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Each time you take a step,
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200 muscles work in unison
to lift your foot,
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propel it forward,
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and set it down.
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It's just one of the many thousands
of tasks performed by the muscular system.
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This network of over 650 muscles
covers the body
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and is the reason we can blink,
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smile,
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run,
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jump,
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and stand upright.
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It's even responsible for
the heart's dependable thump.
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First, what exactly
is the muscular system?
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It's made up of three main muscle types:
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skeletal muscle, which attaches
via tendons to our bones,
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cardiac muscle, which is only
found in the heart,
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and smooth muscle, which lines
the blood vessels and certain organs,
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like the intestine and uterus.
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All three types are
made up of muscle cells,
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also known as fibers,
bundled tightly together.
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These bundles receive signals from
the nervous system
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that contract the fibers, which in turn
generates force and motion.
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This produces almost all
the movements we make.
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Some of the only parts of the body
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whose motions aren't governed
by the muscular system
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are sperm cells,
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the hair-like cilia in our airways,
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and certain white blood cells.
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Muscle contraction can be split
into three main types.
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The first two, shortening muscle fibers
and lengthening them,
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generate opposing forces.
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So the biceps will shorten
while the triceps will lengthen or relax,
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pulling up the arm and making it
bend at the elbow.
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This allows us to, say, pick up a book,
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or if the muscle relationship
is reversed, put it down.
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This complimentary partnership
exists throughout the muscular system.
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The third type of contraction
creates a stabilizing force.
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In these cases, the muscle fibers
don't change in length,
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but instead keep the muscles rigid.
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This allows us to grip a mug of coffee
or lean against a wall.
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It also maintains our posture
by holding us upright.
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Skeletal muscles form the bulk
of the muscular system,
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make up about 30-40% of the body's weight,
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and generate most of its motion.
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Some muscles are familiar to us,
like the pectorals and the biceps,
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Others may be less so,
like the buccinator,
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a muscle that attaches your cheek
to your teeth,
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or the body's tiniest skeletal muscle,
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a one-millimeter-long tissue fragment
called the stapedius
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that's nestled deep inside the ear.
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Wherever they occur, skeletal muscles are
connected to the somatic nervous system,
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which gives us almost complete control
over their movements.
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This muscle group also contains
two types of muscle fibers
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to refine our motions even further,
slow-twitch and fast-twitch.
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Fast-twitch fibers react instantly
when triggered
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but quickly use up their energy
and tire out.
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Slow-twitch fibers, on the other hand,
are endurance cells.
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They react and use energy slowly
so they can work for longer periods.
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A sprinter will accumulate more
fast-twitch muscles in her legs
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through continuous practice,
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enabling her to quickly, if briefly,
pick up the pace,
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whereas back muscles contain
more slow-twitch muscles
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to maintain your posture all day.
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Unlike the skeletal muscles, the body's
cardiac and smooth muscles
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are managed by
the autonomic nervous system
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beyond our direct control.
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That makes your heart thumb roughly
3 billion times
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over the course of your life,
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which supplies the body
with blood and oxygen.
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Autonomic control also contracts
and relaxes smooth muscle
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in a rhythmic cycle.
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That pumps blood through the smooth
internal walls of blood vessels,
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enables the intestine to constrict and
push food through the digestive system
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and allows the uterus to contract
when a person is giving birth.
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As muscles work, they also use energy
and produce an important byproduct, heat.
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In fact, muscle provides
about 85% of your warmth,
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which the heart and blood vessels
then spread evenly across the body
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via the blood.
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Without that, we couldn't maintain
the temperature necessary
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for our survival.
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The muscular system
may be largely invisible to us,
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but it leaves its mark on almost
everything we do,
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whether its the blink of an eye
or a race to the finish line.