Your body has about 650 muscles in it! Can you imagine that? Some of them are working all the time, without you having to think about it. They make up almost half your body weight!¹ Muscles are made up of elastic tissue, kind of like a rubber band. Only instead of stretching like a rubber band, muscles contract, or get shorter. That's how they do their job. Then they go back to their normal length.

You have three kinds of muscle. One is cardiac (car-dee-ack) muscle, and it's found in your heart. It pumps blood all through your body. Another kind is smooth muscle. It is in places like your stomach, intestines, and bladder. Both cardiac and smooth muscles are involuntary (in-voll-un-tare-ee), which means they work without you needing to think about it. The third kind of muscle is skeletal (skell-e-tal) muscle. It is attached all over your skeleton and is what makes your arms, legs, back, hands, feet, and face move. It is voluntary (voll-un-tare-ee), which means that you have to think about it to make it move.

Regular exercise keeps all your muscles strong, not just the ones you might think about. Aerobic exercise is exercise that makes your heart beat faster, which strengthens it. Strength training keeps your muscles toned and can build them up. Building up muscles gives you more power and strength and helps your body stay fit.

Endurance is the measure of your body's ability to keep up an activity without getting tired. The more endurance you have, the longer you can swim, bike, run, dance, and play a sport before tiring out. When muscles are worked nonstop for 15 minutes or more, they need more oxygen to be at their best. Your level of endurance refers to how well your heart and lungs can pump oxygen-rich blood to your muscles. The more oxygen, the better your muscles work and the less tired you get.

Aerobic exercise improves your endurance because it uses a lot of oxygen. Aerobic exercise is exercise that increases your heart rate. Running, dancing, inline skating, biking, power walking, playing soccer, jumping rope, aerobics classes, and swimming are good examples of aerobic exercise.

Aerobic exercise strengthens your heart and lungs and boosts your energy level. It also burns more fat and calories and can reduce the risk of heart disease, high blood pressure, and diabetes.

Healthy Body Composition

Having the appropriate amount of fat is important for your health. Too little or too much fat can cause major health problems. You need to eat a well-balanced selection of foods on a regular basis. Regular physical activity builds muscles and helps control the amount of fat you have. Strength-building activities create stronger muscles, which use energy throughout the day. The more you contract your muscles, the more you improve your strength.

Strength

Dynamic Contraction	Static Contraction

Strength is the ability to move a muscle against resistance. The more strength you have, the easier it is to do physical tasks.

When a muscle tries to move and meets resistance, a contraction results. There are two kinds of contractions—static and dynamic. Static contractions happen when there is so much resistance that the object doesn't move, like if you were pushing on a house. Static contractions strengthen only the muscle that's contracting at that one angle. Dynamic contractions happen when you use your muscles to actually move something. Lifting weights, pushing the vacuum, or moving furniture cause dynamic contractions. The more intense the contraction, the bigger the improvement in strength.

To become stronger, your muscles need to push against some sort of resistance. Sometimes the resistance can be your own weight, like in push-ups, chin-ups, squats, and lunges. Free weights also can be lifted, but they can be dangerous if you don't know how to use them. Make sure to talk to an instructor first. Make sure to talk to a strength trainer first.

Flexibility

Having a flexible body allows your joints and muscles to move through their full range of motion. When you are flexible, your joints have some give, which allows you to bend and reach with ease. The best way to become more flexible is by stretching. The correct way to stretch is to slowly extend your body to the point where you feel tension, not pain, in the muscle. Hold it for 20 seconds. It's very important that you don't bounce because that could damage the muscle fibers. Be sure to stretch before and after work-outs.

Being flexible can help to prevent injuries like pulled muscles. If you have tight muscles and you try to move your body in ways you aren't used to, you run the risk of tearing muscles, ligaments, or tendons. A muscle that's strong and limber is more elastic and ready to deal with stress than a muscle that's tight.

For a cool interactive muscle resource, check out:
MEDLINEplus. X-Plain Patient Education: "Muscles"

The President's Council on Physical Fitness and Sports: "FunFit Kids"

The President's Council on Physical Fitness and Sports: "Get Fit! A Handbook for Youth Ages 6-17"

Powerful Bones. Powerful Girls: The National Bone Health Campaign: "Staying Strong: Fitness Fun"

The Family Doctor: "Exercise: How To Get Started"

KidsHealth: "I Don't Like Sports, So How Can I Stay Fit?"

KidsHealth: "It's Time to Exercise!"

Kidnetic

¹KidsHealth. "Your Multi-Talented Muscles," last referenced 8/12/03.
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Last reviewed 10/10/01