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Skeletal muscle fibers can be further subdivided into slow
and fast-twitch subtypes depending on their metabolism and corresponding
action. Most muscles are made up of combinations of these fibers, although the
relative number substantially varies.
Slow Twitch (Type 1)
Slow-twitch fibers are designed for endurance activities that require long-term, repeated contractions, like maintaining
posture or running a long distance. The ATP required for slow-twitch fiber contraction is generated
through aerobic respiration (glycolysis and Krebs cycle), whereby 30 molecules
of ATP are produced from each glucose molecule in the presence of oxygen. The reaction
is slower than anaerobic respiration and thus not suited to rapid
movements, but much more efficient, which is why slow-twitch muscles do not tire
quickly. However, this reaction requires the delivery of large amounts of oxygen
to the muscle, which can rapidly become rate-limiting if the respiratory and
circulatory systems cannot keep up.
Due to their large oxygen requirements, slow-twitch fibers are associated with large numbers of blood vessels, mitochondria, and
high concentrations of myoglobin, an oxygen-binding protein
found in the blood that gives muscles their reddish color. One muscle with many slow-twitch fibers is the soleus muscle in the leg (~80% slow-twitch), which plays a key role in standing.
Fast Twitch (Type II)
Fast-twitch fibers are good for rapid movements like jumping
or sprinting that require fast muscle contractions of short duration. Unlike
slow-twitch fibers, fast twitch-fibers rely on anaerobic respiration (glycolysis
alone) to produce two molecules of ATP per molecule of glucose. While much
less efficient than aerobic respiration, it is ideal for rapid bursts of movement since it is not rate limited by need for oxygen. Lactate (lactic acid), a byproduct of anaerobic respiration, accumulates in the muscle tissue
reducing the pH (making it more acidic, and producing the stinging feeling in
muscles when exercising). This inhibits further anaerobic respiration. While this may seem counter-intuitive, it is a feedback cycle in place to
protect the muscles from over-exertion and resultant damage.
As fast-twitch fibers generally do not require oxygenation, they contain fewer blood vessels and mitochondria than slow-twitch fibers and less
myoglobin, resulting in a paler colour. Muscles controlling eye movements contain
high numbers of fast-twitch fibers (~85% fast-twitch).
Determination and Alteration of Muscle Type
While there is evidence that each person has a unique proportion of fast-twitch
versus slow-twitch muscles determined by genetics, more research is required.
Regardless, repeated exercise that prioritizes one type of muscle fiber use
over the other can lead to improvements in an individual’s ability to perform
that activity through alterations in the number and composition of fibers associated with
improvements in the respiratory and circulatory systems.