The Stretch Shortening Cycle
©Copywrite 2003 David Woodhouse
A ‘Stretch Shortening Cycle’ (SSC) occurs when an active muscle lengthens immediately prior to shortening. SSCs are extremely common in sport, examples include running, jumping and throwing. Force and power produced in a SSC exceed that in a concentric only movement (COM) (3). In this synopsis I will discuss the mechanisms that have been proposed to explain the phenomena.
In a SSC the muscles are able to achieve a high level of tension prior to the concentric phase of the movement (4). In a COM, extension occurs as soon as the muscles apply sufficient force to extend the relevant joints, and long before maximal tension can be generated (2). This means that velocity at any given joint angle during extension is greater in a SSC than in a COM (10). Certain insects overcome this by utilising ‘catch’ mechanisms in order to allow tension to develop prior to initiation of the movement (5). This can be simulated in human jumping by having a partner resist extension in the bottom position and then release once maximum isometric tension has been achieved (2).
During the eccentric phase of a SSC the tendons stretch and convert potential energy into stored elastic energy (7). This is then reutilised to enhance performance in the subsequent concentric phase (4). The magnitude of the stored energy is a function of the stiffness of the elastic elements (6). In a counter movement jump for example, optimal elastic energy is attained when the tendon stiffness is just less than the force induced by the body’s centre of mass falling under gravity. A stiffer tendon allows greater work to be performed by the muscle during the concentric phase and also minimises ‘coupling time’ (time between stretching and shortening). As coupling time increases, greater stored elastic energy is lost through hysteresis and hence power output decreases (6). The ability of a muscle to store elastic energy is positively correlated to length of its’ tendon. The use of elastic energy during cyclic movements (e.g. running) also increases the economy of the system by recycling potential energy and hence decreasing metabolic cost (4).
It has been suggested that elastic energy is also stored in parallel within the muscle. Recent research however suggests that for fast SSCs with minimal joint flexion (e.g. sprinting), trained subjects have minimal muscle lengthening and therefore little elastic energy will be stored through this mechanism (12). This is possible because the stiffness of their muscles exceeds that of the tendons (10). The absence of significant muscle lengthening also allows the muscle’s length-tension and force-velocity relationships to remain close to optimum (Force is greatest when velocity is zero, and when muscle length is just above resting (7)). The amount of stretch a muscle undergoes during the eccentric phase depends on the rate of force development, which is a trainable parameter (10). An additional training adaptation that results from drop jumping (a mode of plyometrics) is pre-activation of the leg extensor muscles prior to impact (8). This increases the time to develop force and minimises muscle deformation.
Muscle spindles, arranged in parallel, respond to stretch by inducing a myostatic reflex in an effort to keep the muscle close to its’ resting length (10). This reflex increases neural drive and in turn maximum force above that possible in a COM (2). It has been hypothesised however that, due to the time taken to initiate the reflex, SSCs faster than 13ms do not benefit from the mechanism (4). The opposing, protective feedback system is known as the Golgi tendon reflex. The Golgi tendon organs, arranged in series with the muscle, respond to increased tension by inhibiting maximum force development (11). One of the adaptations to plyometric training therefore, is to maximise the stretch reflex and to minimise Golgi tendon inhibition (10)
Stretching an active muscle induces a potentiation within individual cross bridges that allows them to produce greater force (4). The magnitude of the potentiation is positively correlated to the speed of stretch and negatively correlated with coupling time (2). As discussed previously however, there is actually minimal muscle lengthening during SSCs and therefore this mechanism may provide only a limited contribution.
One final explanation is that, subjects performing COMs may have sub optimal intermuscular coordination (2). This is because SSCs are actually the most natural way of performing most movements (10). Familiarisation training in COMs should therefore be considered in any research investigating the difference between SSC and COMs performance in vivo (e.g. CMJ Vs SJ).
Currently there is significant disagreement between researchers concerning the existence and contribution of each of the mechanisms outlined here. It may be that that they are specific to the movement and influenced by factors such as load, range of movement, fibre type and speed. Further research is necessary to help us improve our understanding of this fascinating topic.
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- Bobbert, M.F., G.M. Gerritsen, , M.C.A. Litjens, A.J. Van Soest. 1996. Why is counter movement height greater than squat jump height? Medicine and Science in Sport and Exercise. 28: 1402-1412.
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