Muscles actually work in pairs. A muscle can only pull in one direction so it needs another muscle to pull in the opposite direction in order to return a bone to its original position. When you lift your forearm, the biceps muscle shortens to lift the bone.
When you straighten your arm the triceps muscle pulls it back again and the biceps relaxes. The same action takes place in your legs when you walk and run, and when you move your fingers and toes.
Facts and features
Muscle anatomy diagram - front view
Muscle anatomy diagram - back view
Muscles and Exercise
Fast and slow twitch muscles
Structure and contraction
Skeletal, smooth and cardiac muscle