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!