Mechanisms of the Stretch-Shortening Cycle (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).