by Strength Training
Kinetic Select August 2020
The following is an exclusive excerpt from the book Strength Training, Second Edition, published by Human Kinetics. All text and images provided by Human Kinetics.
Plyometric training is a popular form of training for speed, power, and starting strength. The term stretch–shortening cycle exercises is starting to replace the term plyometrics, and it describes this type of resistance exercise more accurately (Knuttgen and Kraemer 1987).
The stretch–shortening cycle (SSC) refers to a natural part of most movements; the cycle is a sequence of eccentric, isometric, and concentric actions. It is characterized by an eccentric motion leading into a ballistic concentric motion. For example, in the bench press exercise, if you were to start the lift from your chest, you would use an isometric and then a concentric action only. However, if you were to start the lift with the bar at arm’s length, you would use an eccentric, then isometric, then concentric action. Depending on the length of the isometric action, the concentric portion of the lift can be much easier because of the SSC. That is, the longer you isometrically hold the weight before the concentric action, the larger the decrement in the SSC and therefore the more difficult the concentric portion of the lift will be.
Bounding, leaping, and medicine ball throws are common plyometric or SSC exercises (figure 3.3). The key to plyometric training is using the SSC to allow for an enhanced concentric contraction mediated by the preactivation during the eccentric action. Thus, the speed of the eccentric muscle action is vital to the concentric repetition. The ability of the SSC to increase power output depends on load, time, and the ability of the muscle to induce a force-enhancing prestretch of the muscle.
When the sequence of eccentric to concentric actions is performed quickly, the muscle is stretched slightly before the concentric action. The slight stretching stores elastic energy. This elastic energy is added to the force of a normal concentric action, which is one of the common explanations given for why more forcible concentric action results after an SSC. The other common explanation is that a reflex results in quicker recruitment of muscle fibers or recruitment of more muscle fibers involved in the movement.
It is easy to demonstrate how an SSC results in a more powerful concentric action. Perform a normal vertical jump (i.e., a countermovement jump). During this type of jump, you bend at the knees and hips (eccentric action), then quickly reverse direction and jump (isometric followed by concentric action). A countermovement jump involves an SSC. Now perform a jump by bending at the knees and hips, stopping for three to five seconds in that position, and then jumping. This is called a noncountermovement jump; it does not involve an SSC and results in a jump that is not as high as a countermovement jump (i.e., a jump involving an SSC). You can also demonstrate the effect of an SSC by throwing a ball for distance with a normal overhand throwing motion. Then throw a ball for distance starting from the end of the wind-up position (no SSC). The normal throwing motion will produce a throw of greater distance.
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