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Fast Twitch Muscle Fibers vs. Slow Twitch Muscle Fibers...What it All Means E-mail
Written by Jeff Behar, MS, MBA   

Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.

It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorized into Type IIa and Type IIb fibers.

These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types. On average, we have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.

Slow Twitch 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 which is a high-energy substrate used for quick, explosive movements. They also have low glycogen content, and an abundant amount of triglycerides (the stored form of fat). Type 1 fibers 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, slow-twitch fibers are used for aerobic activities requiring low-level force production, such as walking and maintaining posture. Most activities of daily living use slow-twitch fibers.

Fast Twitch Fibers 


Fast-twitch (FT or Type II) fibers are characterized by quick contraction times and a low resistance to fatigue. The activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head of the contractile proteins of these fibers enable these fibers to contract quicker than Type 1 fibers. Fast-twitch fibers 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.

Slow-twitch muscle fibers, have the lowest firing threshold and are recruited first. Demands for larger forces are met by the recruitment of increasingly larger motor units. The largest motor units that contain the fast-twitch fibers have the highest threshold and are recruited last. If your workout intensity is low, slow twitch these motor units may be the only ones that are recruited. If the workout intensity is high, such as when lifting heavy weights or performing drop down sets slow- twitch motor units are recruited first, followed by fast-twitch A and fast- twitch B, if needed.

Every one's proportion of both is different. Which do YOU have more of?

An indirect method that can be used in the weight room to determine the fiber composition of a muscle (rather than a muscle biopsy) is to perform as many repetitions at 80% of your max weight as you can. If you do fewer than seven repetitions, then the muscle group is likely composed of more than 50% fast-twitch fibers. If they can perform 12 or more repetitions, then the muscle group has more than 50% slow-twitch fibers. If you can do between 7 and 12 repetitions, then the muscle group probably has an equal proportion of fibers.

Because lifting weights requires the use of many muscles at once, this method does not work for individual muscles, just muscle groups. In order to determine the fiber-type composition of an individual muscle, a needle biopsy of the muscle of interest must be performed.

Your fiber type proportion will play a major role in the amount of weight you can lift, the number of repetitions that you can complete in a set or interval workout, and the amount of muscle mass that you will be able to develop. For example, an bodybuilder with a greater proportion of slow-twitch fibers will not be able to lift as heavy as will an a bodybuilder with a greater proportion of fast-twitch fibers and therefore will never attain as high a level of muscularity or as strong as will the fast twitch - fibered bodybuilder.

Although the type of fiber cannot be changed from one to another, training can change the amount of area taken up by the fiber type in the muscle. In other words, there can be a selective hypertrophy of fibers based on the type of training.
Therefore, by recruiting more fibers through high intensity (be it more reps. or more weight) can increase muscle mass. So they key here is INTENSITY (not just heavy weight).

Depending on the specific intensity used in training, the muscle may change to a 75% fast twitch area and a 25% slow twitch area. So when more fast twitch muscle fibers are recruited the mass of fast twitch fibers will be greater than that of slow twitch fibers. This will result in a muscle mass gain (measured by the circumference of the muscle).

Conversely, if you train for high rep muscular endurance, the fast-twitch fibers will atrophy while the slow-twitch fibers hypertrophy, causing a greater area of slow-twitch fibers. The endurance capabilities of the muscle will then increase proportionately while its strength and mass will decrease (you lose mass). This is why you get weak and loses some size quickly when you stop training (because slow-twitch fibers are lower in mass than fast-twitch fibers).

Training with a low or moderate intensity will not necessitate the recruitment of the fast-twitch -B muscle fibers. Therefore, the training intensity must, be high. But how heavy a weight and how many repetitions should you use?
So, for gains in muscular strength and mass you should train with heavy weights and few repetitions because using heavy weights recruits the fast-twitch fibers, which are capable of producing a greater force than the slow-twitch fibers. Remember hypertrophy (muscle breakdown needed for muscle growth) will only occur in those muscle fibers that are overloaded.

To increase maximum strength your goal will be to get 5 to 8 reps with 80% of your max. If the aim of training is to increase muscle size (hypertrophy) with moderate gains in strength, then 6 to 12 repetitions should be used.




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