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Skeletons can be remarkably flexible but muscles are needed to move them. Various sports competitors and athletes are seen in action using their muscles. Muscles are used every time we move. There are more than 600 muscles in the human body. Muscles are attached to the bone by tendons and work in pairs to allow for smooth movement. To move a joint, one muscle contracts while the other muscle relaxes and becomes longer.

After watching the explanation of the muscles working to raise or lower the forearm, students could make a model of a human arm using card for the bones and string to act as the muscles. If the arm is hinged (using a split pin) at the elbow, the students could experiment with getting the string to control the elevation of the arm - like a muscle. The students could draw diagrams of their favourite sports star, annotating the muscles used and in which direction a contraction or extension causes movement.

Students could also try to think of other muscle pairs in the body which have to work in tandem during movement. Ask students to draw diagrams of these muscles in two positions of movement to show how one muscle is contracted while the other is extended, and vice versa.