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Muscles are the driving force of all the movements
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in the body, varying from lifting, running, walking, and even organ function. The muscular
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system consists of three major muscle types: cardiac muscles, which are responsible for
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the function of your heart, smooth muscles, which are responsible for the function of
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all your other organs, and skeletal muscles, which are responsible for, as the name suggests,
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the movement of your bones. Both smooth and cardiac muscles function involuntarily, meaning
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they operate by themselves. Skeletal muscles, on the other hand, function voluntarily and
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are under our conscious control. There are three different skeletal muscle
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fiber types known as Type I, IIa, and IIx fibers. The difference of size, color, contractual
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speed, contractual force, and energy source classifies each fiber. Type I fibers, also
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known as slow twitch fibers, are the smallest fiber types with a darkish red color. It has
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a fairly slow twitch speed and produces a relatively small amount of force when contracted.
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It has high amounts of mitochondria, which are orgnelles within each cell that uses oxygen
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to produce energy. Although the force generated is fairly small, type I fibers are highly
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fatigue resistant, allowing it to be active for long periods of time. They are the primary
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fibers used during low-intensity activities with steady oxygen consumption, such as walking,
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jogging, or aerobics. Type IIa fibers, aka moderate fast-twitch
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fibers, are also red but intermediate in size. These larger fibers typically use a combination
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of oxygen and glucose, as sources of energy. This combination allows for quicker contracting
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speed and higher force output compared to type I fibers, however, fatigue resistance
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isn't as high. These fibers are typically activated during anaerobic activities that
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are moderate in duration, such as a mile run, swimming, and short-distance cycling.
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Type IIx fibers, aka fast-twitch fibers, are white in color due to a low oxygen capacity
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and by far the largest fiber type. It makes up for the lack of oxidative capacity by having
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extremely high levels of glucose in its stored form of glycogen, producing the fastest twitch
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speeds and the most force. The downside, though, is that the fiber fatigues quickly, burning
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out after 15 to 30 seconds. High-impact, heavy resistance activities such as lifting weights
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and sprinting, will activate Type IIx fibers the most.
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For any activity, your muscles follow a certain recruiting order. The slow-twitch, low-force,
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fatigue-resistant Type I fibers are always activated first. When Type I fibers are maxed
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out, Type IIa fibers are activated, and then after those are maxed out, Type IIx fibers
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are then activated. This order, known as Henneman's size principle, helps minimize muscle fatigue
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and allows precise motor control by using no more than the force necessary to complete
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a movement.
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Everyone has a genetically determined amount of each muscle fiber type. Some people are
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born with a predominant amount of a certain muscle fiber, making them effective with activties
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that favor those fibers. There have been some findings that suggests that type IIx fibers
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can change into type IIa fibers with proper training, however, this might simply be due
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to type IIx fibers showing higher oxygen capacity through physical adaptation. They are, ultimately,
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still Type IIx fibers. Based on your exercises, which muscle fibers
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do you primarily train? Leave your answers in the comment section below!