Plyometric Training & Stretch Shortening Cycle (SSC)

This article reviews the research relating to Plyometric Training and the Stretch Shortening Cycle (SSC). The article is intended to provide Strength & Conditioning Coaches with an overview of the the SSC, best coaching strategies, testing and monitoring of the SSC, specificity of exercise selection along with other important training concepts related to this method of training. The content used in this article forms some part of the tests and systems used on KG Elite Performance Strength & Conditioning courses in Ireland.
For more information, visit: http://www.kgelite.ie/
Table of Contents
1. Introduction
2. Classification of SSC
3. SSC Performance Tests & Monitoring
4. Plyometric Training Coaching Model
5. Practical Application & Conclusion
6. References
Introduction
Early research on human muscles by Komi and Bosco (1978), and Komi (1984; 1986) demonstrated that concentric muscle work was increased when preceded by active stretch (eccentric action). This phenomenon is known as the stretch-shortening cycle (SSC) and has been extensively studied over the last number of decades. The SSC is a natural type of muscle function in which muscle is stretched (eccentric phase) immediately before being contracted (concentric phase). Two mechanisms are the basis of both, voluntary and involuntary motor processes involved in the stretch-shortening cycle. The first mechanism (the neuromuscular model) is based on the so-called stretch reflex, which is also called the muscle spindle reflex or myotatic reflex, and on the Golgi tendon organ reflex (GTO reflex; Radliffe and Farentinos, 1999). The second mechanism of the stretch shortening cycle (the mechanical model) involves the use of the elastic energy of the muscle-tendon complex. Examples of SSC actions include natural movements seen in everyday life such as walking or running or in sport such as sprinting, changing direction or wind-up movements such as throwing a ball. Although we will all have greater heights to which our SSC potential can reach (mainly due to genetic make-up), the SSC is a trainable quality which can be developed by the Strength & Conditioning Coach to facilitate sports performance.
Classification of Stretch Shortening Cycle (SSC)
Schmidtbleicher (1992) has suggested that the SSC can be classified into two types, these include:
• Fast SSC
• Slow SSC
The fast SSC is characterized by short contraction times (<0.25s) with small angular displacements and can be observed in exercises such as depth jumping or hopping exercises. The slow SSC involves longer contraction times (0.25-0.50s), larger angular displacements and can be observed in exercises such as countermovement jumps or box jumps. The contraction times during both slow and fast exercises are typically determined through the use of electronic jump mats or force plates, and serve as an important variable when looking to develop the SSC.
Testing & Monitoring of the SSC
Slow SSC Tests:
1. Countermovement Jump (CMJ)
2. Standing Long Jump (SLJ)
Fast SSC Test:
1. Reactive Strength Index (RSI)
CMJ Test
The CMJ is the typically the preferred choice of concentric power measurement from the Strength & Conditioning Coach, given it uses an electronic jump mat and there is a greater volume of research to validate it’s usage. Jump height on the mat is measured and recorded in cm of height eg 35cm. To perform the test, hands are placed on the hips, and stay there throughout the test. The athlete squats down and then immediately jumps vertically as high as possible, landing back on the mat on both feet at the same time. The take-off must be from both feet, with no initial steps or shuffling and they must land on the balls of their feet not their heels. They must also not pause at the base of the squat. The best result of at least three attempts is recorded – athletes get a fourth and final jump if improvements are being made on each of their three previous jumps.

SLJ Test
The SLJ is a very useful alternative to the CMJ for coaches who do not posess an electronic jump mat. The athlete stands behind a line marked on the ground with feet slightly apart. A two foot take-off and landing is used, with swinging of the arms and bending of the knees to provide forward drive. The subject attempts to jump as far as possible, landing on both feet without falling backwards. The best result of at least three attempts is recorded – athletes get a fourth and final jump if improvements are being made on each of their three previous jumps.
Reactive Strength Index (RSI) Test
Reactive strength is a representation of the fast SSC potential. It assesses an athletes’ ability to change quickly from an eccentric to a concentric contraction and their ability to develop maximal forces in minimal ground contact time. Typically RSI has been measured using drop jumps from a box onto an electronic jump mat. RSI is a ratio between ground contact time and height jumped, as a result both these variables need to be considered in conjunction with the overall RSI score. The electronic mat measures ground contact time in the drop jump directly and calculates jump height based on the athlete’s "flight time".

Plyometric Training & Coaching Model
Plyometic exercises can be classified into slow and fast stretch-shortening cycle exercises (see above). In addtion to this, they can be further broken down into jumps, hops and bounds (see below). At Elite Performance Institute (EPI), we look to initially improve movement quality first by coaching the "fundamental" jumps before then progressing onto what we call "performance" jumps, hops & bounds. We want our athletes to establish correct movement and co-ordination of the fundamental jump exercises (CMJ, SJ, tuck jump and wall jump & reach) before progressing athletes onto more demanding plyometric exercises.
Jump = two legged take off followed by two legged land
Hop = single leg take off followed same single leg land
Bound = single leg take off followed by opposite single leg land

Practical Applications & Conclusion
Before Strength & Conditioning Coaches begin to prescribe plyometric exercises to their athletes, it is recommended they conduct some SSC profiling using CMJ/SLJ and RSI tests. In doing so they will have a greater understanding of their athletes SSC needs. Once profiled, they should then consider the force-velocity (FV) curve with regards their Strength & Conditioning program and whether they want to only focus solely on reactive strength adaptations or include reactive strength development with explosive or max strength training as part of a concurrent training program.
PI recommends athletes who have limited SSC training begin with slow SSC exercises before progressing onto higher intensity SSC exercises. In addition, whether they are developing slow of fast SSC exercises it is also recommended the S&C Coach applies an appropriate "coaching model" to his athletes training. This will facilitate their co-ordination development as well as reducing the risk of injury.
References
1. Flanagan EP and Comyns TM. The use of contact time and the reactive strength index to optimise fast stretch-shortening cycle training. Strength Cond J 30: 33–38, 2008.
2. Komi PV, Bosco C. Utilization of stored elastic energy in leg extensor muscles by men and women. Med Sci Sport, 1978; 10: 261-265.
3. Komi PV. Neural mechanisms in strength and power training. Conference Book of International Conference on Weightlifting and Strength Training, K. Hakkinen (ed.), Lahti, Finland, 1998; 11-16.
4. Komi PV. Physiological and biomechanical correlates of muscle function: effects of muscle structure and stretch-shortening cycle on force and speed. Exer Sport Sci Rev, 1984; 12: 81-121.
5. Komi PV. Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. J Biomech 33:1197–1206, 2000.
6. McClymont D. The use of the reactive strength index as an indicator of plyometric training conditions. In: Reilly T, Cabri J, and Arau´ jo D, eds. Science and Football V: The Proceedings of the Fifth World Congress on Sports Science and Football. Lisbon, Portugal, 11–15 April 2003. New York:
Routledge; 2008. pp. 408–416.
7. Schmidtbleicher D. Training for power events. In P.V Komi (Ed.),The Encyclopeadia of Sports Medicine. Vol 3: Strength and Power in sport (pp. 169-179), 1992. Oxford, UK: Blackwell.

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