The word ‘plyometrics’ was first coined by US track and field coach Fred Wilt in the 1970’s, ‘plio’ meaning more, and ‘metric’ meaning measure signifies that the idea of plyometric training is to get greater effects from the exercises performed (Verkhoshansky, 2006). The origins of plyometric training come from the Russians ‘shock training’ (Verkhoshansky, 2006), which was limited to variations of depth jumping, a movement requiring limb stiffness, eccentric control and a highly developed stretch shortening cycle. A plyometric exercise can be defined as a movement that incorporates the stretch shortening cycle (SSC); this cycle has to include a rapid eccentric receiving phase, an amortization (controlling) phase and concentric returning phase (Turner & Jeffreys, 2010). Exercises that fall into this category include depth jumping, hurdle jumping, single leg hopping and bounding, an exercise without these three phases cannot be referred to as a true plyometric and should be defined as ‘concentric jumping’ (McBride, McCaulley, & Cormie, 2010).
Verkoshansky (Verkhoshansky & Siff, 2009) suggests a model known as dynamic correspondence; this model states that for an exercise to be sport specific it must be similar to the sporting movement in the criteria shown in Table 1. Of these criteria, the rate and time of force development (RFD) is the most important when it comes to plyometric training. RFD is effectively the slope of the force-time curve and is the speed at which muscle can contract and force can be produced (Aagaard, Simonsen, Andersen, Magnusson, & Dyhre-poulsen, 2002; Baker, 1996). For the SSC to be optimized the amortization phase of a plyometric must be performed in a certain time frame, for a ‘slow’ plyometric (for example a countermovement jump) this is 0.25>0.85 seconds, whereas for a ‘fast’ plyometric (for example a depth jump) it is <0.25 seconds (Turner & Jeffreys, 2010; Markovic & Mikulic, 2010). Adaptations can vary from training the SSC at different speeds (Schenau, Bobbert, & de Haan, 1997) and therefore it is important for an athlete to use the correct exercise to elicit the adaptations that correspond to their sport.
| Table 1 | |
| Dynamic Correspondence Table | |
| Dynamic correspondence quality | Definition of quality |
| Amplitude and direction of movement | Related to the direction and range of movement of a particular segment in the given skill. |
| Accentuated region of force production | The range of movement (or joint angle) where maximal torque is developed. |
| The dynamics of effort | The intensity of the training modality should either match or exceed that found in the sporting movement. |
| Regime of muscular work | Determining the type, speed, and nature of movement e.g. cyclical, static, explosive etc. |
| The rate and time of force development | Particularly important in regards to highly dynamic, time constrained skills.Key kinetic qualities include rate of force development, impulse and power. |
The mechanical model of the SSC includes three parts (Hill, 1938), a contractile component (CC), a parallel elastic component (PEC) and a series elastic component (SEC). The CC is comprised of actin and myosin these are proteins that are responsible for muscle contraction and are key elements of the sliding filament theory. The PEC consists of the epimysium, perimysium, endomysium, and sarcolemma (connective tissue); it is the part that exerts a resistive force when a muscle is passively stretched (when the muscle is relaxed). The SEC includes the structural proteins (i.e. titin), cross-bridges and tendons and is where elastic energy (EE) is stored when a muscle is under tension. In most studies the researchers look at the tendon as being the most important part of the mechanical model as they can extend and store energy and shorten and release it (Kubo, Kawakami, & Fukunaga, 1999; Turner & Jeffreys, 2010), however there are other significant factors such as the stretch (myotatic) reflex.
The myotatic reflex is the neurophysiological aspect of plyometrics, it is the body’s involuntary response to external stimuli that forces the muscles to stretch and return in a powerful spring-like fashion (Turner & Jeffreys, 2010). It involves two key parts known as proprioceptors, the muscle spindles and the golgi tendon organs (GTOs). Muscle spindles are found in the belly of the muscle in the intrafusal fibers which are innervated by gamma motor neurons and detect changes in a muscles length. When the muscle is stretched a signal is sent to the spinal cord via changes in the rate of action potentials to stimulate alpha motor neurons which then send a reflex nerve impulse to the extrafusal fibers to control this lengthening by causing the muscle to contract. The faster the initial stretch occurs, the greater the potentiation and the more forceful the following contraction will be.
GTO’s are found at the point where the skeletal muscle fibers insert into the muscles tendons; its main function is to respond to tendon tension. Each tendon organ is innervated by type 1b sensory nerve fibers, these sense when force is generated within the muscle and cause nerve impulses to be sent to the spinal cord in the form of changes in the action potentials. These synapse with the alpha motor neurons and, if a harmful level of tension is developed, the golgi tendon organ will inhibit/turn off the relevant muscle. Progressive plyometric training and proprioceptive neuromuscular facilitation (PNF) stretching has been show to help to inhibit the GTO’s and therefore allow the extensor muscles of the movement concerned to exert the maximum possible force (Kyrolainen et al., 2005).
The effects derived from the actions of muscle spindles and GTO’s define the three previously mentioned stages, the eccentric phase, the amortization phase and the concentric phase. The eccentric loading phase is typically from landing and involves a high velocity muscle lengthening/pre-stretch, this is where the agonist muscle is stretched, muscle spindles are stimulated and EE is stored in the SEC. The amortization phase is the ‘pause’ between the first and last phases, it is during this period that the muscles must resist and overcome the eccentric forces in order to produce concentric force, it is also the point where nerve signals reach the spinal cord and the alpha motor neurons send a nerve impulse to the agonist muscle. The final concentric phase is where the agonist muscle shortens due to the stimulation from the alpha motor neurons and ‘returns’ the stored EE from the SEC in the form of greater force output (McCarthy, Wood, Bolding, Roy, & Hunter, 2012). The degree of this force output will depend on level of joint flexion, applied force (gravity, bodyweight and potentially external load), and the athletes ability to handle the eccentric loading. The SSC’s effects are highly effective for approximately 0.85 seconds and then decrease by 55% after one second, the EE lost in this time is dissipated mostly as heat energy (Wilson, & Lichtwark, 2011; Cronin, McNair, & Marshall, 2001).
Contraction speed varies depending on the type of muscle fiber concerned, the seven currently known types are I, IC, IIC, IIAC, IIA, IIAX, and IIX. The type I’s are known as slow twitch (ST) fibers, these have slower contraction speeds, less potential to generate force per cross sectional area and slower relaxation times that type II’s (they can retain cross-bridge attachments for longer). The type II’s are the fast twitch (FT) muscle fibers, they contract faster, generate greater force per cross sectional area and relax faster. A study by Bosco, Tihany, Komi, Fekete, & Apor (2008) found that subjects with predominantly FT fibers benefited from fast SSC movements (< 0.25s) with smaller joint angles such as depth jumps, widely spaced hurdle jumps and pogo’s. On the contrary, subjects with more ST fibers benefited from movements with a longer ground contact time (slow SSC) such as high hurdles and countermovement jumps. The reasoning behind this according to the researchers is that the relaxation times in ST fibers are longer, allowing them to maintain cross-bridge attachments (the connection between the myosin heads and actin) for longer, allowing the fibers to use elastic energy more effectively (Bosco, Tihany, Komi, Fekete, & Apor, 2008; Verkhoshansky, & Siff, 2009; Schmidtbleicher, 1992).
The speed of a plyometric exercise can be influenced by a number of factors; a key factor is the type of surface the exercise is performed on. A surface too compliant will generally increase the ground contact time and the time to peak vertical force (McNitt-Gray, Yokoi, & Millward, 1994) possibly due to, aside from the biomechanical reasons, time spent regaining balance and stability due to increased proprioceptive demand. The national strength and conditioning association (NSCA) position statement states that all plyometric exercises should be performed on a firm, shock absorbent surface (Kutz, 2001) rather than an overly hard surface or one with a lack of any resilience in order to prevent impact-induced injuries to bodily tissues and restrict the length of the amortization phase respectively (Jensen, & Ebben, 2007). Appropriate surfaces fall into two categories, compliant and non-compliant. A compliant surface can be defined as one that permits a high degree of shock absorption when compared to a non-compliant surface; examples of compliant surfaces include mini-trampolines, thick matting and sand. A non-compliant surface is the opposite of the above and can be defined as one that permits less shock absorption and is more resilient, although it must still conform to the NSCA guidelines; examples include wrestling mats and grass.
Aquatic plyometric training has recently started to become more common in sport-related fields ranging from strength and conditioning to sports rehabilitation, this is mainly due to the fact that joint impacts and eccentric muscle activation are decreased in water due to its viscosity and buoyancy, leading to less injuries but very little difference in training effects (Miller et al., 2007; Robinson, Devor, Merrick, & Buckworth, 2004). Training in water has positive effects on muscle power due to the added resistance of the water in plyometric exercises and the fact that training against water resistance always elicits a concentric contraction; more muscle activation is required to overcome this resistance and therefore any horizontal or lateral jumps especially will require greater physical effort (Miller, Berry, Gilders, & Bullard, 2001; Robinson, Devor, Merrick, & Buckworth, 2004; Thein & Brody, 1998). A six week study (Miller et al., 2007; Miller, Berry, Gilders, & Bullard, 2001) found that there were very little significant differences in the post-testing results between groups that performed plyometrics in waist deep water and chest deep water. The main difference was that the chest deep water group found greater force and power production improvements, presumably because more body mass was submerged meaning there was greater resistance. Thein and Brody (1998) stated that an optimal water temperature for aquatic training should be between 26 and 28°C to prevent heat related issues as water based exercise can cause increases or decreases in body temperature more readily that on land due to increased conduction, convection and radiation.
It is expected that both land-based and aquatic plyometric exercises would improve the strength, quality and elasticity of connective tissues such as tendons and ligaments due to the increased demand for eccentric control of forces and the greater concentric power output during the stretch shortening cycle (Verkhoshansky & Siff, 2009). Additionally plyometrics have been proven to increase vertical jump height (Baker, 1996; Miller et al., 2010), physiologically this is believed to be due to neuromuscular adaptations including altered motor unit recruitment, improved reflex potentiation and potentially hypertrophic increases in cross-sectional area (in type I and II muscle fibers) (Vissing et al., 2008). The key difference in terms of tissues between land and aquatic plyometrics is the degree of ensuing muscle soreness. Land-based plyometrics are associated with increased delayed onset muscle soreness (DOMS), an inflammatory response which is connected and is part of the body’s adaptation process for strength development, when compared to concentric jumping (Robinson, Devor, Merrick, & Buckworth, 2004). This inflammation is caused by free radical production, an increase in levels of cytokines, phagocyte penetration into the muscle to remove dead cells and muscle micro trauma (Chatzinikolaou et al., 2010). Greater muscle micro trauma is present in land-based plyometrics (levels increase according to surface compliance) because the higher muscle tension during intense eccentric contraction from shock absorption causes high levels of collagen breakdown (Tofas et al., 2008). Aquatic plyometrics have been shown (Arazi, Coetzee, & Asadi, 2012; Shiran, Kordi, Ziaee, Ravasi, & Mansournia, 2008) to cause less muscle micro trauma and inflammation whilst contributing the same amount to performance enhancement. The practical application of this is that if athletes use aquatic plyometrics in their training it will provide them firstly with less discomfort from DOMS, and a shorter recovery time than the generally prescribed 48-72 hours for land-based plyometrics (Chatzinikolaou et al., 2010). It also shows the appropriateness of aquatic plyometrics as a therapeutic modality for injured athletes or other populations (Miller et al., 2010).
Plyometrics are suitable, in different forms and to varying degrees, for all populations, including the elderly, children and injured/disabled. From a rehabilitation point of view plyometrics can be used to restore stability, strength and speed in athletes who are recovering from injuries (Chu, 1999). Plyometric training does this by establishing better intermuscular co-ordination during dynamic movements (Hutton & Atwater, 1992), increasing muscle spindle sensitivity (Swanik, 1999), bone density, and tendon strength. Performing plyometrics in an aquatic setting will incorporate all of these benefits besides, potentially, bone density due to decreased impact forces.
Benefits to performing plyometrics in water are that injured athletes will be more comfortable, relaxed and have a higher pain tolerance and as such will be able to perform exercises that are more effective than those which they could perform or would be willing to perform on land (Miller, Berry, Bullard, & Gilders, 2002). According to Pohl and McNaughton (2003) water is 800 times denser than air and 12 times more resistant (Hoogenboom & Lomax, 2004), and will therefore provide support for approximately 50-54% (Rowsell, Coutts, Reaburn, & Hill-Haas, 2009) of an athlete’s bodyweight and resistance to motion due to buoyancy (Miller, Berry, Gilders, & Bullard, 2001) whilst decreasing compression, vibration and torsional forces which would occur during land-based exercise (Rowsell, Coutts, Reaburn, & Hill-Haas, 2009). Effective SSC adaptations can still be made during aquatic plyometrics as water does not reduce the force required to control the eccentric landing phase or to overcome the water resistance during the concentric jumping phase (Miller, Berry, Bullard, & Gilders, 2002). Adapted aquatic training is beneficial for athletes who are injured at any time during the yearly cycle, but especially during pre-season and in-season when they need to get back to competitive shape quickly following their injury but are unable to take part in land-based activities (Robinson, Devor, Merrick, & Buckworth, 2004; Stan, 2012).
Injury prevention is starting to play a much bigger part in athlete’s strength and conditioning programs; coaches are prescribing huge varieties of exercises from unstable surface training and resistance bands to emphasizing ‘functional’ training more. Plyometric training should, and already is in some cases, be utilized more effectively as a means for prevention of injury as well, especially for the lower limbs, but also for the trunk and upper body. With hamstring injuries being one of the most common in athletes, developing high levels of eccentric strength and control is a key factor in preventing injury during fast cutting movements, rapid deceleration and when landing (Askling, Karlsson, & Thorstensson, 2003). Additionally, balance and proprioception are important components of performance in every sport, especially those that include a unilateral aspect (most sports) and a landing aspect. The knee and ankle are major areas of injury for athletes with poor balance and proprioception (Ekstrand & Gillquist, 1983), and injuries that occur in these areas, such as anterior cruciate ligament (ACL) tears, can keep an athlete out of competition for a long period.
As previously stated, plyometric training will improve eccentric control in the hamstrings, increase muscle spindle sensitivity (and therefore proprioception) and will, as with any load-bearing exercise, improve strength and stability around joints. Aquatic plyometric training will also lead to improvements in these areas, although there will be less eccentric load on the lower limbs due to the buoyancy provided by the water (Miller, Berry, Gilders, & Bullard, 2001). Performing plyometrics in the water may actually be better than doing them on land in terms of improving joint stability as there is, due to the movement of the water, a greater balance and stability challenge especially during single leg movements.
Plyometrics are widely accepted as an effective form of training for athletes of almost every sport, especially ones that involve a lot of jumping movements such as volleyball, and basketball (Martel, Harmer, Logan, & Parker, 2005; Arazi, Coetzee, & Asadi, 2012), they have been shown to improve SSC function, tissue qualities, balance, stability, and eccentric, concentric and isometric strength (McBride, McCaulley, & Cormie, 2008; Pire, 2006). Even blood lactate response, VO2 Max and lactate threshold has been shown to improve as a result of plyometric training (Brown, Ray, Abbey, Shaw, & Shaw, 2010). Plyometric exercises can be adapted to improve different qualities by influencing the ground contact time (and therefore the SSC speed). This can be done by increasing loads, changing the surface compliance, rest periods and jump heights and distances. It is also possible to differentiate between the benefits of horizontal and vertical plyometric exercises. Horizontal movements are generally used for single leg drills, with the intention to develop landing stability and mechanics through improved intermuscular co-ordination. Vertical movements are utilized with bilateral drills to develop more height and power from a stationary position (bilaterally an athlete has greater balance and control and hence is a better position to develop high power from) through intramuscular adaptations. The only major difference between land-based plyometrics and aquatic plyometrics is that there is no research on the benefits of horizontal plyometrics in water.
Siff (2003) stated that aquatic plyometrics (and any other water based explosive training) should not replace land-based plyometrics as it does not drive the same neuromuscular adaptations. However more recent studies (McCaulley et al., 2009) and the research cited throughout the previous pages have stated that aquatic plyometric programs can provide at least equal, and in some cases, greater benefits that that of land-based plyometric programs. Plyometrics in the water cause less DOMS, are safer than land-based plyometrics, promote interest and give variety to athletes and can allow for more regular training. They also provide almost identical physiological adaptations, and in some cases even better adaptations, for example, the waters buoyancy reduces the mass of the participant, meaning a reduced ground contact time and faster jump time. Additionally the water resistance (viscosity) causes a greater SSC concentric contraction, increasing lower limb power. Negative aspects are that specialized equipment is required that some coaches may not have access to, adequate space is required in a swimming pool or aquatic environment of appropriate depth and strong coaching skills are required especially if a larger group of athletes are being trained. Overall aquatic plyometric training has been shown to be equally effective as land-based plyometrics and has applications to injury prevention, rehabilitation, and strength and conditioning. Coaches should consider utilizing periods or sessions of aquatic plyometrics in their athlete’s macrocycles in order to prevent injury, monotony and to improve performance.
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Hobbs Performance 