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Robert U. Newton Ph.D.
Most powerful activities involve a counter movement during which the muscles involved are first stretched and then shortened to accelerate the body or limb. This action of the muscle is called a stretch shortening cycle (SSC) and involves many complex and interacting neural and mechanical processes. A great deal of research has been directed toward the study of the stretch shortening cycle because it has been observed that performance is greater in SSC movements than if the activity is performed with a purely concentric action. In a cross-sectional study, Bosco et al. (1982) observed differences between squat jump (SJ) and counter movement jump (CMJ) heights of 18%-20%. A SJ is a purely concentric jump initiated from a crouch position. The CMJ is initiated from a standing position and the athlete performs a preparatory dip movement then jumps upwards. Most exercise scientists and coaches believe that greater performance in SSC movements is due to recovery of elastic energy and increased muscle activation due to the stretch reflex. Such is the theory proposed by scientists such as Komi, Bosco, Gollhofer, and Schmidtbleicher. However, recent research and a growing body of knowledge reveals that these notable experts may have got it wrong.
This question remains a contentious issue in biomechanics but at least six possible explanations may be offered:
1. It may be that subjects are simply not used to performing SJ and as such do not have the proper coordination to perform the jump with optimal control. Therefore the jump will be less than the maximum achievable height as determined by the properties of the neuromuscular system.
2. In the SJ the muscles are unable to achieve a high level of force prior to the start of the concentric contraction. When performing maximal voluntary contractions, it takes time before the muscle force has reached its maximum. This delay can be avoided by allowing the muscle to build up to a maximally activated state prior to the start of the concentric contraction, either by performing an isometric contraction (isometric pre-load) or during a counter movement as in a CMJ.
3. There is the possibility for the storage and reutilisation of elastic energy. During the counter movement in a CMJ the active muscles are pre-stretched and absorb strain energy. This energy is temporarily stored in the series elastic elements and then reutilised during the concentric phase resulting in an increased work output for the CMJ over the SJ.
4. The muscle stretch that occurs during the counter movement triggers spinal reflexes, which help to increase muscle stimulation during the concentric phase. The increase in stimulation results in increased contraction force during the concentric phase and thus greater jump height.
5. It is possible that the pre-stretch of the active muscle alters the properties of the contractile machinery. This enhancement has been termed potentiation with the effect being greater with increased speed of stretch and decreasing with time elapsed after the pre-stretch.
6. There is a considerable interaction between the contractile mechanics and the tendinous recoil of the musculo-tendinous unit. Due to the elastic nature of tendon, the additional force present at the start of the concentric phase following the stretch or eccentric phase results in relatively greater tendinous extension with less myofibrillar displacement. Therefore, in SSC movements there is the potential for the muscle fibers to be displaced less and thus be operating closer to an optimal length. Using the same reasoning, it is also feasible that the recoil of the tendinous structure would allow the velocity of shortening of the contractile element to proceed more slowly with a corresponding enhancement to force production due to the force-velocity characteristics of muscle contraction.
It is the relative contribution of each of these six explanations that is currently being debated extensively in the field of Exercise Science. Biomechanists such as Bobbert, Huijing, Van Ingen Schenau, Walshe suggest that the greater achievement in CMJ compared with SJ is mainly due to the fact that the counter movement allows the extensor muscles to build up active state and force prior to shortening. In SJ, shortening starts as soon as the level of muscle stimulation is increased above that required for maintenance of the starting position and consequently less force and thus less work is produced over the first part of the shortening distance.
The possibility of poor coordination in the SJ has been ruled out as research has shown that there is no movement disintegration i.e. the muscle forces were converted to whole body acceleration with efficiency equal to that of the CMJ.
There is no indication that muscle stimulation in CMJ is enhanced by neural responses triggered by the pre-stretch because the EMG levels during the start of the concentric phase are not different between the SJ and CMJ. However, it is possible that EMG is facilitated during the eccentric phase thus contributing to the development of a high level of active state and muscle force before the start of push-off.
With regard to storage and reutilisation of elastic energy, Bobbert et al. (1996) argue that if the concentric angular displacement is the same, an increase in the amount of elastic energy stored at the start of the concentric phase merely reduces the amount of energy to be produced by the contractile elements. This is because lengthening of the series elastic elements occurs at the expense of the length over which the contractile elements can do work. Thus, stored elastic energy increases the efficiency of doing positive work, but not the total amount of positive work that can be produced.
Similarly, the role of stretch potentiation of the contractile machinery is thought to be minimal in CMJ. In fact, most people who study reflexes concede that the force evoked during a stretch reflex is small and probably of minimal significance during high-force contractions.
Bobbert et al. (1996) did not address the issue of interaction between the contractile mechanics and the tendinous recoil, however, Walshe et al. (1997) found that this mechanism was not a possible explanation for the differences in SSC and concentric only (CO) performance.
In summary then, it would appear that the difference in CMJ and SJ height is due primarily to the fact that the countermovement allows the subject to attain greater joint moments at the start of the upward movement. This results in greater forces exerted against the ground and subsequently an increase in impulse (F x t) and thus acceleration of the whole body upward. The other mechanisms proposed appear to play at best a secondary role in the enhancement of performance by the SSC.
This article is dedicated to the memory of Dr. Gerrit Jan van Ingen Schenau who passed away 2nd April, 1998.
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Last updated Tuesday, June 19, 2012