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Fast Twitch and Slow Twitch Muscle Fibers Apr 11, 2012

Fast Twitch and Slow Twitch Muscle Fibers

Have you ever wondered how a track team participant decides if they are best suited for sprints or distance events? Usually the athlete’s muscle fiber make-up decides what the best events are for them and how well they excel at it.

Our bodies’ muscular skeletal are contain two main types of muscle fibers: slow twitch (Type I) and fast twitch (Type II). The differences between the two types help to determine how the body responds to physical activity due to how each type of muscles contract. For the general population, our muscle fibers are generally 50/50 on the two types.

muscle fiber

Slow twitch (Type I) muscles fibers use oxygen efficiently and produce more fuel (known as ATP) that fuels the body that helps to sustain more endurance and allows you to go for a long time without fatigue. Think of cross country runners or long distance cyclists.

Fast twitch (Type II) is more anaerobic and generate quick burst of speed and/or strength and fatigue quickly. They produce about the same amount of force as slow twitch per muscle contraction but fire much more rapidly. Fast twitch muscle fibers assist sprinters and power lifters to generate a lot of force quickly.


There is some evidence that specific training can change muscle fiber from one type to the other. Keep in mind that genetic differences may be dramatic at the elite levels of athletic competition, but following the principles of conditioning can dramatically improve personal performance of a typical athlete. With consistent endurance training, muscle fibers can develop more and improve their ability to cope with and adapt to the stress of exercise.

Jill Purvis –

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