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Skeletal Muscles—Move That Body! Skeletal Muscles— Move That Body! Your body is packed full of muscles. In fact, muscles make up about half of your weight. Some muscles are large and thick, like those that lift your legs. Others are small and thin, like those that control your eyes. Some muscles work inside you day and night to deliver materials around your body. But all muscles have one thing in common—they move! Muscle Contraction All muscles are made of special cells that allow them to move. Muscle cells can contract, or squeeze together, to become shorter and thicker. When the cells contract, they can do work. If every muscle cell in your body contracted at once, they could lift 25 tons! Muscle cells are grouped into separate muscles. Each muscle is responsible for specific movements. There are about 656 muscles in your body, and more than 600 of them move your skeleton. These are called skeletal muscles. Skeletal muscles are the muscles you use to climb the stairs, wash your hair, or chew your food. Most things you do during the day require skeletal muscles. Let’s take a closer look at how these muscles work. When muscles contract, they get shorter and thicker. Muscle Connections Skeletal muscles connect to your bones by tendons. Tendons are strong, flexible cords. They often attach one end of a muscle to a bone that is stable. The other end attaches to a part that can move— when the muscle contracts, it pulls the movable part toward it. You can feel some of your tendons. Let one arm hang down at your side, palm forward. Bend your arm slightly and use your other hand to press inside the bend in your elbow. Can you feel a tough cord? This tendon connects your biceps muscle to bones in your lower arm. Another tendon connects the upper end of the biceps to your shoulder bone. These arm muscles attach to bones by tendons (shown in blue). Muscles Working Together Skeletal muscles often work in pairs. When one contracts, the other relaxes. Muscles can only pull bones; they can’t push them. So muscles on different sides of a bone pull it in different directions. Discovery Education Science © 2007 Discovery Communications, LLC Page 1 of 2 Skeletal Muscles—Move That Body! Now let’s check out how some muscles in your arm move. Place your left hand on your right biceps and bend and straighten your elbow. Can you feel your biceps contracting and relaxing? Did you notice that when you contract, or flex, your biceps feels shorter and thicker? This causes your lower arm bone to move up. A muscle on the back of your arm (your triceps) contracts to straighten it back out. Hold your arm very straight—can you feel your triceps tightening? Your muscles work hard to move your body, so it’s important to keep them strong and healthy by exercising and eating right. Did you know that your muscles are about 75 percent water? Most of the remaining part is made of protein. Drinking plenty of water and eating protein-rich foods, like nuts and meat, will help keep your muscles in tip-top shape! Discovery Education Science Your biceps and triceps work as a team to move your lower arm. © 2007 Discovery Communications, LLC Page 2 of 2