What is the Stretch Shortening Cycle (SSC)?
The stretch shortening cycle (SSC) refers to the ‘pre-stretch’ or ‘countermovement’ action that is commonly observed during typical human movements such as jumping. This pre-stretch allows the athlete to produce more force and move quicker. Though there is controversy surrounding the mechanics responsible for the performance improvements observed from using the SSC, it is likely to be a combination of the active state and the storage of elastic energy within the tendon. Due to the negative effects of the electromechanical delay, it may be suggested that training methods which improve muscular pre-activity, such as plyometric and ballistic training, may be beneficial for improving athletic performance.
Athletes have been shown to jump 2-4cm higher during the countermovement jump (CMJ) than they can during the squat jump (SJ). This is simply because the CMJ incorporates a pre-stretch dropping action when compared to a squat jump – which initiates the movement from a static position without the use of a pre-stretch. This pre-stretch, or ‘countermovement’ action is known as the stretch-shortening cycle (SSC) and is comprised of three phases (eccentric, amortization, and concentric).
The Stretch Shortening Cycle is described as a rapid cyclical muscle action whereby the muscle undergoes an eccentric contraction, followed by a transitional period prior to the concentric contraction. This muscle action is also sometimes referred to as the reverse action of muscles. The action of the SSC is perhaps best described as a spring-like mechanism. This is seen by compressing like a coil which causes it to rebound and jump off a surface or in a different direction. Increasing the speed at which the coil is compressed or how hard it is pressed down (amount of force applied) will result in the spring jumping higher or farther. This is known as the ‘rate of loading.’ Increasing this will often mean the spring will jump higher or farther. Therefore, a jump which incorporates a ‘run-up’ will often allow an athlete to jump higher or farther than a jump from a static position because of an increase in the rate of loading.
The SSC does not only occur during single-bout jumping or rebounding movements, but also during any form of human movement when a limb changes direction. For example, during walking, jumping, running, twisting or even lowering and then raising your arm. As the limbs are continuously changing direction there is a constant use of the SSC in order to change the direction the limb is moving. As some movements are much faster than others (e.g. sprinting vs. walking), there are great differences in the speed of the SSC. Consequently, the SSC has been separated into two categories based upon the duration of the SSC:
Fast-SSC: <250 milliseconds
Slow-SSC: >250 milliseconds
A long jump is typically classified as a fast-SSC movement as it has a ground contact time of 140-170 milliseconds. Whereas race walking, which has a ground contact time of 270-300 milliseconds is commonly classified as a slow-SSC movement.
Mechanisms of the Stretch-Shortening Cycle
There are numerous neurophysiological mechanisms thought to contribute to the SSC, some of which include: storage of elastic energy, involuntary nervous processes, active state, length-tension characteristics, pre-activity tension and enhanced motor coordination. There are three primary mechanics responsible for the performance enhancing effects of the SSC. These three mechanisms are Storage of Elastic Energy; Neurophysiological Model; and Active State. For our purposes and this book, we will focus on the Active State of SSC.
The active state is the period in which force can be developed during the eccentric and amortization phases of the SSC before any concentric contraction occurs. For example, during the ‘countermovement’ or ‘dropping’ action of the CMJ, the active state is developed during the eccentric and amortization phases. The belief is that exercises which possess longer eccentric and amortization phases of the SSC will allow more time for the formation of cross-bridges, therefore enhancing joint moments, and thus improving concentric force output. Increasing the amount of force, and the time available for force to be developed, typically leads to a concurrent increase in the impulse (Impulse = Force x Time). In other words, increasing the force application will lead to improvements in power output and therefore athletic performance.
There is widespread agreement to suggest that the active state is largest contributor to the performance enhancing effects of the SSC, as it allows for a greater build-up of force prior to concentric shortening
The SSC, otherwise known as the reverse action of muscles, is a spring-like mechanism shown to enhance athletic performance both in explosive- and endurance-based sports. Well-trained athletes appear to possess better stretch-shortening cycle capacities than less- or non-trained individuals, and thus highlights the necessity to optimize this property to enhance athleticism. Despite the long-list of mechanisms proposed to influence the effects of the SSC, the active state is commonly believed to be the primary contributor. The time to develop mechanical force is negatively affected by the electromechanical delay, and thus attempts to maximize muscular pre-activity via training methods (e.g. plyometrics) should be implemented.