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Stretching FAQ 1.2.2 - Fast And Slow Muscle FibersStretching FAQ 1.2.2 - Fast And Slow Muscle Fibers
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Subject: 1.2.2 - Fast and Slow Muscle Fibers
The energy which produces the calcium flow in the muscle fibers comes from "mitochondria", the part of the muscle cell that converts glucose (blood sugar) into energy. Different types of muscle fibers have different amounts of mitochondria. The more mitochondria in a muscle fiber, the more energy it is able to produce. Muscle fibers are categorized into "slow-twitch fibers" and "fast-twitch fibers". Slow-twitch fibers (also called "Type 1 muscle fibers") are slow to contract, but they are also very slow to fatigue. Fast-twitch fibers are very quick to contract and come in two varieties: "Type 2A muscle fibers" which fatigue at an intermediate rate, and "Type 2B muscle fibers" which fatigue very quickly. The main reason the slow-twitch fibers are slow to fatigue is that they contain more mitochondria than fast-twitch fibers and hence are able to produce more energy. Slow-twitch fibers are also smaller in diameter than fast-twitch fibers and have increased capillary blood flow around them. Because they have a smaller diameter and an increased blood flow, the slow-twitch fibers are able to deliver more oxygen and remove more waste products from the muscle fibers (which decreases their "fatigability").
These three muscle fiber types (Types 1, 2A, and 2B) are contained in all muscles in varying amounts. Muscles that need to be contracted much of the time (like the heart) have a greater number of Type 1 (slow) fibers. According to `HFLTA':
When a muscle begins to contract, primarily Type 1 fibers are activated
first, then Type 2A, then 2B. This sequence of fiber recruitment allows
very delicate and finely tuned muscle responses to brain commands. It
also makes Type 2B fibers difficult to train; most of the Type 1 and 2A
fibers have to be activated already before a large percentage of the 2B
fibers participate.
`HFLTA' further states that the the best way to remember the difference between muscles with predominantly slow-twitch fibers and muscles with predominantly fast-twitch fibers is to think of "white meat" and "dark meat". Dark meat is dark because it has a greater number of slow-twitch muscle fibers and hence a greater number of mitochondria, which are dark. White meat consists mostly of muscle fibers which are at rest much of the time but are frequently called on to engage in brief bouts of intense activity. This muscle tissue can contract quickly but is fast to fatigue and slow to recover. White meat is lighter in color than dark meat because it contains fewer mitochondria.
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