The Effect of Plyometric Training on Muscle Strength - Research Paper Example

The effect of plyometric training on muscle strength and speed performance Exercise training interventions generally aim to maximise the physiological effects that lead to better performance. All athletes seek to improve enormously, through various types of exercise, their skeletal muscle especially in the arm, legs, and torso that are important for the bodily movements. Plyometric training is a widely used technique for enhancing athletic performance. Plyometric exercises are also known to decrease injury risk especially in female athletes. Motion is the basis of life in humans and the organ directly related to motion is muscle. All our bodily movements, including involuntary physiological movements, are performed by muscles. Adequate skeletal muscle strength is essential for normal physical functioning, while enhanced muscle strength is an absolute requirement for competitive athletes. Muscle quality is defined as muscle strength, (i.e., maximal voluntary contractile force or torque) per unit regional muscle mass (Newman et al., 2003). Muscle mass is known to correlate with strength measured as maximum isometric force exerted in healthy adults (Maughan et al., 1983). Muscles are of different shapes and sizes and function in different roles. Large muscles such as the hamstrings and quadriceps, generally control motion. Muscles possess characteristics such as excitability or the ability to respond to stimuli (e.g., nerve impulse), contractibility or the ability to undergo shortening or stretching of length, and elasticity, that is, the ability to regain original length and shape after contraction or extension (The Muscle Physiology Laboratory, University of California at San Diego, http://muscle.ucsd.edu/musintro/jump.shtml). The human muscle consists of subunits called fascicles that are made up of fasciculi which are bundles of singly innervated muscle fibres linking one tendon to the other (Paul, 2001). Tensional forces developed by muscle fibres are transmitted to bones via tendinous structures, generating natural and complex movements through the actions of the joints. The viscoelastic properties of tendon allow for a dynamic interaction between the muscle and the tendon, enabling muscles to contract isometrically and generate force during human movements (Fukunaga et al., 2002). Sarcolemma or the muscle cell membrane maintains a membrane potential, like the neuronal membrane (Davies and Nowak, 2006). Also, like in the nerve cell membrane, impulses are transmitted along muscle cell membranes. The motor nerve generates the action potential that travels along the sarcolemma and through the transverse tubular system deep into the muscle to excite the muscle fibre. The only function of impulses in muscle cells is to cause contraction. The complex interaction between the central nervous system, muscle–tendon unit and bony structures produce joint movement. The mechanism of muscle contraction Skeletal muscle contraction is dependent on nervous impulse. An important property of skeletal muscle is its ability to contract to varying degrees, and the degree of contraction is proportional to the number of motor units in the muscle that is stimulated. The muscle fibres are composed of innumerable thread-like myofibrils which are the actual myocyte contractile apparatus. The myofibrils themselves are made up of an infinite number of bands or sarcomeres which are the functional units of the muscle. A sarcomere consists of at least 20 known proteins present in an ordered and almost crystalline arrangement (Becker et al., 1997). Overlapping thick and thin filaments called (myo)filaments, are arranged end-to-end within the sarcomere. Two proteins namely, actin and myosin which are involved in the process of muscle contraction are the constituents of the two filaments. The thick filament is made up of about 400 myosin molecules, and interacts with actin filaments containing about the same number of actin monomers (Szent-Györgyi, 2004). The myosin heads are known as cross-bridges because they can bind to and move along actin in the thin filament. In simple biochemical terms, during the cross-bridge cycle, actin combines with myosin and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate (http://muscle.ucsd.edu/musintro/ bridge.shtml). Myosin, actin and ATP together constitute the contractile system. ATP represents potential energy, therefore, the interaction of the two proteins with ATP converts chemical energy into mechanical work. The actin-myosin reversible binding is the molecular basis for force production and movement in muscle cells. The amount of force produced by the muscle is dependent on the proportion of myosin binding to actin which, in turn, is tightly regulated by the troponin complex, tropomyosin, concentration of Ca++ in the muscle cell cytoplasm, and flux of Ca2+ ions (Becker et al., 1997). The timing of muscle contraction is thus controlled by calcium ions released from the sarcoplasmic reticulum. A low Ca2+ concentration in the sarcoplasm activates the muscle, while removal of Ca2+ results in relaxation (Szent-Györgyi, 2004). Sarcomere structural changes during muscle contraction Electron microscopy has revealed that the cross-striated muscle is organised in sarcomeres that extend from one Z-line to the next. A sarcomere is arranged with the thick myosin filaments sandwiched between two actin filaments. The thick filaments are symmetrical at the ends, with a bare zone in the centre (the H zone) in which there are no cross-bridges. Right spatial relationships of muscle structural proteins within the sarcomere lead to productive muscular contraction. The sliding filament theory explains the contraction of voluntary muscles as being due to mutual sliding (that is, relative movement) of the two sets of filaments, the thick (containing the protein myosin) and the thin (containing the protein actin), without any change in filament lengths (Szent-Györgyi, 2004). However, changes in the sarcomere length do occur during muscular contraction. In a relaxed muscle, actin and myosin filaments lie side by side and the H zone has maximum width. During the process of contraction, the actin and myosin myofilaments interact, thereby causing filament sliding. The actin filaments present in the two halves of the sarcomere slide to the centre of the myosin filament that is, the H zone, causing the shortening of the sarcomere, while the thick myosin filament binds a thin actin filament at each end. In the fully contracted muscle, with the ends of the actin myofilaments overlapping, the H zones disappear, while the I band (the space between consecutive sarcomeres) narrows. The myosin-actin interaction is reversible because the affinity of myosin to actin is greatly reduced by ATP binding due to which myosin separates from actin (Szent-Györgyi, 2004). Following the detachment of myosin, hydrolysis of ATP to ADP and inorganic phosphate occurs, producing energy to contract the muscle. In the relaxed muscle, the myosin heads are bound to tropomyosin. Troponin has binding sites for calcium ions and when bound to Ca, the shape of troponin molecule changes and its position shifts which tugs at the tropomyosin to which it is attached. The movement of tropomyosin away from myosin leaves the field open for actin to bind to myosin, and the process of muscle contraction gets underway again. Force production during muscle contraction Skeletal muscle movement is voluntary. The skeletal muscle fibres are generally classified as type I (oxidative/slow) or type II (glycolytic/fast) fibres. All the different types of muscle fibres are present in all muscles in varying amounts. The myosin heavy chain (MHC) isoform patterns have shown that limb skeletal muscles contain one slow fibre (Type I), and three types of fast fibres (Type IIa, IIb and IIx). These fibres have marked differences with regard to contraction, metabolism, and susceptibility to fatigue (Wang et al., 2004). According to Booth and Thomason (1991), Type I fibres have the ability to withstand fatigue since they have ample ATP for oxidative metabolism, being rich in mitochondria. Thus, athletes genetically have a higher percentage of slow twitch fibres tend to excel in events requiring endurance, such as the marathon race. Another feature of Type I fibres is the presence of large numbers of capillaries. Type I muscles are red in colour because of the presence of high concentrations of myoglobin, a protein normally produced by oxidative muscle fibres. Type IIb fibres are the fastest fibres to contract. They are dependent on glycolysis for their enegy requirements as they are poorly endowed with mitochodria and oxidative enzymes. Type IIa and IIx are between type I and IIb in their oxidative capacity and contraction functions. Type IIb are concentrated in the arms and legs. Peak power output is considerably greater in those possessing a predominance of fast fibres. However, although the fast fibres can generate very powerful contractions over a short period of time, they also fatigue fast. Classification of voluntary muscular contractions The generation of twitch tension in the muscle fibres through the action of actin and myosin cross-bridge cycling leads to changes in the length of the muscle. Skeletal muscle contractions are classified according to changes in length or resultant force as concentric, eccentric and isometric contractions. This classification corresponds to motor, shock absorber and stabiliser aspects of muscle function (Timm, 1991). The term “contraction” is a misnomer inasmuch as all types of muscular contraction do not actually cause shortening of muscle. Only concentric contractions cause the muscle to actively shorten. In the eccentric muscle action, the muscle fibers actively lengthen, while in the isometric contraction, muscle actively holds its fixed length (http://www. neuromus.ucsd.edu/musintro/contractions.shtml). The concentric muscle contraction is accompanied by shortening of the muscle fibres as in contracting to lift the load. For instance, climbing the stairs works the quadriceps concentrically. On the other hand, an eccentric muscle action, such as walking down the stairs, involves the lengthening of the muscle fibres to lower the load. Muscle contraction alteration through exercise training Voluntary muscle contraction is controlled by the central nervous system. The central nervous system transmits the signals in the form of action potentials to the motor neuron that innervates several muscle fibres. Through programs of regular exercise healthy subjects are able to achieve maximal activation of their muscles. This is possible since the mechanical properties of slow and fast muscles can be made to adapt to precise directions from the central nervous system (Fitts and Widrick, 1996). Peak power output is considerably greater in those possessing a predominance of fast fibers. Adult skeletal muscle being highly flexible, it can switch between different fibre types as a result of exercise training or altered motoneuron activity (Booth and Thomason, 1991; Pette, 1998; Hood, 2001). Originally eccentric exercise programs were designed in order to enhance muscle power capabilities for competitive athletic pursuits (Strauber, 1989). The eccentric contraction or the shock absorber function of the muscle also has clinical rehabilitation application as it involves contractile as well as connective tissue activity (Timm, 1991). Comparison of eccentric only with concentric only has shown that strength yields in the former are much higher (Hortobagyi et.al., 1997; Farthing and Chilibeck, 2003). Muscle growth promoted by eccentric contraction training is also better (Farthing and Chilibeck, 2003). However, the American College of Sports Medicine, ACSM (2000) recommends that the resistance-training and strength-training programmes should include concentric as well as eccentric muscle actions since proper muscle strength enhancement results from the activation of the specific muscle fibres recruited by both the protocols. A combined concentric and eccentric program has been shown to yield greater improvements in strength and strength-related tasks compared to a concentric- only training regimen (Colliander and Tesch, 1990). Even the performance of normal activities of daily life by older persons has shown greater improvements with combined eccentric and concentric exercise programme as opposed to concentric-only exercise (Gur et al., 2002). Muscular endurance has also been reported to improve following adoption of a combined concentric and eccentric training using 1–3 sets and 15–20 repetitions (Bird et. al., 2005). Nervous system adaptations occur with training techniques that cause increased recruitment of fast twitch fibers and better synchronization of the muscles involved in the action (Hather et al., 1991). When tension is induced in the muscles, either by contracting or lengthening, it results in increased blood circulation to these muscles bringing in the much needed oxygen and nutrients. Repetitive exercise performance stimulates muscle cell growth (hypertrophy). Studies have shown that eccentric training is superior to concentric training for inducing muscle hypertrophy (Farthing and Chilibeck, 2003). Thus, concentric and eccentric training exercises provide the basis to develop maximum athletic potential. Plyometric training Power in terms of athletic movements is a combination of strength and speed. Since time immemorial, athletes and their coaches have constantly sought training methods that improve power and enhance performance. Optimisation of muscular power output is considered fundamental to successful performance in most athletic and sporting activities. Plyometric training is a distinct method of exercising that is focussed on increasing muscular power or explosiveness. It originated in Russia about 40 years ago when Yuri Verkhoshansky devised what he called as the "shock training." Plyometric exercises enhance the maximum reaction of the trainee athlete through eliciting powerful and rapid eccentric muscular contractions. Hence, plyometric training techniques are used by athletes in all types of sports to increase strength and explosiveness (Chu, 1998). The word Plyometric is formed from two Greek words: Plyo, meaning greater, and metric for measurement (Zanon, 1989). Briefly, the plyometric technique consists of an eccentric action such as the rapid stretching of a muscle which is immediately followed by a concentric, that is, shortening action of the same muscle and connective tissue for the purpose of developing a forceful movement over a short period of time (Chu, 1986; Baechle and Earle, 2000; Chimera et al., 2004). Muscle lengthening immediately preceding a muscle shortening activity is known as a stretch-shortening cycle (Komi, 1992). An exercise protocol that promotes an explosive movement in the athletes following a stretch-shortening cycle is termed plyometrics. (Chu, 1983). Also, theoretically, any exercise that includes prior loading could be described as a plyometric activity. Thus, sprinting is a plyometric activity in the sense that it involves taking one step after another. The plyometric exercises are basically made up of depth jumps and depth drops. The concentric activity in the stretch-shorten cycle pre-stretches the contractile tissue with kinetic energy (Stauber, 1989; Wilson et al., 1991). Such an exercise procedure is intended to provide high-level muscle conditioning and adequately prepare the muscle power functions and performance skills of the athletes to meet the demands of the various sports (Timm, 1991). The principle of stretch-shorten action The stretch reflex inherent in the muscle causes a stretched muscle to contract, but prevents contraction of the antagonist muscle. For example, the contraction of the quadriceps muscle when the patellar tendon is hit with a rubber mallet is because the hitting of the tendon causes the quadriceps to stretch. The contraction here is actually to prevent over-stretching of the quadriceps, and at the same time, contraction of the antagonist hamstring muscle is also inhibited (Chu, 1983). The plyometric technique takes advantage of two different properties of the muscle namely, elasticity and stretch reflex to produce instant explosive force, and improving, over time, the reactive strength. When the muscle lengthens through eccentric action, much of the energy produced is lost as heat (Wilk et al., 1993). However, some part of the energy is also stored by the elastic components of the muscle. The combination of eccentric and concentric actions of the stretch-shorten cycle enables the stored elastic energy to be used to produce more powerful muscular contractions than those produced by a concentric action alone (Assmusen and Bonde-Peterson, 1974, Miller et al., 2002). Tendons and the cross bridges between actin and myosin comprise the series elastic components (SEC) of the muscle that store energy when stretched. When the musculotendinous unit is stretched under the action of the eccentric muscle contraction, the SEC acting as a spring, stretches, storing the elastic energy. In the plyometric technique, when the muscle undergoes a concentric contraction immediately following the eccentric action, the stored elastic energy is released while the muscle and tendons regain their unstretched configuration. The storage and reutilization of elastic energy by the tendinous structures play an important role in enhancing work output and movement efficiency of different types of human activity (Fukashiro et al., 2006). Bosco et al. (1980) have shown that the faster the muscle is stretched eccentrically, the greater the force exerted on the following contraction. Some important observations so far have been that the muscles are able to contract more strongly if they are pre-stretched, and the tension created is also dependent on the muscle length (Faccioni,A. http://163.178.103.176/Fisiologia/general/activ_bas_3/ Plyometric1.pdf). These can be explained thus: when the muscle is stretched, the muscle spindle also lengthens proportionately. The muscle spindles are the primary proprioceptors or nerve endings related to stretching which are located in the tendons and in the muscle fibres. The muscle spindles sense the eccentric stretching of the muscle fibres (that is, change in length as well as the rate of change), and signal the information to the central nervous system. This triggers the stretch reflex whereby the change in muscle length is counteracted by inducing the stretched muscle to contract. The more acute the stretching of muscle, the stronger will be the muscle contractions (Chimera, 2004). According to Woodrup (2008), the shock method or classical plyometrics involving the rapid production of the maximum possible muscle tension through mechanical shock stimulation has been described by Mel Siff as the most effective. This method also involves a relaxed muscular state such as stepping down a box before the induction of intense muscular contraction e.g., bouncing upwards immediately after landing. An important feature of shock training is a very brief transition period consisting of a pause, rather like a reflex action, between the end of the eccentric phase and the start of the concentric phase. This type of dynamic activity is designed to take advantage of the energy available from increased tension due to muscle reflex caused by the impact stimulus, as well as from the release of the elastic energy produced and stored during the eccentric phase to improve speed (Position paper Plyometrics, www.biggerfasterstronger.com). Whenever a plyometric technique is incorporated into a training programme, it is extremely important to expedite the shortening phase of the stretch-shorten cycle since loss of energy produced in the initial eccentric stretching as heat (Wilk et al., 1993) could occur. In other words, to improve muscle strength, and, thereby, performance through plyometrics, the athletes need to explode off the ground as quickly as they can. A sprint start is a complex motor feat. In a sprint start, the block acceleration in the first two steps primarily depends on the activation of the rectus femoris muscle among others. Apart from the reaction to the starter's pistol, explosive power is of primary importance to the athlete. Also, a high proportion of fast muscle fibres which participate in sprints/fast movement is essential for speed development. Plyometric techniques improve reactive strength by utilising the strength-shortening cycle (SSC) movements, besides stimulating the fast-twitch muscle fibres. Optimal leg muscle power is a general requirement for the performance of daily activities. So also, leg muscle power and vertical jumping ability are two critical prerequisites for effective performance in sports and athletics (Potteiger, 1999; Canavan and Vescovi, 2004). The development of speed in athletics depends primarily on genetic make-up of the individual and the efficiency of the training methodology. The vertical jump performance is an important aspect of human muscle power capabilities and its enhancement is possible through various training methods, such as heavy-resistance training (Wilson, 1993); explosive-type resistance training (Adams, 1992); electrostimulation training (Malatesta, 2003) etc. However, most sports scientists and coaches are of the opinion that plyometric training (PT) is a method of choice when improvements in both vertical jump and leg muscle power are in focus. PT essentially involves performance of stretch-shortening cycle (SSC) movements. PT for the lower body, consists of various types of jumping exercises, including drop jumps (DJs), countermovement jumps (CMJs), alternate-leg bounding, hopping and other SSC bouncing exercises (Markovic, 2007a). Studies have shown PT to influence CMJ (countermovement jumps) twice as much as the SJ (squat jump) (Markovic, 2007a). Statistically significant improvements in vertical jump height obtained with PT could be of practically very relevant since the improvement in vertical jump height of ~10% was also matched by similar increase in sprinting performance (Kotzamanidis, 2006; Chimera, 2004; Little, 1996). Data suggest the possibility of the positive influence of PT on vertical jump ability being carried over to other athletic performances, too. From the perspective of the above-discussed results, PT could, therefore, be a desirable programme to enhance especially the vertical jumping ability, and other athletic performance as well. PT has been found to be more effective in improving performance of vertical jumps in the SSC jumps as it boosts the ability of the athletes to make use of the elastic and neural benefits of the SSC (Wilson, 1993). Greater positive effects of PT observed in the slow SSC jumps (particularly the CMJ) as compared to either the concentric-only jumps (ie, SJ), or the fast SSC jumps (ie, DJ) are to be expected because of the specificity of contraction-type training (ie, SSC muscle function) (Markovic, 2007a). On the other hand, the observed difference in the effects of PT between the CMJ and the DJ, is a result of the biomechanical differences between slow and fast SSC jumping exercises (Bobbert, 1990). Significant difference occurs in the mechanical output and jumping performance between slow SSC (large-amplitude movement) vertical jumps like CMJ and countermovement drop jump, and fast SSC (small-amplitude movement) vertical jumps like a bounce drop jump (Walsh, 2004). In other words, the jumping technique (ie, movement amplitude and ground-contact time) should play a crucial role in the design of PT programmes. The meta analysis conducted by Markovic (2007a) on the effects of PT on vertical jump height, with special reference to the type of vertical jump test used has indicated that PT benefits slow stretch-shortening cycle, SSC, jumps more than concentric (SJ, squat jump) as well as fast SSC jumps (DJ, drop jump). Physiology of plyometric training Studies have shown that a more rapid and a more forceful contraction is induced in a muscle that experiences stretching before the concentric contraction (Bosco and Komi, 1980). The reason is that during SSC, as a result of the rapid (eccentric) stretch, elastic energy is produced in the muscles and tendons and stored in the series and parallel connective-tissue components of muscle. The stored energy constitutes an additional force that is released when the muscle recoils quickly as concentric contraction that occurs immediately following the stretch (Asmussen and Bonde-Petersen, 1974). The stretch reflex increases the activity of the muscles experiencing eccentric muscle action, eliciting a more forceful concentric muscle action (Bosco et al., 1981). The stretch reflex, which involves neuromuscular adaptations, facilitates greater motor-unit recruitment (Chimera et al., 2004). Golgi tendon organs normally have a protective function against excessive tensile loads in the muscle. But plyometric training is believed to desensitise the Golgi tendon organs (Hutton and Atwater, 1992), which results in a greater stretching of the elastic components of muscles. In the plyometric exercises, therefore, all the different aspects mentioned above namely, the mechanical aspect (that is, the series and parallel elastic component) additionally aided by Golgi tendon desensitisation, and the neurophysical component (i.e., stretch reflex) are believed to contribute to the enhancement in the rate of force production (Bosco et al., 1981). As the stretch reflex and stored elastic energy combine, they produce a more powerful concentric force than normal. However, the fundamental principle of plyometric training is that the muscle pre-stretching should be quick because when a muscle is stretched quickly it develops greater tension and stores a greater amount of elastic energy within the muscle, than when stretched slowly. If the eccentric contraction is slow, the stretch-reflex mechanism is negated and the exercise loses its plyometric quality. The muscular performance gains that accrue from plyometric exercising techniques are likely to be due to the neural adaptations of the muscle (Wilk et al, 1993). Motor performance greatly improves when a variety of plyometric exercises such as depth or drop jumps, counter-movement jumps, bouncing, leg bounding and hopping and so on, are used (Diallo et al., 2001). Plyometric training has been observed to produce neuromuscular adaptations in the hip adductor muscles that may promote knee joint stability and minimise the risk of knee injuries in female athletes (Chimera et al., 2004). Effects of plyometric training on muscle power output Biomechanical studies of drop jumping have shown that the jumping technique strongly affects the mechanical output of muscles (Bobbert, 1990). For instance, in a variation of the drop jump i.e., the bounce drop jump, the downward movement after the drop is reversed immediately into an upward push-off. This slight modification of technique is speculated to trigger improvement of the power output capacity of muscles. Similarly, the ability to change direction while sprinting is extremely important in most team and individual sports. The belief so far has been that strength and power development would improve change of direction (COD) performance. However, studies that evaluated exercises that are performed bilaterally in the vertical direction including plyometrics and vertical jumping have mostly reported failures of these exercises to elicit improvements in COD performance (Brughelli et al., 2008). Markovic et al. (2007b) studied the effects of sprint training and plyometric training on muscle power and athletic performance, and compared the results of both. Participants who were randomly assigned to a plyometric group, or sprint group, or control group took part in the training that lasted 10 weeks. Muscular strength as well as function were increased by plyometric training but not as much as the sprint training. Plyometric training also increased the agility that is, quickness and reaction time of the participants. As any reduction in the time the foot is in contact with the ground constitutes a more efficient transfer of energy in the direction of movement, plyometric training is an effective means of improving explosive power. Principles of plyometric training To obtain the best possible results vis á vis muscle strength and explosive speed, a plyometric regimen must observe both the universal principles of track and field training such as Progressive Overload, Specificity, Recovery, Individuality, and Variability, and certain basic principles that pertain specifically to plyometrics (Derse and Stolley, 1995). Variation is extremely important in plyometrics, since muscle response is greatest when the stimulus is varied over time. Varying the type or number of repetitions or the intensity of exercises performed provokes greater neuromuscular adaptation leading to enhanced muscle power output. The basic principles that are specific to plyometrics include ensuring basic strength in the athlete to squat 150-200% of his or her body weight before performing depth jumps (Derse and Stolley, 1995); ensuring rapidity of eccentric stretching for reasons discussed earlier; and, ensuring an explosive response to the rapidly executed pre-stretching. In order to minimise injuries related to plyometric training, drills should always be performed while wearing properly cushioned shoes on grass or padded mats or any such soft level surface, avoiding concrete surfaces. One of the main goals of plyometric training is to increase power. The focal point of plyometric exercises designed to enhance muscle power is the concurrent usage of steps that ensure maximum strength and quickness. Explosiveness, not endurance, is of the essence. Power plyometric drills consist of a range of jumping movements from hops to leaps, depth jumps and box jumps being a part of advanced plyometrics (Derse and Stolley, 1995). In the case of speed plyometrics, over- load refers to heightened speed. Movements are executed with considerably more speed than normal, compelling the neuromuscular system to respond more speedily to a stimulus, with a lasting effect. Sprinters, hurdlers, long jumpers, soccer players, basketball players are some of the sportspeople who benefit from speed plyometric training. The fast-twitch fibres of the leg muscles play a key role for the hurdlers. Hence, plyometrics exercises would be of great help. For distance runners, proper running procedure is more important than speed. Rhythm drills including skipping, running with high knee lift, air kicks, running butt kicks, fast feet running among many more are suitable for speed development as well as for running mechanics. The exercises generally consist of the same basic movement but are performed differently to develop rhythm, power, or speed (Derse and Stolley, 1995). Explosive power and quick reaction to the starter's pistol are of utmost importance to the athlete, while a sprint start is a complex motor stereotype. In a sprint start, the block acceleration in the first two steps primarily depends on the activation of the various quadriceps muscle components. Sleivert and Taingahue (2004), studying the relationship between sprint start performance muscle strength and power variables, concluded that concentric and not plyometric force development plays a crucial role in sprint start performance. They further observed that sprint acceleration was proportional to maximal concentric muscular contraction. However, it is only the start from a stationary position that requires a concentric muscular action. The subsequent strides would benefit from power plyometrics since the eccentric contraction preceding the next concentric contraction increases power potential in the muscles of the calves, thighs and hips. Plyometric exercises such as hopping and bounding are said to develop explosive ability (i.e., initial acceleration) while enhancing leg stiffness. Chelly and Denis (2001) have shown that leg power correlates with both the initial acceleration and maximal running velocity during track sprinting. But leg stiffness relates to maximal velocity and not acceleration. In another interesting study conducted with national level 100m sprinters, Bret et al. (2002) observed that sprinters who had the greatest leg stiffness produced the highest acceleration not at the start but after completing the initial 30m. These results further prove that concentric muscular strength expression is a key acceleration determinant, while plyometric power which rises when leg stiffness is greater could be taken as an indicator by the sprint athlete to decide when to use a fast eccentric pre-stretching muscular contraction so as to achieve an enhanced power output from the subsequent concentric contraction. Conclusion The physiology and principles of plyometric training consisting of exercises that enable achievement of speed and muscular strength required in almost all athletic and field sport events are explained. The biomechanical aspects of plyometric exercises are discussed in the backdrop of the role played by the central nervous system- muscle–tendon interaction during human movement. References Adams K, O’Shea JP, O’Shea KL, et al., 1992. The effect of six weeks of squat, plyometric and squat-plyometric training on power production. 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