Muscles Work in Pairs

Your body moves because of skeletal muscles. These are the muscles attached to your bones, and their special job is to contract, which means to get shorter and pull.

A muscle can only pull. It cannot push. When a skeletal muscle contracts, it tugs on a tendon, which is a tough cord that connects the muscle to a bone. The pull moves the bone, and that is how your body moves.

So how can you bend your arm and then straighten it again? One muscle cannot do both jobs, because pulling works in only one direction. Your body solves this problem by using muscles in pairs. While one muscle in the pair contracts and pulls, the other muscle relaxes. Then they swap jobs to move the bone back.

Your upper arm is a great example. The bicep sits on the front of your upper arm, and the tricep sits on the back. To bend your arm, the bicep contracts and pulls while the tricep relaxes. To straighten your arm, the tricep contracts and pulls while the bicep relaxes. Two muscles working as a team, taking turns, give you smooth movement in both directions.

Skeletal muscles do more than start movement. They also stop movement and hold your body steady, like when you stand still without wobbling. Every step you take, every ball you throw, and every smile you make happens because muscles contract, pull on tendons, and move bones, always working together in pairs.

Think about all the ways your body can move. You can bend your elbow, kick a ball, turn your head, and wiggle your toes. Each of these movements happens at a joint, which is the place where two bones meet.

Every movement at a joint is caused by muscles. Muscles attach to the bones on both sides of a joint. When a muscle on one side contracts, it pulls its bone, and the joint moves. When that muscle relaxes and the muscle on the other side contracts, the joint moves back. Muscles take turns contracting and relaxing, and that teamwork is what lets you move in so many ways.

Scientists give names to these movements. Bending a joint so two bones come closer together is called flexion, like bending your elbow to bring your hand toward your shoulder. Straightening the joint again is called extension. Lifting your arm out to the side is called abduction, and bringing it back to your body is called adduction. Turning a bone around, like twisting your forearm, is called rotation.

The type of joint decides which movements are possible. Your elbow works like a hinge, so it mostly bends and straightens, just like a door. Your shoulder is a ball and socket joint, so it can move in many more directions.