Muscles are either fast-twitch or slow-twitch. Fast-twitch muscles contract quickly to generate lots of power. Slow-twitch muscles contract slowly and generate less power, but they work for longer without tiring. A healthy body has an equal split of fast- and slow-twitch muscle.
Slow-twitch muscle fibers have high concentrations of mitochondria and myoglobin. Although they are smaller than the fast-twitch fibers, they are surrounded by more capillaries. This combination supports aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal (aerobic) exercise activities.
Fast-twitch fibers have a high threshold and will be recruited or activated only when the force demands are greater than the slow-twitch fibers can meet. Fast-twitch fibers can generate more force, but are quicker to fatigue when compared to slow-twitch fibers. Strength and power training can increase the number of fast-twitch muscle fibers recruited for a specific movement. Fast-twitch fibers are called “white fibers” because do not contain much blood, which gives them a lighter appearance than slow-twitch fibers.
Genetics determines how much of each muscle-fiber type you possess; however, identifying whether you are fast- or slow-twitch dominant would require an invasive muscle biopsy. Therefore, if you find that you tend to enjoy more endurance-based activities and that they are relatively easy for you, you probably have a greater number of slow-twitch fibers. Conversely, if you really dislike going for long runs, but enjoy playing sports that rely on short bursts of explosive movements, or if you like weight training because it is relatively easy, you are probably fast-twitch fiber dominant.
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