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When referring to a movement about a joint in the body, the muscles that contract to produce that movement are known as the agonist muscles. Also known as prime movers, agonists are the primary muscles that initiate this movement. The movement may also be assisted by muscles known as synergists or secondary movers. Agonist muscles are opposed during a specific movement by another group of muscles known as antagonists, which must lengthen to allow the agonists to contract and shorten to allow a return to resting muscle length.
The function of all skeletal muscle is to produce movement about a joint or joints. As muscles typically attach to one bone, cross a movable joint like the elbow, and attach to another bone on the other side of the joint, a contraction by that muscle will pull the bones toward each other, causing the joint to move. Muscle contraction is initiated by the central nervous system, with the brain sending an impulse along nerve vessels known as motor neurons that innervate the agonist muscles, telling them to, for instance, bend the elbow to lift a glass of water. Once the muscles receive this signal, they produce a contraction that is either concentric, as in lifting the water glass, eccentric, as in lowering it back to the table, or isometric, as in holding it at the mouth to drink.
Every movable joint of the body is surrounded by groups of agonist muscles, with different agonists bringing about movements in different directions. At the shoulder, the deltoid muscle is the agonist that lifts the arm away from the body, with different portions of the muscle activating depending on whether the arm is lifted forward, sideways, or backward. The same muscle also contracts eccentrically to lower the arm back down, meaning that it lengthens to decelerate the arm, preventing it from simply dropping against its own weight and gravitational forces. If the arm is lowered against resistance, however, as in pulling the arm downward through water, an opposing set of muscles contracts to produce this movement, the same muscles that act as antagonists to the deltoid: the pectoralis major and latissimus dorsi.
The same system of antagonist and agonist muscles opposing one another is in place at every movable joint of the body. To extend or straighten the knee against resistance, the quadriceps in the anterior thigh must shorten and therefore are the agonists, while the hamstrings in the posterior thigh must lengthen to allow this movement to occur and therefore are the antagonists. Conversely, to flex or bend the knee against resistance, the hamstrings become the agonist muscles and the quadriceps the antagonists. Other examples of agonist muscles include the biceps brachii muscle during elbow flexion; the triceps brachii during elbow extension; the gluteus maximus during hip extension, or lifting the leg backward; and the iliopsoas during hip flexion, or lifting the leg forward.