Do you ever think about what body parts let you pick up and play your musical instrument with ease? Or what parts help you smile at your friend when you see him heading down the hall? Or what parts help your heart pump blood all day, every day?

If you're thinking it's your multi-talented muscles, you're right! You have about 650 muscles in your body, and they are constantly at work. Some muscles are controlled by you, while others do their job without you thinking about them at all. Still others are found only in your heart and no other place in the body. So let's get on the move and check out your muscles!

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 (say: car-dee-ack) muscle, and skeletal (say: skell-ih-tull) muscle.

Smooth Move
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. Luckily, this doesn't happen very often.

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 your blood vessels.

A Hearty Muscle
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 (say: 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.

Seeking Skeletal Muscle
So you know which muscles help you digest food, and which ones pump blood - but what about the ones that help you do everything else, from kicking a soccer ball to spooning dog food into your dog's dish? The muscles that allow you to do these types of things are called your skeletal muscles. They are also sometimes called striated (say: 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. They 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 (say: 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 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 the muscle in 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 massive 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 attach directly to bone like they do in the rest of the body; instead, they 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. Stick out your tongue and wiggle it to see the power of those strong, flexible tongue muscles.

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 (say: peck-tore-al-iss) major and pectoralis minor are the muscles that are found on each side of your upper chest. These are usually called pectorals (say: 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 muscles, down under your rib cage, are your rectus abdominus (say: reck-tuss ab-dom-in-uss) muscles, or abdominals (say: ab-dom-in-uls). 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 (say: bye-sepp). 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 femoris (say: kwad-rih-sepps fee-more-iss), 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 (say: glue-tee-us max-ih-muss), the muscles that are under the skin and fat in your behind!

Time for Tendons
Skeletal muscles are held to the bones with the help of tendons (say: ten-duns). 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.

Making Muscles Marvelous
Your muscles are amazing - they keep your heart beating, let you move many parts of your 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 muscle, aerobic (say: air-oh-bick) exercise is where it's at. Aerobic exercise is great for your heart, because when you exercise and bring in more oxygen to your 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!

Want to see a muscle in action? (You will need Shockwave to see this.)

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Important Note: All the information on KidsHealth.org is for educational purposes only. The place to get specific medical advice, diagnoses, and treatment is your doctor.