What Are Plyometrics And Why Do We Utilise Them??
Plyometrics, or "stretch-shortening" exercises expose the body to a repulsion stimulus that increases power, speed, improves coordination, and accommodates the body to explosive movements and forces (15,6). They are a popular exercise in the preparation of track and field athletes (5). The purpose of plyometric training is to teach the neuromuscular system to respond instantaneously to the stretch reflex with as fast and forceful a muscle contraction as possible (9). The greater the strength of the athlete and the faster that athlete can activate or "turn on" that strength, the more powerful the athlete (9,17,10). Power production is a key determinant of success in sport (4). Athletes need to use training techniques that are focused on transitioning the strength gained from heavy (but slow) resistance training to high-velocity movements (4). Plyometric training done with rapid force absorption (e.g., landing from a jump) and force generation (push-off from a landing) is critical to help the athlete convert strength into power (4).
How Does It Work??
During the deceleration phase (e.g. landing from a jump or bound) the stretch reflex is triggered in those muscles that are causing the deceleration (9,11). The stretch reflex is a protective response that keeps rapidly stretched or loaded muscles from tearing (9). When engaged, the stretch reflex causes muscles that are being stretched to contract (10). During this braking process, the muscles momentarily store elastic energy like a stretched rubber band, which has the potential to "snap back." The faster one voluntarily contracts muscles that have been automatically activated by one’s own stretch reflex, the greater the combined force output from released elastic energy and voluntary concentric muscle action (9, 10). Therefore, plyometric exercises enable our muscles to deliver maximum force in the shortest possible time (9) (Power = strength x speed).
During the deceleration phase (e.g. landing from a jump or bound) the stretch reflex is triggered in those muscles that are causing the deceleration (9,11). The stretch reflex is a protective response that keeps rapidly stretched or loaded muscles from tearing (9). When engaged, the stretch reflex causes muscles that are being stretched to contract (10). During this braking process, the muscles momentarily store elastic energy like a stretched rubber band, which has the potential to "snap back." The faster one voluntarily contracts muscles that have been automatically activated by one’s own stretch reflex, the greater the combined force output from released elastic energy and voluntary concentric muscle action (9, 10). Therefore, plyometric exercises enable our muscles to deliver maximum force in the shortest possible time (9) (Power = strength x speed).
Plyometric Pre-requisites
Athletes should first be able to demonstrate a body weight squat with good posture and limited forward trunk lean while maintaining neutral knee alignment; otherwise, these functional deficits are likely to be exacerbated when plyometric exercise is implemented because of the high forces (4). Furthermore, the athlete must have full range of motion and an adequate base level of strength, endurance, and neuromuscular control in order to properly perform plyometric exercise without developing or reinforcing bad techniques (4,6). Plyometrics require a strength base to be able to perform them safely and effectively (5). Without a sufficient strength base and the flexibility to back it up, plyometrics may pose more of an injury threat than a potential gain in explosive power (9). The general consensus in the literature for lower body plyometric training is that athletes must be able to perform a 1RM of a 1.5 to 2 x body weight squat. If the ratio is below 1.5, the athlete should continue with a strength training program and perform low to medium level plyometrics until achieving the required ratio (15,1,4). Conversely, literature (4) states that a squat of 60% of body mass five times within five seconds is adequate before he/she can begin plyometric training. This particular modality may be a better demonstration of the elasticity and torque capacity of the involved tissues. For the upper extremity, the strength levels include the ability to perform a bench press equal to body mass or to perform five hand-clap push-ups (4,9,5,15,1). Although injury risk is low, not adhering to pertaining requirements and proper progressions will increase the risk of injury (1).
Shock Method/Depth Jumps
The origin of plyometrics was referred to as the ‘shock method’ (18). Athletes would drop from a height and upon impacting with the ground they would experience a ‘shock’ which would immediately be followed by a jump (take off) (14).
This is the most advanced and intense form of plyometrics (1). If the athlete conducts a vertical take-off after a jump with the aim of flying as high as possible, these conditions force the central nervous and physiological systems to exceed the ordinary boundaries (18). The creation of such conditions in the training process is the forced intensification of the work regime which becomes a potent training stimulus. Under these conditions the body mobilises any innate mechanism designed by nature to be available for these and even more complex, extreme situations (18).
Research (Fig: 1) by Verkhoshansky (18) demonstrates the limits and diminishing returns with regards to intensity recommendations.
Fig 1: The maximal values of power output (N), Coefficient of reactivity (R) and the minimal ground time (T) were reached in depth jumps executed from the height of 75cm. The maximal level of effort (Fmax) was reached in the depth jump executed from the height 95-115cm (18). Verkhoshansky concluded that when focusing on explosive power, the ideal height to drop from was about 75cm.
Counter Movement Jumps
Athletes need to use training techniques that are focused on transitioning the strength gained from heavy resistance training to high-velocity movements (4).
Research (18,13) concludes that adding additional weight to the counter movement jump decreases the height of the jump and consequently, the coefficient of reactivity and the working effect of the take-off movement. This results in a decrease in plyometric performance. Literature (12) states that the counter movement jump is the most reliable and valid field test for the estimation of explosive power of the lower limbs in physically active men.
Complex Training
Complex training refers to alternating between a heavy strength training exercise and similar plyometric exercises on a set for set basis (2). The athlete may perform a set of heavy squats followed by a set of squat jumps, or a set of heavy bench press followed by a set of clapping push ups (5,19). This can be done with either exercise being performed first (16). The aim of complex training is to maximally recruit the nervous system with the heavier training and to use that enhanced recruitment (called post activation potentiation or PAP) to improve performance on the plyometric exercises (5). Research on complex training is mixed. Studies looking at athletes find that it enhances power output (3,8) whereas studies looking at untrained college students find that it has no effect (7). This reinforces the fact that athlete status and strength levels are important for plyometric training effectiveness (5).
Summary
The literature is quite clear about how to progress athletes/clients into plyometric training. The true power increases lie in depth jumps. Depth jumps stimulate the elastic properties in the connective tissue which increase tensile tendon strength. Providing the athlete/client is conditioned correctly and meets the recommended pre-requisites, then a smart and steady shock method program can be implemented alongside a current strength program.
References
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2) Alves, M., Vilica, J.M., Antonio-natal, R., Abrantes, C. and Sampaio, J. (2010) "Short-term effects of complex and contrast training in soccer players 'vertical jump, sprint, and agility abilities,. Strength and conditioning. Vol. 24, No. 4: 936-941.
3) Baker, D. (2003) Acute effect of alternating heavy and light resistance on power output during upper-body complex power training. strength and conditioning. Vol. 17, No. 3: 493-497.
4) Chu, D. and Myer, G. (2013) How Plyometrics Work. Human Kinetics. No. 206: 6569-6575.
5) Cissik, J. (2011) A Fresh Look At Plyometrics. Track Coach. No. 195: 6234-6236.
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11) Makaruk, H., Czaplicki, A., Sacewicz, T. and Sadowski, J. (2014) The Effects Of Single Versus Repeated Plyometrics On Landing Biomechanics And Jumping Performance In Men. Biology of Sport. Vol. 31, No. 1: 9-15.
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13) McBride, J., Nimphius, S. and Erickson, T. (2005) The Acute Effects Of Heavy-Load Squats And Loaded Countermovement Jumps On Sprint Performance.. Strength and conditioning. Vol. 19, No. 4: 725-963.
14) Nunn-cearns, G. (2013) Plyometris/Jump Training. Modern Athlete and Coach. Vol. 51, No. 1: 15-19.
15) O'connor, D. and King, J. (1999) Application of Plyometrics to the Trunk. Human Kinetics. Vol. 4, No. 3: 36-40.
16) Stone, M., Sands, W., Pierce, K., Ramsay, M. and Haff, G. (2008) Power and Power Potentiation Among Strength–Power Athletes: Preliminary Study. Sports Physiology and performance. No. 3: 55-67.
17) Vassil, K. and Bazanovk, B. (2011) The effect of plyometric training program on young volleyball players in their usual training period. Journal Of Human Sport and Exercise. Vol. 7, No. 1: 34-42.
18) Verkhoshansky, Y. (2012) Shock Method and plyometric: Updates and indepth examination. [Online] Available from: http://www.verkhoshansky.com/ [accessed 16 February 2015]
19) Wilcox, J., Larson, R., Brochu, K. and Faigenbaum, A. (2006) Acute Explosive-Force Movements Enhance Bench-Press Performance in Athletic Men. International Journal Of Sports Physiology and Performance. Vol. 1, No. 3: 261-270.
