Muscles work by contraction only. To move a bone to and fro, there should therefore be two sets of muscles. When one set contracts, it moves the bone in one direction, and when the other set contracts it moves the bone in the opposite direction. These two sets of skeletal muscles work in opposition to each other i.e. They are antagonistic to each other. Bending a hinge joint is called flexion, and straightening the joint, extension. The muscle that causes flexion is known as the flexor, that which causes extension, the extensor. At the elbow, the biceps muscle is the flexor while the triceps muscle is the extensor. The biceps lie in front of the humerus. Its upper end has two tendons (hence the same biceps) which have their origin on the scapula. The lower end of the biceps has a single tendon inserted on the radius. The triceps muscle lies behind the humerus. It has its origin on the scapula and the humerus as three tendons, hence the name triceps. Its lower end continues as a single tendon inserted on the olecranon process of the ulna. The contraction of the biceps causes the forearm to be pulled upward towards the scapula.