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MUSCLE CONTRACTION CLASSIFICATION Although a muscle contracts in one manner—it shortens and lengthens—there are three classifications of muscle contraction. An individual can train using one or more of these classifications in order to improve strength and muscular endurance fitness. These classifications are isotonic, isokinetic and isometric conditioning. Isotonic Isotonic movement is a contraction that results in free, dynamic movement at a joint in the body. Isotonic training uses the more traditional resistance training equipment, such as barbells, dumbbells, pulleys, resistance machines, and heavy calisthenics such as situps, push-ups, squats, etc. Isotonic movement occurs in two ways: Concentric Contraction A concentric contraction occurs when the resistance or weight is less than the force exerted by a muscle or muscle group. This causes the muscle or muscle group to shorten. When an individual performs a biceps curl, pulling the weight toward the body, the biceps shorten—a concentric contraction. Eccentric Contraction The muscle is also capable of exerting tension while lengthening, known as eccentric contraction. This typically occurs when the muscle produces a braking force to slow down the movement produced by gravity (such as when lowering the weight during a biceps curl). It also occurs when the resistance is greater than the force exerted by the muscle or muscle group. On a now-canceled American television series, a couple of the show’s characters, “Brian” and “Joe” were in the weight room. Brian decided to use the leg press machine. After Brian had just performed the concentric portion of his movement—the quadriceps shortened to extend his legs—Joe loaded so much weight onto the machine that Brian could no longer hold his legs in an extended position. Brian’s legs started to bend—his quadriceps lengthened—an eccentric movement. Because the body can handle more weight eccentrically, some individuals use eccentric movements to improve their performances. Isokinetic Although isokinetic conditioning, like isotonic training, results in dynamic movement at a joint, isokinetic machinery regulates the movement so that, no matter how much force is used, the speed of the exercise remains the same. In joint motion, muscles involved in the movement are stronger at certain angles and weaker at others. In isokinetic exercise, the resistance of the machine adjusts as needed throughout the movement to ensure that the speed remains constant. Because isokinetic machinery accommodates the user, it is often used in rehabilitative settings. Isokinetic training generally requires special equipment. Isometric In isometric training, the muscle contracts against an immovable resistance. An example of this is pushing against a wall—although the muscle exerts a force, it does not cause the limb to move. An advantage to isometric training is that large gains in muscular strength can occur. The disadvantage, however, is that the strength gain is specific to the angle at which the exercise is performed, so several angles should be used for each joint when performing isometric training.