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Body Works What Makes Your Body Move? Exploration Connection: Your muscles Your skeleton allows your body to move, but what makes it move? Your muscles. Some muscles are “voluntary.” They are the ones you control, the ones that let you exercise—or relax. Some muscles are “involuntary.” They just do their job with no effort on your part—like your heart muscles. How do you think muscles make your body move? What muscles immediately come to mind when you think of your own body? Your body has muscles wherever your bones can move. Some muscles are near the surface, like those that move your arms and legs. You can feel them tense and bulge as you move. Some are deep inside you, like the diaphragm and rib muscles that help you breathe, or the stomach and intestinal muscles that push food along as it is digested. Muscles are unusual tissues that can change the energy in their cells to pulling power. Most muscles are attached to bones by strong white cords called tendons. One end of the tendon is attached to the muscle, the other end to the bone. Muscles move bones by pulling on tendons. Muscle movement is controlled by nerves which, in turn, are controlled by signals from the brain. At the right signal, the long, thin cells inside the muscle become shorter and thicker. As the muscle shortens, it pulls the bone it is attached to forward. Exploration: Make a muscle model. Cut the index card in half. Draw arm bones and label them. Punch three holes in each half, as shown. Fasten the cards together with the paper fastener. Tie a ribbon to each hole on the lower arm bone, and then thread them through the holes on the upper arm bone. Pull on the loose end of one ribbon at a time. Observe what happens. Now design your own model of an arm muscle. For example, how could you use empty paper towel tubes? What else would you need? Biceps Try your model out and make any alterations you think might improve it. Interpret your results. Triceps ▲ Biceps muscle contracted Biceps muscle relaxed Biceps Triceps You have no muscles in your fingers and toes. Look at this picture and see if you can tell what moves your toes. Tendons • What happened to the “bones” when you pulled the “muscles” in your model? • Do arm muscles push or pull? Do they both work at the same time? • How would your arm work if it had only one muscle? • Did your model teach you anything else about your muscles? Communicate your results. • Write up your exploration and invite a family member to try it. 42 Muscles 43