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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 complementary 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 thump
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 it's the blink of an eye
or a race to the finish line.