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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 can vary greatly.
Slow twitch (Type 1)
Slow-twitch fibres are good for endurance activities like maintaining
posture or long distance running which require long term or repeated
contraction. The ATP required for slow-twitch fibre contraction is generated
through aerobic respiration (glycolysis and Krebs cycle) whereby 30 molecules
of ATP are produced from each one of glucose in the presence of oxygen. The reaction
is slower than anaerobic respiration, which is why it is 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 oxygen
to the muscle which can rapidly become rate limiting if the respiratory and
circulatory systems cannot keep up.
Due to this requirement for large amounts of oxygen slow-twitch fibers are associated with large numbers of blood vessels, mitochondria and
high concentrations of myoglobin, an oxygen binding protein related to hemoglobin
found in the blood, which lends them a red coloration. An example muscle with a
large number of slow-twitch fibers would be the soleus muscle (~80% slow-twitch) of the leg which plays a key role in standing.
Fast twitch (Type II)
Fast-twitch fibers are good for rapid movements like jumping
or sprinting which 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. Whilst much
less efficient than aerobic respiration it is not rate limited by a requirement
for oxygen making it ideal for rapid bursts of movement. A by-product of anaerobic
respiration is lactate (lactic acid) which accumulates in the muscle tissue
reducing the pH (making it more acidic, and producing the stinging feeling in
muscles when exercising) which inhibits further anaerobic respiration. Whilst
this may seem counter-intuitive it is in fact a feedback cycle in place to
protect the muscles from over-exertion and damage.
As fast-twitch fibers generally do not require oxygenation
they contain fewer blood vessels and mitochondria than slow-twitch fibers and less
myoglobin giving them a paler colour. Muscles controlling eye movements contain
high numbers of fast-twitch fibers (~85% fast-twitch).
Determination and alteration of muscle type
There is some evidence that the proportion of fast-twitch
versus slow-twitch muscles of an individual is partly genetic in nature. That
is, we are born with a unique proportion of such muscles that suit us to
particular types of physical activity. This is not without debate, however.
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 fiber composition and number associated with
improvements in associated systems such as respiration and circulation.