Muscle shapes
Muscles have four general shapes. When
you use these muscles, whether in sports or
in your everyday routine, try to picture their
movement as the shape described.
n
Circular muscles are ring-shaped and
located around your mouth and eyes. The
muscles around your eyes are unique because
they are both voluntary and involuntary. Why
do you think that is? Think about all of the
important movements these tiny muscles
make every day.
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The involuntary muscles in your stomach,
called your abdominals, are an example of flat
muscles. Your forehead muscles are also flat
muscles. Both of these muscles stretch flat
across these parts of your body.
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Your biceps and triceps muscles in your
arms are shaped like spindles, thick in the
middle and thin at the ends. When you flex
your bicep, for example, you can see the bulge
in the middle of the muscle. Another name for
spindle-shaped muscles is fusiform muscles.
n
Your deltoid and pectoral muscles, found
across your shoulders, are triangular in shape.
When you lift an item from a high shelf or just
raise your hand in class, you are using your
deltoids and pecs. These groups of muscles
are called triangular because the origin of the
muscle (the part that attaches to the bone) is
wider than the other end. Triangular muscles
are also called convergent muscles.
Your muscles:
Keeping
you strong and active!
J
ust as there are different types of joints,
your body also has different types of
muscles. Voluntary, or skeletal, muscles
are controlled by you. When you
consciously move, you are working your
voluntary muscles. They are made of long
layers of fibers that look like light and dark
stripes when viewed under a microscope.
Voluntary muscles make up about 40 percent
of an adult’s body weight. They are connected
to your bones by tendons, or cords made of
strong tissue. Tendons enable your bones to
move when you contract your muscles.
Voluntary muscles work in groups each time
you move. The muscle that creates the move-
ment is called the agonist, or prime mover.
The agonist works with the antagonist,
a muscle that creates the opposite effect on
the bone. You might have heard of protago-
nists and antagonists in stories. While those
opposing forces work against each other,
your agonist and antagonist muscles are
opposing forces that work together to
create a movement.
Involuntary muscles, on the other hand,
move by themselves. For example,
muscles in your heart, called
cardiac muscles, keep your
blood flowing. Unlike most
other muscles, cardiac
muscles are connected
only to each other,
and not to bone.
Cardiac muscles can stretch only in certain
ways like other voluntary muscles, but have
the powerful force of voluntary muscles.
Involuntary muscles in your digestive system
are hard at work after the voluntary muscles
in your mouth chew food. Most of the
muscles of the organs in your digestive
system are called smooth muscles because
of their smooth, uniform appearance
(in contrast to the striped
appearance of volun-
tary muscles).
Your lower jawbone is the only bone in your head you can move.
