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!