Plyometric exercises are a commonly used and fantastic tool for increasing strength and power (Chu, 1998). The term plyometric refers to an action that facilitates a maximal muscle contraction as quickly as possible. Such exercises consist of a rapid pre-stretch (lengthening/eccentric contraction) of the musculature involved, immediately followed by a powerful shortening (concentric contraction) (Komi, 1992). Several mechanisms are said to lead towards a greater force output than voluntary contraction alone (Winkleman, 2010). This article focuses on the primary mechanisms that contribute towards the stretch shortening cycle (SSC) and discusses programming implications for plyometric exercise.
The countermovement jump (CMJ) is an example of plyometric exercise; however everyday examples can be observed in running, jumping and hopping. The CMJ is often used as a measure of reactive strength/power (Newton, Rogers, Volek, Häkkinen, & Kraemer, 2006) and the SSC is pivotal to its movement (Winkleman, 2010).
The following video demonstrates a CMJ:
During the counter movement jump, the athlete accelerates towards the ground by bending at the knee, ankle and hip whilst either throwing their arms downwards or placing their hands on their hips. This rapidly lowers the centre of mass and stretches the quadriceps, calf muscles (gastrocnemious and soleus) and glutei (Soest & Bobbert, 1993). The athlete then immediately jumps into the air by forcefully contracting said muscles and extending the legs.
Voluntary Contraction: The brain stimulates muscle contractions by sending impulses via the central nervous system and efferent nerves to the appropriate motor units. Each muscle is made up of many motor units whereby a motor unit consists of one motor neuron and the muscle fibres it activates. When activated, all muscle fibres within the motor unit will contract together (Baechle & Earle, 2008).
Such voluntary contractions and motor control will direct the initial downwards movement during the CMJ by primarily stimulating the appropriate motor units of the hamstrings, hip and knee flexors whilst controlling the eccentric contractions (lengthening) of quadriceps and glutei etc. Subsequent voluntary concentric contraction of the quadriceps, calf muscles, hamstrings and glutei will then cause an extension of the lower limbs and consequently the athlete will jump into the air. However it is utilisation of the SSC through this countermovement that enables a greater force to be exerted than compared with a voluntary concentric contraction alone as seen in a regular squat jump (Bobbert, Gerritsen, Litjens, & Van Soest, 1996).
SSC: The SSC potentiates a greater recruitment of muscle fibres by utilising the stretch reflex (Bosco, Viitasalo, Komi, & Luhtanen, 1982) and combined with the elastic components of muscles and tendons it contributes towards greater force production (Hill, 1970). The stretch reflex can be described as a neural response and protection mechanism to the muscles, whereas muscle elasticity is attributed to its mechanical structure.
Spindle Response during stretch reflex: During a sudden lengthening of a muscle, in this case primarily the quadriceps, sensory organs (muscle spindles) located in the muscle belly (around the intrafusal fibres) release an impulse via Ia afferent nerves to the spinal cord. The size of this impulse depends on the magnitude of the stretch and if large enough, an automatic protective response will be initiated via the alpha motor neurons, causing the extrafusal muscle fibres to contract; thus shortening the quadriceps and extending the leg. This response takes place much quicker than voluntary muscle activation (Wilson & Flanagan, 2008) and enables the stimulation of additional muscle fibres contributing towards greater force production (Bosco et al., 1982). The diagram depicts this feedback mechanism:
Simultaneously the Ia afferent will also transmit an inhibitory response to the antagonist muscle, in this case the hamstrings group, causing them to relax and allowing the quadriceps to contract. This response is known as reciprocal inhibition.
Mechanical model: Pictured below is Hill’s (1970) mechanical model of skeletal muscle which is still commonly used (Winkelman, 2010). The contractile element (CE) is the main method of generating force and is stimulated by corresponding motor units as previously discussed. The series elastic component (SEC) mostly consists of tendons which store elastic energy when stretched. The parallel elastic component (PEC) is made up of the surrounding plasma membrane and connective tissue; it contributes towards muscle stiffness by exerting a passive force against muscle lengthening.
When the muscle tendon complex is stretched, elastic energy is stored in the SEC and can contribute towards force production if a subsequent concentric contraction is performed immediately. If the concentric contraction does not immediately follow the eccentric contraction then this elastic energy will be lost as heat (Baechle & Earle, 2008). Also during a fast SSC movement, the SEC can perform a large amount of work that has been shown to not be possible in slower movements (Kubo et al., 2000). Consequently when the SEC contracts faster, it also allows the CE more time to generate a greater force (Böhm, Cole, Brüggemann, & Ruder, 2006), optimizing its rate of force development (RFD). It should also be noted that the longer length of time it takes to perform the CMJ has been suggested to contribute more to the increased force production than the elastic components of muscle structure as the eccentric loading activates the muscle sooner leading to a greater RFD during the concentric contraction (van Ingen Schenau, Bobbert, & de Haan, 1997).
Practical Application: Due to the greater force generating capabilities of plyometric exercise and greater stress on the musculotendinous unit through eccentric loading, their programming must be considered carefully (Chu, 1998). For the lower limbs it is suggested to follow a natural progression starting with non countermovement jumps that develop explosive strength and neuromuscular efficiency, before progressing to the CMJ and finally depth jumps where the athlete starts by stepping off a box and jumping as soon as they land on the floor (Winkelman, 2010). This series of progressions where the height of the box can be increased will gradually demand a greater contribution from the SSC. The initial prestretch will occur faster as a result of greater eccentric loading, stimulating a larger impulse from the muscle spindles that will lead to the recruitment of more motor units and thus a greater force production which should increase jump height (Bosco et al., 1982).
The ability to exert a maximal force as quickly as possible transfers to most sports and therefore plyometric exercises are a very useful tool in developing the capacities of the SSC for improving sports performance.
Baechle, T. R., & Earle, R. W. (2008). Essentials of strength training and conditioning. Human Kinetics Publishers.
Bobbert, M. F., Gerritsen, K. G. M., Litjens, M. C. A., & Van Soest, A. J. (1996). Why is countermovement jump height greater than squat jump height? Medicine & Science in Sports & Exercise, 28(11), 1402–1412.
Böhm, H., Cole, G. K., Brüggemann, G.-P., & Ruder, H. (2006). Contribution of muscle series elasticity to maximum performance in drop jumping. Journal of Applied Biomechanics, 22, 3–13.
Bosco, C., Viitasalo, J. T., Komi, P. V., & Luhtanen, P. (1982). Combined effect of elastic energy and myoelectrical potentiation during stretch-shortening cycle exercise. Acta Physiologica Scandinavica, 114, 557–565.
Chu, D. A. (1998) Jumping into plyometrics. Champaigh, IL: Human Kinetics
Hill, a. V. (1970). First and last experiments in muscle mechanics. Cambridge, Eng.; United Kingdom: Cambridge Univ. Press.
Komi, P. V. (1992). Stretch-shortening cycle. In, Komi, P.V. (ed.), Strength and power in sport, Oxford, Blackwell Scientific Publications, 1992, p. 169-179. United Kingdom.
Kubo, K., Kanehisa, H., Takeshita, D., Kawakami, Y., Fukashiro, S., & Fukunaga, T. (2000). In vivo dynamics of human medial gastrocnemius muscle-tendon complex during stretch-shortening cycle exercise. Acta Physiologica Scandinavica, 170, 127–135.
McGuigan, M. R., Doyle, T. L. A., Newton, M., Edwards, D. J., Nimphius, S., & Newton, R. U. (2006). Eccentric utilization ratio: Effect of sport and phase of training. Journal of Strength & Conditioning Research, 20, 992–995.
Newton, R. W., Rogers, R. A., Volek, J. S., Häkkinen, K., & Kraemer, W. J. (2006). Four weeks of optimal load ballistic resistance training at the end of season attenuates declining jump performance of women volleyball players. Journal of Strength & Conditioning Research, 20, 955–961.
Soest, A. J., & Bobbert, M. F. (1993). The contribution of muscle properties in the control of explosive movements. Biological Cybernetics, 69, 195–204.
van Ingen Schenau, G. J., Bobbert, M. F., & de Haan, A. (1997). Mechanics and energetics of the stretch-shortening cycle: a stimulating discussion. Journal of Applied Biomechanics, 13, 484–496.
Wilson, J. M., & Flanagan, E. P. (2008). The role of elastic energy in activities with high force and power requirements: a brief review. Journal of Strength and Conditioning Research, 22, 1705–1715.
Winkelman, N. C., (2010). The difference between the countermovement and non-countermovement jump: Implications on performance. Professional Strength & Conditioning, 20, 4-8.