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  • Muscles are the driving force of all the movements

  • in the body, varying from lifting, running, walking, and even organ function. The muscular

  • system consists of three major muscle types: cardiac muscles, which are responsible for

  • the function of your heart, smooth muscles, which are responsible for the function of

  • all your other organs, and skeletal muscles, which are responsible for, as the name suggests,

  • the movement of your bones. Both smooth and cardiac muscles function involuntarily, meaning

  • they operate by themselves. Skeletal muscles, on the other hand, function voluntarily and

  • are under our conscious control. There are three different skeletal muscle

  • fiber types known as Type I, IIa, and IIx fibers. The difference of size, color, contractual

  • speed, contractual force, and energy source classifies each fiber. Type I fibers, also

  • known as slow twitch fibers, are the smallest fiber types with a darkish red color. It has

  • a fairly slow twitch speed and produces a relatively small amount of force when contracted.

  • It has high amounts of mitochondria, which are orgnelles within each cell that uses oxygen

  • to produce energy. Although the force generated is fairly small, type I fibers are highly

  • fatigue resistant, allowing it to be active for long periods of time. They are the primary

  • fibers used during low-intensity activities with steady oxygen consumption, such as walking,

  • jogging, or aerobics. Type IIa fibers, aka moderate fast-twitch

  • fibers, are also red but intermediate in size. These larger fibers typically use a combination

  • of oxygen and glucose, as sources of energy. This combination allows for quicker contracting

  • speed and higher force output compared to type I fibers, however, fatigue resistance

  • isn't as high. These fibers are typically activated during anaerobic activities that

  • are moderate in duration, such as a mile run, swimming, and short-distance cycling.

  • Type IIx fibers, aka fast-twitch fibers, are white in color due to a low oxygen capacity

  • and by far the largest fiber type. It makes up for the lack of oxidative capacity by having

  • extremely high levels of glucose in its stored form of glycogen, producing the fastest twitch

  • speeds and the most force. The downside, though, is that the fiber fatigues quickly, burning

  • out after 15 to 30 seconds. High-impact, heavy resistance activities such as lifting weights

  • and sprinting, will activate Type IIx fibers the most.

  • For any activity, your muscles follow a certain recruiting order. The slow-twitch, low-force,

  • fatigue-resistant Type I fibers are always activated first. When Type I fibers are maxed

  • out, Type IIa fibers are activated, and then after those are maxed out, Type IIx fibers

  • are then activated. This order, known as Henneman's size principle, helps minimize muscle fatigue

  • and allows precise motor control by using no more than the force necessary to complete

  • a movement.

  • Everyone has a genetically determined amount of each muscle fiber type. Some people are

  • born with a predominant amount of a certain muscle fiber, making them effective with activties

  • that favor those fibers. There have been some findings that suggests that type IIx fibers

  • can change into type IIa fibers with proper training, however, this might simply be due

  • to type IIx fibers showing higher oxygen capacity through physical adaptation. They are, ultimately,

  • still Type IIx fibers. Based on your exercises, which muscle fibers

  • do you primarily train? Leave your answers in the comment section below!

Muscles are the driving force of all the movements

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C2 US fiber muscle oxygen fatigue skeletal activated

Muscle Fibers Explained - Muscle Contraction and Muscle Fiber Anatomy

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