While all motor units behave in the same
way, this cannot be said for all muscle fibers. Not all muscle fibers
have the same biochemical (metabolic) functions. Every muscle fiber can
function under both anaerobic and aerobic conditions, although some
muscle fiber work better under anaerobic conditions, while others muscle
fiber work better under aerobic conditions.
The muscle fibers that rely on and use
oxygen to produce energy are aerobic, type I, red, or slow-twitch (ST)
fibers. The muscle fibers that do not require oxygen are anaerobic, type
II, white, or fast-twitch (FT) muscle fibers. Low-twitch (ST) and
fast-twitch (FT) fibers exist in relatively equal proportions within the
body, and this 50/50 relationship is not thought to be greatly affected
by strength training and bodybuilding.
The recruitment of muscle fibers is
load-dependent. During moderate and low intensity activity, slow-twitch
(ST) fibers are recruited as work horses. As the load increases, a
greater number of FY fibers are activated during a contraction.
The distribution of muscle fiber types
can vary, both within the same muscle and among different muscle.
Usually, the arms tend to have a higher percentage of fast-twitch (FT)
fibers than the legs: biceps 55% fast-twitch (FT), triceps 60%
fast-twitch (FT), whereas soleus (calves) 24% fast-twitch (FT) muscle
Fiber type composition (i.e. proportion
of fast-twitch (FT)T fibers within a muscle) plays an important role in
strength training and bodybuilding. Muscles containing a high percentage
of fast-twitch (FT) fibers are capable of quicker and more powerful
There are no clear differences in
muscle fiber distribution between female and male athletes. An inherited
propensity toward more fast-twitch (FT) muscle fibers might indicate
which individuals are genetically suited to strength training and
bodybuilding. While genetics is an important factor in determining
success, it is not the only one. Regardless of one's genetic endowments,
every individual, through intensive training and proper nutrition, can
improve his or her muscle size, tone and definition.
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