What are characteristics and/or functions of fast and slow twitch muscle fibers?

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Many differences exist when looking at the characteristics and functions of both slow and fast twitch skeletal muscle fibers. Classically, muscle fibers are broken into four subtypes. Slow twitch fibers, and three unique types of fast twitch fibers, known as fast fatigue resistant, fast fatiguable, and fast intermediate, respectively. 

Slow twitch muscle fibers show a number of characteristics different from the other fast twitch muscle fiber types. These muscle fibers are also known as type I muscle fibers. Slow twitch muscle fibers will contract much more slowly than fast twitch, however, they also have a very high resistance to fatigue. This is because they contain a form of myosin that uses ATP very slowly, leading to slow contraction. These fibers contain more enzymes for oxidative pathways, but use less ATP than fast twitch fiber types. They function to play a role when contraction needs to be maintained over a long period of time, for example in maintaing posture. 

Fast twitch fibers, often also described as type II muscle fibers, contract much more quickly than slow twitch fibers. This is because fast twitch fibers make use of a different form of myosin that can use ATP rapidly, creating quick "fast" contraction of the fibers.

The first subtype of the fast twitch muscle fibers is the type IIA, or fast, fatigue resistant type. These fibers contain many mitochondria which can generate large numbers of ATP, allowing them to maintain fast contraction over a prolonged period of time. 

The second subtype of fast twitch muscle fibers is the type IIB, also known as fast fatiguable muscle fibers. These fibers also use up ATP rapidly to contract fast, but do not have the oxidative capacity (i.e. not as many mitochondria) to generate the ATP necessary to contract for long periods of time. Because of this, they rely on anaerobic glycolysis to generate the majority of their ATP. Due to this factor, these fiber types are activated for short durations at maximal intensities.

Finally, the final group of the fast twitch muscle fibers is known as the fast intermediate group. This group serves as a classification type for all the fast twitch fibers that do not traditionally fit under the definition of type IIA or type IIB fibers. These muscle fiber types contract fast, but do generate as much force as type IIB fibers, and cannot contract for as long as type IIA fibers.

Hope this helps!

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