Fast Twitch and Slow Twitch Muscle Fibers

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I have had a question that is interesting about the difference between different muscle fibers and tried to get a good answer. Here is the differences between fast twitch and slow twitch muscle fibers and the difference that they make to your workouts and fitness level.

One slightly confusing area of exercise is the idea that a muscle can do both push a lot of once or move many times. Look at the difference between doing a set of squats compared to running or the difference in the leg muscles of a bodybuilder compared to a triathlete.

All types of skeletal muscle are constructed from densely knit fibers. These muscle fibers can be looked at as Fast Twitch and Slow Twitch muscle fibers. Which kind are you looking for when you are doing s? Which ones do you want to have when you are running distance?

Slow Twitch Muscle Fibers (Type 1 Muscle fibers)

The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Slow twitch muscle fibers are

  • smaller in diameter
  • red in color
  • depend on oxidative phosphorylation for their ATP supply
  • are highly vascularized (better blood supply)
  • have more mitochondria
  • more myoglobin

Fast Twitch Muscle Fibers (Type 2 Muscle fibers)

Muscle Fibers

Muscle Fibers

Fast twitch fibers are responsible for the speed of muscular contraction, and a fast twitch response is the ability of a muscle to rapidly contract to a specific distance over a short period of time.

Therefore, any training program that conditions your muscles to go from a state of complete relaxation to an immediate state of contraction is a speed training program.

Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

Fast twitch muscle fibers

  • larger
  • white
  • glycolysis is the source of ATP
  • less vascularized
  • less mitochondria
  • less myoglobin

Say you were to help someone lift a heavy couch up a flight of ten stairs. You would use your hands as grips and let your legs do all the work. On the first step your legs will start to recruit type IIa fibers. By the 2nd or 3rd step your nervous system does not recruit more motor units. This being the case the first set of fibers rest and more type IIa fibers are recruited. Along with these, a number of type IIb fibers are called into play (to maintain fluent motion up the stairs).

As your journey continues more type IIa and type IIb fibers are recruited until by the last step they have all come into play. Your muscle fibers weren’t twitching at maximum speed until the end of the stairs when they neared failure. The faster a muscle fiber twitches the greater the force is. At the beginning, the fibers weren’t forced to twitch at maximum frequency to overcome the , but at the end they had to produce as much force as possible to overcome the . This is how recruitment is designed to maintain a certain amount of force.

Fast-twitch fibers are activated by their neurons at a rate ten times faster than the rate of activation for slow-twitch fibers. The distribution of fast- versus slow-twitch fibers in the muscles is primarily an inherited characteristic, determined by the genetic coding of each person. While it is common for a person to have muscles with a relatively even distribution of fast-and slow-twitch fibers, some persons inherit a tendency to a significantly greater number of one type of fiber over the other. Such persons tend to excel in the sports best suited to their muscular composition.

I have to thank a couple of sites for some of this info. Especially bodybuilding.com and Sports Medicine about.com as well Wiki Answers.

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