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Muscle fibers types and exercise article

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9 months ago #1
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Here is an article I thought it might answer questions such as should I do low reps, high reps and so on. If you have any questions fire up. It's a kind of long article but stick with it to the end.

Muscle Fiber Types and Training

By Jason R. Karp, M.S.

How skeletal muscles adapt to a repeated stimulus depends, to a large extent, on the inherent characteristics of the muscles themselves. Specifically, the types of fibers that make up individual muscles greatly influence the way your athletes will adapt to their training programs. There is a reason why some athletes can sprint faster and get bigger muscles more easily than others, and why some athletes are able to run for much longer periods of time without fatigue. In order to design training pro- grams that will work best for each of your athletes, it is important for the coach to understand at least some of the complexity of skeletal muscles.


Humans have basically three different types of muscle fibers. Slow- twitch (ST or Type I) fibers are identified by a slow contraction time and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content, Energetically, they have a low supply of creatine phosphate (a high-energy substrate used for quick, explosive movements), a low glycogen content, and a wealthy store of triglycerides (the stored form of fat). They contain few of the enzymes involved in glycolysis, but contain many of the enzymes involved in the oxidative pathways (Krebs cycle, electron transport chain). Functionally, ST fibers are used for aerobic activities requiring low-level force production, such as walking and maintaining posture. Most activities of daily living use ST fibers.
Fast-twitch (FT or Type II) fibers are identified by a quick con- traction time and a low resistance to fatigue. The differences in the speeds of contraction that gives the fibers their names can be explained, in part, by the rates of release of calcium by the sarcoplasmic reticulum (the muscle's storage site for calcium) and by the activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head of the contractile proteins. Both of these characteristics are faster and greater in the FT fibers (Fitts & Widrick, 1996; Harigaya & Schwartz, 1969).
Fast-twitch fibers are further divided into fast-twitch A (FT -A or Type IIA) and fast- twitch B (FT -B or Type lIB) fibers. FT -A fibers have a moderate resistance to fatigue and represent a transition between the two extremes of the ST and FT -B fibers. Structurally, FT -A fibers have a large motor neuron and fiber diameter, a high mitochondrial density, a medium capillary density, and a medium myoglobin content. They are high in creatine phosphate and glycogen and medium in triglyceride stores. They have both a high glycolytic and oxidative enzyme activity. Functionally, they are used for prolonged anaerobic activities with a relatively high force output, such as racing 400 meters.
Fast-twitch B fibers, on the other hand, are very sensitive to fatigue and are used for short anaerobic, high force production activities, such as sprinting, hurdling, jumping, and putting the shot. These fibers are also capable of producing more power than ST fibers. Like the FT -A fibers, FT -B fibers have a large motor neuron and fiber diameter, but a low mitochondrial and capillary density and myoglobin content. They also are high in creatine phosphate and glycogen, but low in triglycerides. They contain many glycolytic enzymes but few oxidative enzymes. Table 1 summarizes some major characteristics of the three fiber types.

Table 1: Characteristics of the Three Muscle Fiber Types
Fiber Type Slow Twitch (ST) Fast Twitch A (FT-A) Fast Twitch B (FT-B)
Contraction time Slow Fast Very fast
Size of motor neuron Small Large Very large
Resistance to fatigue High Intermediate Low
Activity used for Ae

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