An isokinetic contraction is a specific type of concentric muscle contraction that occurs when the muscle contracts and shortens at the same speed throughout the entire range of motion. A concentric contraction happens when the force of the muscle overcomes any external forces and the length of the muscle shortens. To produce one, special equipment is usually needed to gauge the speed of the contraction and keep it steady regardless of the force being exerted. This allows the muscle to maintain maximum force throughout the entire movement.
Because it causes the muscle to exert maximum effort during the whole range of motion, isokinetic contractions are viewed as a superior way to build strength evenly throughout the length of the muscle. It is difficult to train this way, however, because it requires special and often expensive equipment capable of monitoring and controlling the speed of the contraction. Therefore, most strength-building exercises use the second and more common type of concentric contraction, called an isotonic contraction.
In an isotonic contraction, the force of the muscle fibers remains constant but the speed may vary. A common example is curling the bicep to lift a weight. The downside to this type of training is that, due to the varying leverage at different joint positions during the range of motion, the muscle tends to develop the most strength only in the area where the most force is exerted.
Some muscle strengthening exercises use methods other than isotonic or isokinetic contraction, such as isometric or eccentric contraction. An isometric contraction occurs when the external resistance and the muscle exert equal force, and the length of the muscle remains the same, such as when pushing against a wall. Isometric training is sometimes used as a strength-building exercise, as in the plank position in yoga, but it is not as effective as concentric training because the muscle tends to only gain strength in the position in which it is trained.
An eccentric contraction is the opposite of a concentric contraction. It occurs when the external resistance exerts more force than the muscle, so the muscle increases in length. An eccentric contraction may happen when a person lifts a weight that is too heavy, and the bicep extends even though he or she tries to keep it flexed. Since the force exerted is too much for the muscle to handle, eccentric contraction carries the highest risk of muscle injury and is not an ideal exercise method.