over your muscle fibers? Most people are born with a 50:50 ratio of slow-twitch to fast-twitch muscle fibers. The composition really lies on a continuum, with many fibers having some of both characteristics. Whether you’re dominant in one area depends primarily on genetics, explains Scott Trappe.
Men’s Fitness provides a muscle-fiber test that can help you determine where you fall on the spectrum. Click here for the test.
It may be possible to change the composition of your muscle fibers to some extent with training. Although the scientific community doesn’t fully agree on this. Several months’ worth of long, slow runs can convert fibers to slow-twitch. Likewise, shorter, faster intervals alternated with long periods of rest can convert some fibers to fast-twitch. However, the conversion possible through training is probably only about 10 percent of the total muscle fibers, according to Trappe.