So now we come muscle action terminology. The terminology used to describe the movement actions of muscles, is really relevant because as you will see on the next lesson, this understanding helps you select resistance exercises with a high muscle action specificity. During the performance of sports skills an active muscle can produce different types of movements. Sometimes tension is produced in the muscle but the joints don't move. This movement is referred to as Static. The sprint start is an obvious application of highly tense muscles that produce no movement. When the active muscle causes either a decrease or an increase in the joint angles, the resulting movement produced is referred to as, Dynamic. The term Concentric identifies movement when the muscle fibers are shortening and joint angles are decreasing. And when muscle fibers tense while concurrently lengthening, the joint angle increases. When I straighten my arm for example, the biceps muscle is lengthening. The term used to describe this phenomenon is Eccentric. The term Isometric describes a situation when there is a muscle tension, but no joint movement. In other words, no work is being done. Now in all three situations, the muscle and its tendons are involved. The combination of the muscle and tendon is referred to as the Muscle Tendon Complex. So you can see, that there are many complexities when describing muscle actions. And there are other complexities as well. So, let's just talk about a few more. First, the muscle fibers may be shortening while the tendon lengthens. That is the muscle and tendon, have different actions. And second, there is variation in the mechanical force production of a muscle as it moves through its range of motion. And third, a muscle cannot produce a uniform or isotonic force. That is, there is no such thing as natural isotonic movement. Fourth, the word contraction, in describing muscle action, is quite often applied incorrectly. Not all muscle actions involve a contraction. We've already discussed eccentric muscle action when referring to the lengthening of the muscle fibers. In the strength training literature, this is referred to as a contraction. The exact terminology used is eccentric contraction. Now by definition, contractions means the process of becoming smaller. Eccentric muscle action is therefore not a true contraction. There is also the situation when considerable muscle tension is being produced, but there is no external joint movement. The term isometric contraction is used to describe this situation. The muscle itself contracts, however the tendon stretches. And the muscle tendon unit as a whole therefore, results in no external joint movement. Contraction, by definition therefore, only applies to concentric muscle activity. Eccentric and isometric muscle tendon actions are not contractions in the true meaning of the word. And number five, the term isokinetic is used to describe constant rate of movement. Now, a muscle does not shorten at a constant velocity while performing sports skills. It is possible to use specially designed exercise machines to control a limb's velocity, and this makes the movement, theoretically, isokinetic. However, uniform limb velocity does not occur when performing sports movements. And six, the forces typically produced while performing a sport skill involve a combination of concentric, eccentric and isometric actions. When running or jumping for example, the quadriceps muscle of the thigh, moves through eccentric action first as the foot strikes the ground. And this is followed by a brief isometric action, that stops the downward movement of the body. And then there's a concentric contraction during the propulsion phase. The culmination of eccentric, concentric action forms the stretch-shortening cycle. The stretch-shortening cycle is just an economical way to enhance the performance of the muscle. And we're going to return to this notion in the next lesson. And finally, in some instances gravity is the force causing the eccentric action of muscle. Such as when a gymnast is moving into the iron cross. In this case, maintaining isometric action becomes important to performing this skill effectively. Indeed, isometric actions are a significant part of the skill itself.
