What Are Fast-Twitch Muscle Fibers?

If you were to look at your muscles under a powerful microscope, you would see that they are made up of a bunch of different fibers.   These fibers, which are called myocytes, are made up of small strands of proteins which are called myofibrils. These strands are what give your muscle strength and allow them to move so well. The individual proteins in the strands can pull on each other, allowing them to contract.

Take an even closer look at your muscles and you will realize that not all of the fibers are the same. There are three types of fibers in your muscles: Fast Twitch Type I, Fast Twitch Type II, and Slow Twitch muscle fibers.


Why “Slow” and “Fast” Twitch?

The reason that muscle fibers are called “slow” and “fast” twitch is because of how fast they are able to contract, or twitch.

Slow twitch muscle fibers generally contract at a twitch rate of 10-30 per second. By contrast, fast-twitch muscle fibers generally have a twitch rate of 30-70 twitches per second.

If you remember back to your physics class, energy is related to speed. Basically, the faster a force occurs, the more energy will be produced. Because of this property, fast-twitch muscle fibers are used for tasks which require explosive energy – like being able to jump high.

It is very important to note that fast-twitch muscle fibers do NOT produce more force than slow-twitch muscle fibers. Their abilities to produce explosive force all has to do with their ability to contract much faster than slow-twitch muscle fibers.


Properties of Slow and Fast Twitch Muscle Fibers

Slow-Twitch Muscle Fibers

  • Contract slowly
  • Small size
  • Use energy efficiently
  • High amount of mitochondria
  • Uses fat stores for energy
  • Good for endurance activities

Fast-Twitch Muscle Fibers Type A 

  • Contract quickly
  • Medium size
  • Use fat and glycogen stores for energy
  • Resist fatigue and recover quickly
  • Good for quick, low-intensity repetitive activities like lifting lighter weights
  • Medium amount of mitochondria

Fast-Twitch Muscle Fibers Type B

  • Contract very quickly
  • Very large size
  • Tire very quickly
  • Low amount of mitochondria

Because fast-twitch muscle fibers have to act so quickly, they use up a lot of energy. Thus, the body mostly relies on its slow-twitch muscle fibers. These muscle fibers are very efficient in using energy and don’t tire as easily.

For training purposes, you really don’t need to know the differences between Type A and B Fast-Twitch Muscle fibers. Just know that Type B is more important for explosive energy, whereas Type A will help you produce less-explosive results over an extended period of time.


Fast and Slow-Twitch Muscle Fiber Composition in the Body

Most people have a balance of fast and slow-twitch muscle fibers in their body.   It has been noted, however, that some athletes have more of one type of muscle than another. For example, Olympic high jumpers tend to have more fast-twitch fibers whereas distance runners tend to have more slow-twitch muscle fibers.

There is a lot of debate as to whether athletes can alter the composition of their muscle fibers through training, or whether some people simply have more of one type of muscle fiber due to genetics (thus making them better able to play certain sports). However, most evidence does support the idea that you can turn one muscle fiber type into another. For example, biopsies have shown that endurance runners have almost no Type B Fast-Twitch muscle fibers.   It appears that their Type B fibers are turning into Type A fibers.  However, we do know that Type B fibers will not turn into Slow-Twitch fibers, which is good news for athletes like basketball players and jumpers.

What is clear is that it takes a very specific type of workout to build up your Fast-Twitch muscle fibers, and also a proper diet to support the energy they require. This is what is talked about in Vert Shock in more detail.  Even if you are unable to turn one type of muscle fiber into another, the workouts can still help you make  your fast-twitch muscle fibers stronger so you have more explosive energy.


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