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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!

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Muscle Fibers Explained - Muscle Contraction and Muscle Fiber Anatomy

374 Folder Collection
Linda Chung published on November 12, 2018
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