Muscles are arranged around the skeleton so as to bring about a variety of movements. The two general types of arrangements are the opposing antagonists and the cooperative synergists.

Antagonists are opponents, so we use the term antagonistic muscles for muscles that have opposing or opposite functions. The biceps brachii is the muscle on the front of the upper arm. The origin of the biceps is on the scapula (there are actually two tendons, hence the name biceps), and the insertion is on the radius. When the biceps contracts, it flexes the forearm, that is, bends the elbow. Recall that when a muscle contracts, it gets shorter and pulls. Muscles cannot push, for when they relax they exert no force. Therefore, the biceps can bend the elbow but cannot straighten it; another muscle is needed. The triceps brachii is located on the back of the upper arm. Its origins (the prefix tri tells you that there are three of them) are on the scapula and humerus, and its insertion is on the ulna. When the triceps contracts and pulls, it extends the forearm, that is, straightens the elbow.

Joints that are capable of a variety of movements have several sets of antagonists. Notice how many ways you can move your upper arm at the shoulder, for instance. Abducting (laterally raising) the arm is the function of the deltoid.

Adducting the arm is brought about by the pectoralis major and latissimus dorsi. Flexion of the arm (across the chest) is also a function of the pectoralis major, and extension of the arm (behind the back) is also a function of the latissimus dorsi. Without antagonistic muscles, this variety of movements would not be possible.

You may be familiar with range-of-motion (or ROM) exercises that are often recommended for patients confined to bed. Such exercises are designed to stretch and contract the antagonistic muscles of a joint to preserve as much muscle function and joint mobility as possible.

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