The Muscular System
What Are Muscles Made of?
Since you have 650 muscles in your body, you might think that all together,
they're pretty heavy. And you're right! Your muscles make up about one half of
your body's weight. For example, that means if you weigh 60 pounds, about 30
pounds of you is pure muscle!
Muscles are all made of the same material, a type of muscle fiber that is
elastic (sort of like the material in a rubber band). Hundreds or even thousands
of these small fibers make up each muscle. You have three different types of
muscles in your body: smooth muscle, cardiac muscle, and skeletal muscle.
Smooth Muscles
Smooth muscles are sometimes also called involuntary muscles. You can't control
this type of muscle - these muscles work involuntarily, which means that your
brain and body tell these muscles what to do without you even thinking about it.
You can't use your smooth muscles to make a muscle in your arm or jump into the
air.
So what are smooth muscles good for then, you ask? Lots! Smooth muscles are in
your stomach, and they work by contracting (this means they tighten up) and
relaxing to allow food to pass into your small intestine. If you're like a lot
of kids and you've ever thrown up, then you've felt the smooth muscles in your
stomach at work. The muscles push the food back out of the stomach so it comes
up through the esophagus and out of the mouth. Because they are involuntary
muscles, you don't have any say in the deal.
All smooth muscles are usually set up in sheets, with one behind the other.
Smooth muscles are also found in your bladder, and they contract to allow you to
hold urine in until you're ready to urinate. They are also found in a woman's
uterus, which is where a baby develops - these involuntary muscles help to push
the baby out of his mother's body when it's time for him to be born. Smooth
muscles are at work behind the scenes in your eyes to keep them focused. And
tiny smooth muscles are even found in the walls of some of your blood vessels.
Skeletal Muscles
Skeletal muscles are also sometimes called striated (stry-ay-ted) muscle
(striated is a fancy word meaning striped), because the light and dark parts of
the muscle fiber make them look striped.
Skeletal muscles are also known as voluntary muscles - these are the muscles
that you can control. Your leg won't bend to kick the ball unless you want it
to, and your dog's dinner won't find its way into the dish unless you tell your
muscles to make it happen.
These muscles help to make up the musculoskeletal (muss-kew-low-skell-it-ull)
system - the combination of your muscles and your skeleton. Together, the
skeletal muscles work with your bones to give your body power and strength. In
most cases, a skeletal muscle is attached to one end of a bone. It stretches all
the way across a joint (the "hinged" place where two bones meet), and then
attaches again to another bone.
Want to see a skeletal muscle in action? Try making a muscle with your arm. The
muscle in the top half of your arm contracts, and it pulls the bones in the
lower half of your arm along with it. After you make the muscle, let your arm
relax. What happens to the lower part of your arm as you relax your upper arm?
Now try bending your leg, as if you were getting ready to kick a ball. The
muscles in the back of your thigh contract, and they pull your leg bones up and
back. Pretend to kick that imaginary ball now - and as you relax the muscles in
back of your thigh and contract the muscles in the front, the bones in your
lower leg will come down.
Skeletal muscles come in many different sizes and shapes to allow them to do
many types of jobs. The biggest and most powerful muscles are in your back, near
your spine. These muscles help keep you upright and standing tall. They also
give your body the power it needs to lift and push things. So the next time you
need to push that dog of yours into the car for his vet appointment, say thank
you to your back muscles!
Muscles in your neck and the top part of your back aren't as large, but they are
capable of some pretty amazing things: try rotating your head around, back and
forth, and up and down to feel the power of the muscles in your neck. These
muscles also hold your head high.
Experiment to see what you can do with other muscles in your body - how many
directions can you move them in? Try doing an imaginary hula-hoop to see how the
muscles around your abdomen and hips work. Sit cross-legged to check out how the
biggest muscles in your legs pull on the bones to get them in the right
position.
And most importantly, don't forget to find a mirror and check out the muscles in
your face! The muscles in people's faces don't all attach directly to bone like
they do in the rest of the body; instead, many of them attach under the skin.
(The same goes for some other primates like gorillas, monkeys, and chimpanzees.)
This allows you to contract your facial muscles just a tiny bit and make dozens
of different kinds of faces. Even the smallest movement can turn a smile into a
frown. You can raise your eyebrow to look surprised, or wiggle your nose. And
while you're looking at your face, don't pass over your tongue - a muscle that's
only attached at one end! Your tongue is actually made of a group of muscles
that work together to allow you to talk and help you chew food.
Tendons
Skeletal muscles are held to the bones with the help of tendons.
Tendons are cords made of tough tissue, and they work as special connector
pieces between bone and muscle. The tendons are attached so well that when you
contract one of your muscles, the tendon (and bone) moves along with it.
Some parts of the body have very small bones and not a lot of space, so the
tendons in these parts are specially shaped to fit the area. Can you guess what
parts? If you're thinking hands and feet, you're right!
The tendons in your hands and feet are shaped like long, skinny ropes. They
extend all the way to the ends of your fingers and toes to allow you to bend and
move them. These rope-shaped tendons give these parts power without making them
too big. They get their power from muscles that are higher up in the arm or the
leg. Test the true strength of the tendons in your hands by making a fist or
grasping a pencil. And see what you can do with the tendons in your feet - like
curling your toes or standing on tiptoe.
Cardiac Muscles
Ever thought about why your heart can pump blood day in and day out? It's
because the heart is a - you guessed it - muscle. The muscle that makes up the
heart is called the myocardium (my-oh-car-dee-um). It is sometimes also called
cardiac muscle. The thick muscles of the heart contract (tighten up) to pump
blood out and then relax (loosen up) to let blood back in after it's circulated
through the body.
Just like smooth muscle, cardiac muscle works all by itself with no help from
you. The brain and nervous system tell the heart muscle what to do and how fast
to pump blood, and the heart muscle does the rest. This means pumping blood
every minute of the day, every day of their lives
Many Different Muscles
Because there are so many skeletal muscles in your body, it would be impossible
to name every single one - it would be like calling out 650 names in class! But
some of the more important skeletal muscles may be ones that you've heard of
already.
In each of your shoulders is a deltoid (say: dell-toyd) muscle. Your deltoid
muscles help you move your shoulders every which way - from swinging a softball
bat to shrugging your shoulders when you're not sure of an answer.
The pectoralis (peck-tore-al-iss) muscles are found on each side of your
upper chest. These are usually called pectorals (peck-tore-uls), or pecs for
short. When many boys hit puberty, their pectoral muscles become larger. Many
athletes and bodybuilders often have large pecs, as well.
Below these pectorals, down under your rib cage, are your rectus abdominus (reck-tuss
ab-dom-in-uss) muscles, or abdominals. They are also sometimes called abs for
short.
Remember before when you made a muscle with your arm? The muscle in your upper
arm is called your bicep . When you contract your bicep, you can
actually see it push up under your skin.
And when you pulled your leg down and through to do that pretend kick? The
muscle that you contracted in the front of your thigh is called quadriceps (kwad-rih-sepps)
or quad, for short. Many people who run, bike, or play certain kinds of sports
can develop their quads and make them get large and strong.
And when it's time for you to take a seat? You'll be sitting on your gluteus
maximus, the muscles that are under the skin and fat in your behind!
Building Muscles
Your muscles are amazing - they keep your heart beating, let you move your whole
body, and more. It's a good idea to take care of your muscles - and even better,
it's fun, too!
When it comes to building cardiac and skeletal muscle, aerobic
exercise is where it's at. Aerobic exercise is great for your heart, because
when you exercise and bring in more oxygen to your skeletal muscles, your heart
becomes stronger. Since it's a muscle, it can become more and more powerful -
and sometimes even a bit bigger! It's easy for kids to get aerobic exercise:
things like bike riding, in-line skating, swimming, soccer, dancing, and jumping
rope are just some examples of aerobic activity.
And to keep your skeletal muscles smiling, be sure to give them a good workout,
too. When you exercise your skeletal muscles, it makes them stronger and
sometimes larger. You can be active and have more fun for a longer time when
your skeletal muscles are in shape - you're able to tug harder during tug of war
and kick harder in karate. To build up your skeletal muscles, try things like
push-ups or pull-ups, rowing in a boat, half sit-ups, running, or bike riding.
Your pecs, abs, biceps, and quads will thank you!
One thing that's important to remember when exercising your skeletal muscles is
to give them a proper warm-up. Muscles and tendons are usually pretty tough, but
if they are pulled or twisted before they are warmed up, they can get damaged or
strained. Usually damage to a muscle or tendon is only temporary, but it's best
to be kind to your muscles ahead of time by taking 5 or 10 minutes to stretch
them out before you exercise or play a sport. Be sure to sit properly when
you're working or playing on the computer, and remember to take breaks - both of
these things help prevent you from straining your muscles, too. Keeping your
muscles marvelous means you will feel magnificent!