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.
What is the Stretch-Shortening Cycle (SSC)?
Athletes have been shown to jump 2-4cm higher during the countermovement jump (CMJ) than they can during the squat jump (SJ) (1). 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 (2). This pre-stretch, or ‘countermovement’ action is known as the stretch-shortening cycle (SSC) and is comprised of three phases (eccentric, amortization, and concentric) (Figure 1) (3). Images A-B display the eccentric phase, image C demonstrates the amortisation phase, and images D-E represent the concentric phase of the SSC.
The SSC 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 (4). This muscle action is also sometimes referred to as the reverse action of muscles (5). The action of the SSC is perhaps best described as a spring-like mechanism, whereby compressing the coil causes it to rebound and therefore jump off a surface or in a different direction (Figure 2). 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’, and 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 (6, 7, 8).
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
Table 1 provides some examples of common exercises and their potential SSC classification. As displayed in Table 1, a long jump is typically classified as a fast-SSC movement as it has a ground contact time of 140-170 milliseconds (9). Whereas race walking, which has a ground contact time of 270-300 milliseconds is commonly classified as a slow-SSC movement (10).
As measuring the duration of the SSC at each contributing joint (e.g. ankle, knee, hip) during a jumping exercise is problematic, researchers have often questioned the ability to measure the SSC indirectly by analysing the ground contact times. As a result, researchers have looked for relationships between ground contact times and coupling time*. Strong relationships have been found between coupling time and exercises with ground contact times ranging from 270-2500ms (16, 17). However, no relationships were observed in exercises with ground contact times of 400-800ms (17). This therefore questions the reliability of classifying exercises with ground contact times of <850ms into particular SSC categories based upon their ground contact duration. For example, simply classifying race walking as a slow-SSC movement because it has a ground contact time between 270-300ms. Though this is often common practice, understanding the issue with doing so is important.
*Coupling time is the amortisation/isometric phase of the SSC which connects the eccentric to the concentric phase – hence the term ‘coupling’, as it couples the two together. Or in other words, coupling time is defined as the transition between the eccentric and concentric phases of the SSC (16).
Mechanisms of the SSC
There are numerous neurophysiological mechanisms thought to contribute to the SSC, some of which include: storage of elastic energy (18, 19, 20, 21), involuntary nervous processes (22, 23), active state (1, 24), length-tension characteristics (25, 26), pre-activity tension (27, 28) and enhanced motor coordination (1, 24). Despite this large list, it is commonly agreed that there are three primary mechanics responsible for the performance enhancing effects of the SSC (2).
These three mechanisms are:
- Storage of Elastic Energy
- Neurophysiological Model
- Active State
Storage of Elastic Energy
The concept of elastic energy is similar to that of a stretched rubber band. When the band is stretched there is a build-up of stored energy, which when released causes the band to rapidly contract back to its original shape. The amount of stored elastic energy (sometimes referred to as ‘strain’ or ‘potential’ energy) is potentially equal to the applied force and induced deformation (5). In other words, the amount of force used to stretch the band, should be equivalent to the amount of force produced by the band in order to return to its pre-stretched state.
In humans, this stretch and storage of elastic energy is instead placed upon the muscles and tendons during movement. However, due to the elastic properties of the tendon, it is commonly agreed that the tendon is the primary site for the storage of the elastic energy (29, 30). Unlike muscles, the tendons cannot be voluntarily contracted, and as a result they can only remain in their state of tension. This means that the muscle must contract and stiffen prior to the beginning of the SSC during ground contact – known as ‘muscular pre-activity’. The muscle must then remain contracted/ stiff during the first two processes of the SSC (eccentric and amortisation phases) in order to transmit the isometric forces into the tendon. This causes the deformation/ lengthening of the tendon and the development of the storage elastic energy.
During the concentric phase of the SSC (often referred to as the ‘positive acceleration’ phase), the muscle is then able to concentrically contract and provide additional propulsive force (2). Failing to stiffen during the eccentric and amortisation phases, means the performance enhancing effect of the SSC will be lost and the joint would likely collapse. This demonstrates the importance of muscle stiffness during the SSC and its ability to improve performance. It also suggests that athletes’ with higher levels of muscular strength can absorb more force (i.e. higher rate of loading), and therefore have a better ability to use the SSC. An abundance of research has demonstrated that stronger athletes have a better ability to store elastic energy over weaker individuals (31, 32, 33). Elite athletes from both power- and endurance-based sports have also been demonstrated to possess a superior ability to store elastic energy (31, 32). Furthermore, efficient utilisation of the SSC during sprinting has shown to recover approximately 60% of total mechanical energy, suggesting the other 40% is recovered by metabolic processes (34, 35). In aerobic long-distance running, higher SSC abilities have also been shown to enhance running economy – suggesting that athletes with a better SSC capacity can conserve more energy whilst running (33, 36, 37). This indicates the importance of the SSC for both energy release and energy conservation. However, this storage of the elastic energy within the tendon cannot last forever, and has been shown to have a half-life of 850 milliseconds (38).
The muscles and tendons contain sensory receptors known as ‘proprioceptors’, these send information to the brain about changes in length, tension and joint angles (39). The proprioceptors within the muscle are known as ‘muscle spindles’, whilst those in the tendon are called ‘Golgi tendon organs’.
When a muscle is forcefully lengthened, the muscle spindles engage a stretch-reflex response to prevent over-lengthening and limit the possibility of injury. The engagement of these muscle spindles is thought to cause an increased recruitment of motor units and/ or an increased rate coding effect (40, 41). An excitation of either or both of these neural responses would lead to a concurrent increase in concentric force output and may therefore explain the performance enhancing effects of the SSC. The increase in concentric force output would therefore then lead to an enhanced power output during sporting movements (e.g. jump), and thus may improve performance. However, many studies have reported no increase in muscle activation after a pre-stretch activity (e.g. CMJ) when compared to non-pre-stretch activity (e.g. SJ) (26, 42, 43). This suggests that muscle spindle reflex activity does not have any impact on the increased force by the SSC (1).
Furthermore, when a muscle is forcefully lengthened, the Golgi tendon organs (GTO) engage an opposite stretch-reflex response to the muscle spindles. Their role is to inhibit (i.e. prevent) the excitation of the muscle spindles during forceful over-lengthening to prevent the possibility of injury (5). Though this may seem as a bizarre trade-off between the muscle spindles and the GTO, the muscle spindles activate when muscle-tendon unit is forcefully lengthened, whilst the GTO activate when the forceful lengthening becomes too large (39).
Due to the inhibitory stretch-reflex response of the GTO, it is thought that this may counteract the contraction action of the muscle spindles. If so, this would mean that the GTO inhibits the high-muscular stiffness needed during the SSC and therefore reduces the concentric force output and subsequent performance (2). In fact, research has shown that muscle activation levels – and therefore muscle stiffness – have been reduced during the early phases of the SSC in individuals who are unaccustomed to intense SSC movements (28). Interestingly however, 4-months of plyometric training has been shown to reduce this GTO inhibitory effect (disinhibition) and increase muscular pre-activity and muscle-tendon stiffness (27). As a result, it appears that effective training methods (e.g. plyometrics) can reduce or even eliminate the potential negative effects observed from the GTO inhibitory effect.
The active state is the period of time 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 amortisation phases. The unpinning 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) (24, 44). 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 (1, 24, 44, 2).
Electromechanical Delay (EMD)
The electromechanical delay (EMD) refers to neural and physiological delay in the production of mechanical force. This simply implies that the muscles cannot generate and transmit force to the skeletal system instantaneously, instead there is a slight delay. A delay in the production of mechanical force can therefore lead to a reduction in performance (24).
Currently there are numerous components which have been suggested to contribute to this delay:
- Finite rate of increase in muscle stimulation by the central nervous system.
- Propagation of the action potential on the muscle membrane.
- Time-constraints of calcium release and cross-bridge formation.
- Interaction between the contractile filaments and the series elastic components.
- Toe-region of the tendon.
As a complete analysis of all these neurophysiological factors are beyond the scope of this article and readily available in exercise physiology textbooks, only the toe-region will be explained.
The toe-region of the SSC, otherwise simply explained as ‘slack within the tendon’ is present at the very beginning of the SSC. To simplify this concept, imagine a coiled up piece of string that is being pulled from both ends in order to straighten it out and create tension. Well this ‘slack’, before the string is straight, is referred to as the ‘toe-region’. It is recognised that this slack within the tendon delays the time in which muscle-tendon stiffness and concentric force can be generated – simply the time taken to straighten the string and create tension (45) (Figure 3). Therefore, the toe-region reduces the time available to generate force during the SSC, and thus reduces concentric force output.
Due to the negative effects of the EMD on mechanical force, it has been proposed that optimising muscular pre-activity may reduce or even counteract the effects of EMD by exciting the muscle and creating muscle-tendon stiffness prior to the start of the SSC (2). As a result, training methods which improve pre-activity, such as plyometric and ballistic training, may be beneficial for the optimisation of the SSC (27).
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 SSC capacities than less- or non-trained individuals, and thus highlights the necessity to optimise this property to enhance athleticism. Despite the long-list of mechanisms proposed to influence the effects of the SSC, the active state is commonly believe to be the primary contributor. The time to develop mechanical force is negatively affected by the electromechanical delay, and thus attempts to maximise muscular pre-activity via particular training methods (e.g. plyometrics) should be profound.
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