Video narration
When you bend your arm to lift a schoolbag, what actually moves it? Your bones give your arm shape, and your skeleton supports you and protects some organs, but bones alone cannot lift the bag. At your elbow, the bones meet at a joint, a place where bones can move against one another. Muscles attach to bones by tendons, strong cords that connect muscle to bone. Here's the key: bones do not move themselves, and muscles don't push. To bend your elbow, the biceps contracts-tightens and pulls through its tendon-while the triceps relaxes. That pull swings your forearm upward, carrying the schoolbag with it. To straighten your arm, the action reverses: the triceps contracts and the biceps relaxes. So what lifts the bag? Your biceps contracts and draws the forearm up at the elbow while the triceps relaxes.