The muscular system is composed of specialized
cells called muscle fibers. Their predominant function
is contractibility. Muscles, where attached to bones
or internal organs and blood vessels, are responsible
for movement. Nearly all movement in the body is the
result of muscle contraction. Exceptions to this are
the action of cilia,
the flagellum
on sperm cells, and amoeboid
movement of some white blood cells.
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The integrated action of joints, bones, and skeletal muscles
produces obvious movements such as walking and running. Skeletal
muscles also produce more subtle movements that result in
various facial expressions, eye movements, and respiration.
In addition to movement, muscle contraction also fulfills
some other important functions in the body, such as posture,
joint stability, and heat production. Posture, such as sitting
and standing, is maintained as a result of muscle
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contraction. The skeletal muscles are
continually making fine adjustments that hold the body
in stationary positions. The tendons of many muscles
extend over joints and in this way contribute to joint
stability. This is particularly evident in the knee
and shoulder joints, where muscle tendons are a major
factor in stabilizing the joint. Heat production, to
maintain body temperature, is an important by-product
of muscle metabolism. Nearly 85 percent of the heat
produced in the body is the result of muscle contraction.
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