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Types of Muscle Fibers
flow necessary to supply oxygen to muscles during exercise. Smooth muscle also supplies the visceral organs in the gastrointestinal tract.
Skeletal muscle will generate heat because of the breakdown of ATP. Whenever ATP is broken down, heat is produced. This heat is easily noticed during exercise, particularly when there is sustained muscle contraction. When there is extreme cold, muscles will shiver due to random skeletal muscle contractions. This shivering expends ATP energy and produces heat necessary to keep the body warm when it is challenged by a cold environment.
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TYPES OF MUSCLE FIBERS
There are three different types of muscle fibers in the human body. There are slow oxidative fibers or SO fibers that contract relatively slowly, using the process of aerobic respiration in order to produce ATP. There are also fast oxidative fibers or FO fibers that contract quickly and use both aerobic and anaerobic (glycolytic) respiration. These tend fatigue more quickly than SO fibers. Finally, there are fast glycolytic fibers or FG fibers that have fast contractions but use anaerobic glycolysis to gain energy. These muscles fatigue the fastest. You should know that humans contain different proportions of these types of muscles.
The speed of contraction will depend on how quickly the ATPase associated with the myosin molecule can cross-bridge and reload. Fast fibers will hydrolyze ATP approximately twice as quickly as slow fibers, resulting in a much quicker cycling process. If a fiber uses ATP made by aerobic pathways, it is considered oxidative. This will be a fiber that is more resistant to fatig0ue. Glycolytic fibers will use ATP made by glycolysis, which does not require oxygen, work more quickly, and fatigue the fastest.
The oxidative fibers will contain more mitochondria than glycolytic fibers because these organelles are the source of ATP energy used by these cells. On the other hand, these tend to be small diameter muscle cells that do not produce a large amount of tension. There is intense blood supply to these cells and a greater amount of myoglobin, which houses the oxygen necessary for these cells. This is why these fibers are called “red muscle.”
These SO fibers or slow oxidative fibers can function for a long period of time, producing isometric contractions that stabilize bones and joints, as well as contractions that happen on a regular basis. They are not used for powerful and fast movement that require rapid cross-bridge cycling. The FO fibers or fast oxidative fibers are considered intermediate fibers that will produce ATP more quickly than slow oxidative fibers and also have high amounts of mitochondria. They do not have a lot of myoglobin in them so their color is lighter. FO fibers are used for walking and other movement but are not good for explosive movement.
FG or fast glycolytic fibers use anaerobic glycolysis as the main source of ATP. Their diameter is large and there is a lot of glycogen in these fibers. There are not a lot of mitochondria in these fibers and not a lot of myoglobin so they are called white muscle fibers. They are good for quick and powerful movements but fatigue quickly. Note that most major muscles will have a mixture of each type of fiber with the predominant fiber type in a muscle determining the primary function of the specific muscle in question.
Red muscle fibers are “slow” fibers that are activated when cycling, marathon running, swimming, and in muscle tone maintenance. Their main energy source is beta-oxidation of fatty acids. As the intensity of the exercise increases, there will be increased muscle dependence on carbohydrates as an energy source.
White muscle fibers are “fast” fibers that are rich in the enzymes that participate in glycolysis as well as in glycogen itself. This is the type of muscle fiber used in sprinting and weight-lifting. The biggest danger of this type of exercise is that it builds up lactic acid, resulting in the fatigue and muscle pain seen when the activity goes on too long. Lactate will be released from these muscles into the bloodstream, where it is carried to the liver in order to be converted back into glucose in a process called the gluconeogenesis. This is called the Cori cycle and allows lactate to serve as an energy source for the body.
Hormones will affect this type of muscle metabolism in various ways. Insulin will increase the entry of glucose and fatty acids into the muscle cells, which explains why diabetics have difficulty with muscle metabolism. Insulin also activates the anabolic processes in the body, which allows for the formation of proteins, glycogen, and
