Plyometric Theories: A Practical Approach
Plyometric exercise has become a necessity in the training regimens of several coaches and athletes. In the recent past, only a few coaches and athletes attempted this exercise. However, plyometric exercise has evolved today into a universally recognized and effective tool that can be used to improve power and agility (Campos, 2002). Athletes from all occupations can perform plyometric exercises. Initially, this training was thought as simply box jumping though it encompasses more than jumping. However, strength and conditioning programs that comprise plyometric training have now become more complex and creative due to the success of this training (Campos, 2002). Several theories try to explain the basics of plyometric exercises. These theories majorly aim at explaining the nature of the Stretch-Shortening Cycle (SSC). The main plyometric theories include the mechanical model and the neurophysiologic model. This essay attempts to review the mechanical and neurophysiologic theories of plyometric training and discuss the practical applications in plyometric training as they relate to SSC.
History and Definition of Plyometric Training
Plyometric exercise began gaining popularity in the 1970’s, in the United States. It was referred to as ‘jump training’, which until then, was used mainly in Eastern European bloc countries by most top athletes, in sports like gymnastics, track and field, and weightlifting. Veroshanski was one of the coaches who first published a series of jumping drills. Initially the word ‘plyometric’ meant ‘measurable increase’ from two Greek words plio and more (McGuigan, 2011). Later, one of America’s greatest track coaches, Fred Wilt, coined the term plyometric in 1975.
Plyometric training conditions the body through vigorous exercises. The term plyometric refers to a training procedure based on the belief that pre-stretching a muscle before a concentric contraction can result in a more powerful concentric contraction. The pre-stretching before contraction is referred to as ‘stretch shortening’ cycle. The cycle is considered a natural muscle function and is detectable in several sporting activities like jumping and throwing. Stretch shortening cycle movements compared to non-stretch shortening movements have shown better performance, as a result, of pre-stretching (Ellenbecker, 2003). The influence of pre-stretching is that some mechanisms are responsible for improved contraction force, and these are stretch reflex or storage of elastic energy in the pre-stretching phase of the movement or eccentric.
Plyometric training effectively bridges the gap between strength and power by utilizing the stretch shortening cycle (SSC), to provide an overload to the associated agonist muscles. The speed component involved in plyometric acts as a transition between an athlete’s strength training adaptations and their athletic ability in sport. A plyometric training program can include exercises such as bounding, hopping, jumping variations, push-ups, and medicine ball throws, each of which use quick and powerful movements that incorporate a muscular pre-stretch or counter movement (Aagaard & Simonsen, 2002). This in turn stores and uses energy like an elastic band. Theoretically, it is the mechanical “elastic” components of the muscle tissue and tendons that provide the power production during movements that use the SSC, as well as the associated neuromuscular activity (Bobbert, 1987).
Plyometric exercises commence with a speedy stretch of a muscle and later followed by a speedy shortening of the same muscle, breaking the two periods into the eccentric and concentric phase. This kind of training conditions the nervous system to react faster to the stretch-shortening cycle (Aagaard and Simonsen, 2002). This enhances an athlete’s ability to increase their movement speed and improve their power production. Regular plyometric training can also help in the strengthening of bone and aid in weight control. In addition, plyometric training performed before the start of a season can decrease an athlete’s risk of sport related injuries (Ellenbecker, 2003). Particularly, this can benefit young female athletes who seem to be at a higher risk of knee injuries compared to their male counterparts.
A muscle contracts more rapidly and frequently if stretched earlier than a concentric contraction. A good example is when a jumper first ‘sinks’ before a vertical jump. The muscles that facilitate in the jump stretch for a short while producing a powerful movement, by quickly lowering the center of gravity (Brughelli, 2008). Two models, which are under SCC, help to explain how this phenomenon occurs. These are the mechanical and neurophysiological models. However, it is essential to understand the SSC as these models are based on this cycle.
Stretch-Shortening Cycle
The Stretch-shortening cycle (SSC) involves storage of potential energy in a stretched muscle. For this process, a body segment must be periodically exposed to stretch forces, which occur during typical physical activity: walking, running, or jumping. An external force such as gravity act upon the eccentric/lengthening phase, which is then followed by concentric/shortening phase (Cavagna, 1965). This encompasses the basic functions of the SSC, which involve important pre-activation and activation features. The SSC incorporates the mechanical and neurophysiological models of plyometrics to achieve enhancement of concentric performance. This is done by utilising the energy storage characteristics of the SEC, as well as stretch reflex stimulation and GTO inhibition, to enable maximal muscle fibre recruitment in the shortest time possible (Bret, 2002).
Three distinct phases constitute the stretch-shortening cycle; Phase I: Stretch of the agonist muscle (eccentric phase), Phase II: pause between phase I and II (amortization phase), and Phase III: shortening of agonist muscle fibres (concentric phase). The diagram below illustrates the stretch-shortening cycle for an athlete doing long jump.
The eccentric phase commences at touchdown and is progressive until the movement ends. The amortization phase is the transition from eccentric to concentric phases; it is fast without movement. The last phase is the concentric phase, which follows the amortization phase and entails the entire push-off time until the athlete’s foot leaves the surface (Ellenbecker, 2003). The ultimate goal of plyometric training is a reduction in the amortization time, because a prolonged stretch phase results in a reduction of neuromuscular efficiency and elastic potential energy.
Phase I is where preloading and stretching of the muscle occurs. Here, elastic energy is stored in the SEC, along with the muscle spindles being stimulated. Nervous signals are sent from here to the ventral root of the spinal cord via Type Ia afferent nerve fibres. During this phase, the stretching of the muscles stimulates muscle spindles (Brughelli, 2008). The muscle spindle sends out a signal that finally causes the muscle to contract. Once external forces are applied, the movement transfers to Phase II. This is the point at which there is a rapid transition made between eccentric and concentric action.
Phase II (amortization) can simply be stated as the time that lapses between landing and jumping back. The amortization phase is crucial and should be kept short. During this phase, the Type Ia afferent nerve fibers synapse with the alpha motor neurons in the ventral root of the spine, which in turn stimulates the associated agonist muscle group (Cavagna, 1965). As mentioned before, the shorter this phase lasts, the more powerful the resulting movement will be. Any associated GTO activity will be at its most prominent here, due to the excessive tension generated.
Phase III involves the body’s concentric response to the eccentric and amortization phases; this is where the stored elastic energy in the SEC, and the stimulation of the agonist muscle group via alpha motor neurons, is used to facilitate the resulting concentric muscle action (Andersen, 2001). The potentiation of muscle action to produce greater force, observed during the SSC is not associated with isolated concentric muscle action and is, therefore, exclusive to the SSC and plyometric training program. The diagram below, illustrates the concentric phase of a medicine ball jump squat.
Cornie (2011) has previously categorised SSC movements based on their duration. If the three phases of the SSC occur at less than 250 milliseconds, this is considered a ‘fast SSC’ activity. ‘Slow SSC’ activities are said to occur at more than 250 milliseconds. This is often the case during basic vertical jumps (maximal effort squat jumps and counter-movement jumps). During the movements with slower eccentric phases, the movements rely less on neuromuscular capabilities. Instead, muscle fibers develop a higher level of cross-bridge attachment at a molecular level, which actively prepares the muscle for the transition. Fast SSC movements rely more on the neuromuscular properties associated with the SSC mentioned. The lack of neuromuscular expression during slow SSC muscle action highlights a significant biomechanical difference to fast SSC muscle actions, which will ultimately show different outcomes during sports performance (Cavagna, 1965). From strength and conditioning coach’s point of view, this suggests that the way in which these exercises are performed can be specified to target different mechanisms of the SSC (Cornie, 2011). Therefore, selecting the most appropriate plyometric modality will be vital when considering an athlete’s specific training requirements.
Theories of Plyometric Training
Mechanical Model
Under this model, elastic energy from tendons and muscles is stored due to a rapid stretch. The stored energy is later released after a concentric muscle action follows the stretch. According to Brughelli (2008), the effect is similar to an elastic spring. In this case, the spring is a component of the tendons and muscles referred to as series elastic component (SEC).
When a concentric muscle action follows an eccentric stretch, the energy stored in the muscle is released, resulting in greater total force. Guton A (2000) expanded on this idea by underlying the mechanics of the skeletal muscle; his model portrayed a musculotendinous unit as a simple contractile element, containing a series of elastic component (SEC) and a parallel elastic component (PEC). Between these, the SEC contributes most of the mechanics of plyometrics, due to potential of muscle fibers arranged in series, to ‘deform’ in response to loading, and thereby store potential energy. These muscle fiber formations are generally found in the tendons of a muscle. Bret (2002) suggests that the non-tendinous aponeuroses in more proximal musculature may adopt SEC behavior, as well. This deformation of intramuscular connective tissue could provide a further source of potential elasticity, mostly in muscles that exhibit no tendons.
Mechanical Model of Skeletal Muscle Function
The series elastic component (SEC), when stretched, stores elastic energy, which increase the force produced. The contractile component (CC) is the main source of muscle force during concentric muscle action and the parallel elastic component (PEC) exerts a passive force with un-stimulated muscle stretch (Ellenbecker, 2003).
When eccentric muscle action occurs, elastic energy is stored in the SEC as it lengthens as would a coil spring. Any concentric action that occurs immediately after this event will benefit from contributing elastic energy, which is released as the musculotendinous components return to their natural un-stretched configuration (Guton, 2000). However, if the eccentric or ‘negative’ phase is too long, the energy stored will not benefit the following concentric phase; it will instead dissipate as heat and not act as a contributing factor to the concentric components.
Neurophysiological Model
As previously mentioned the pre-stretch action in the mechanical model of plyometrics involves the storing of SEC elastic energy, which then contributes to the concentric performance of agonist muscles. The body contains receptors or proprioceptors that are sensitive to stretch and tension (Brughelli, 2008). The muscle spindle is one of these proprioceptors and plays a major role in the stretch reflex. The stretch reflex is an involuntary contraction (response) to external stimuli that stretch the muscle (i.e. knee jerk reaction). When the spindle is stretched, it sends a signal to the spinal cord via type Ia nerve fibers. After synapsing with the alpha motor neurons in the spinal cord, impulses travel to the agonist extrafusal fibers, causing a reflexive muscle action (Bosco, 1979).
The strength of the response of the muscle spindle is determined by the rate of stretch. Practically speaking, this means the greater and more quickly a load is applied to the muscle, the more forceful the muscular contraction will be. The stretch reflex is responsible for stimulating muscular proprioceptors, which can facilitate, inhibit and modulate both agonist and antagonist muscle activity (Brughelli, 2008). The primary neurophysiologic factor involved in this model is muscle spindle activity. Muscle spindles are comprised of intra-fusal fibers (modified muscle fibers) that run parallel to the normal extra-fusal muscle fibers as shown in the diagram above (McGuigan, 2011). Since deformation of the muscle spindle activates a neurological response causing the muscle to contract, i.e. rapid stretch. They can be defined as proprioceptive organs that are sensitive to muscular stretch rate and magnitude, which cause potentiation of agonist muscle activity during plyometric work (or when quick stretches are detected). In turn, this increases the force produced by the concentric muscle action, which is an example of agonist muscle activity being facilitated through neurophysiologic means (Guton, 2000).
Muscle contraction is inhibited by Golgi Tendon Organ (GTO), which is another proprioceptor located near the muscle tendon junction. On the other hand, muscle spindles are sensitive to muscle distortion. However, instead of working to facilitate muscular activation, GTOs work to inhibit activation as a protective mechanism against excessive tension (Brughelli, 2008). This occurs during heavy resistance training. Here, the GTO sensory neuron located in the muscle tendon will discharge and inhibit the motor neuron working in the muscle belly, causing the muscle to relax. This limits the speed and force output at which a muscle could contract, therefore, limiting muscular power and stimulating antagonist power (McGuigan, 2011).
There are two ways of increasing a muscles force production: increasing the number of motor units activated, and increasing the rate at which a motor unit is stimulated. Plyometric training influences these two factors affecting force production and speed of that force production. In summary, plyometric exercise trains the neuromuscular system to respond more efficiently (Bosco, 1979). This is beneficial to athletes in several ways.
Practical Applications of Plyometric Training
The results from this investigation give a professional or the strength and conditioning coach a practical and theoretical framework for which to develop and implement lower body plyometric programs while instructing relevant technique. A sprinter can concentrate on the lower extremity limb segment biomechanical positions, which are reflective of maximizing leg triple extension and flexion and creating a symmetrical balanced position (Cornie, 2011). When this athlete concentrates on utilizing soft landings, which stresses their knee flexion, the neuromuscular response generated by the Golgi tendon body and the SSC, enables the athlete to enhance their power outputs and vertical force. In effect, increasing their ability to be more explosive, jump higher and change direction faster (Brughelli, 2008). This is a lower body plyometric training that is applicable for athletes of all sport, as long as the principles of progressive overload are maintained, and appropriate recovery allotted.
Studies have indicated that jump training, combined with resistance training, results in significant improvement in jump scores and long jump scores for youngsters that undertake them compared to groups that only perform strength train. This can be attributed to the learning ability of young children to react to the stimulus of ground contact. Children learn to play skill games such as baseball and tennis at young ages of between 6-8 years (Andersen, 2001). The application of plyometric training has been utilized to develop hand-eye coordination specific to these games. Younger ages are the excellent times of initiating plyometric training because the nervous system is ‘plastic’ and the youngsters can utilize on motor learning, to develop their game skills. Equally important is that those movement skills involving the lower extremities are very crucial in developing athletic ability. Therefore, plyometric training can be used to develop athletic ability for most young athletes (Cornie, 2011).
In essence, plyometric exercises are beneficial to athletes of different fields. They are also applicable to non-athletes who wish to keep fit, although the volume for such participants is often lower. Thus, the duration and complexity of the exercises depends on the participant’s expertise and experience. Here, armatures may only manage a maximum of 80 ground sessions per session, whereas experts can do up to 140 ground sessions in a session. The essence of plyometric exercises is to increase one’s explosive power. The exercises are also beneficial in enhancing the efficiency of transitioning from the eccentric to the concentric phase during contraction through SSC In this case, athletes use muscle elastic energy and myotatic reflex passively and actively respectively. The active myotactic reflex enhances the speed of voluntary reflex to a supra-maximal velocity. This in turn increases the rate of motor muscles activation leading to faster movement.
Although there has been some confusion over the training effects of plyometrics, it is certain that the increase in muscle strength and power is attributable to an increase in muscle elasticity and adaptation in neuromuscular functions. Improved elastic potential in muscle may also be due to the enhancement of the stretch reflex (Aagaard and Simonsen, 2002). This is stimulated during stretch shortening muscle activity, where the muscle spindles are stretched, resulting in stimulation of other nerve impulses, and increased activation of motor units, thus added contractile strength.
Plyometric exercises range from low intensity to high intensity exercises, which can include beginning with a double leg hop to depth jumps. Other activities like jumping jacks, jumping rope and hopscotch can also be characterized as plyometric training since the quadriceps get subjected to a stretch-shortening cycle, every time the feet touches the ground (Bret, 2002). Actually, plyometric training involves natural part of most movement’s evidence by hopping, jumping and skipping. Plyometric training tends to be fun and effective though injury might occur if the volume and intensity of the training exceeds the participant’s ability (McGuigan, 2011).
Any participant should develop an appropriate baseline of strength, before taking part in any plyometric training program. Either, such participants can commence with lower intensity drills and later progress to high intensity drills with time. It can also be crucial to undertake clinical trials to establish the most effective plyometric training for any participant (Bosco, 1979). In performing multiple sets, participants should get adequate rest and recovery between sets in order for them to replenish the energy required to perform the next series of repetitions using the same intensity.
The total effect of plyometric exercises is dependent on two key factors. The training volume needs to be specific to the player in order to be effective. Thus, non-athletes and athletes need to use different volumes appropriate for their levels of experience in order to enhance the success of plyometric exercises. Further, it is essential to regulate the intensity of the plyometric exercises to fit the trainee. This also depends on the experience of the in plyometric training. In addition, exhaustive plyometric training may result in injury rather than being beneficial to the trainee. Thus, regulation and moderation are essential in order for plyometric training to be beneficial.
Plyometric exercises tend to be performed rapidly and explosively, unlike other traditional strength training exercises. Trainers may choose to integrate this exercise into other activities or introduce it during a warm-up period (Bobbert, 1987). It can progress to include single leg hops, throws using lightweight balls and multiple jumps depending upon an individual’s goals and needs. In order to help prevent over training and optimize individual gains, any plyometric training program should be modified overtime. Every participant of a plyometric training program should be provided with information concerning safe training procedures, proper exercise technique and rate of progression. These participants should also wear protective and supportive athletic footwear and perform these exercises on surfaces with some resilience (Ellenbecker, 2003). Plyometrics should be complemented with flexibility, agility, aerobic and strength training, and should not be a stand-alone exercise, but rather should be integrated into a properly designed conditioning program.
Plyometric training are in essence a power training modality in several studies, due to the use of multi-joint exercises characterized by explosive SSC muscle actions. These exercises have been shown to exhibit powerful performance gains in athletes attributable to the neural adaptations that occur during plyometric training (Bret, 2002). Plyometric not only makes an individual powerful; this training can offer several health benefits to a person. In fact, this training is a secure, fun and worthwhile method of conditioning for any participant if age-appropriate measures are taken into place.
Methods of Plyometric Training
Plyometric training can be prescribed through various methods. These include three different modes: lower body, upper body and trunk plyometrics (Andersen, 2001). However, this essay focuses on the lower body mode, as it is the most popular method of training within strength and conditioning research, and is appropriate for any athlete and any sport. Lower body plyometrics are suitable for training maximal power production for such athletic requirements such as horizontal and lateral locomotion, change of direction, jumping and sprinting (Cornie, 2011). There are various training drills to promote these movements:
- Jump drills (e.g. squat jump and tuck jump) – These drills involve jumping and landing in the same spot. Jumps in place, stress the vertical element of jumping and are performed in a repeated manner, with no rests in between the jumps. The time between the jumps is the stretch-shortening cycle’s amortization phase.
- Standing jumps (e.g. vertical jumps and jumps over barriers) – These stresses either vertical or horizontal factor. Standing jumps are maximal efforts with recovery between repetitions. Focus on cross-bridge formation enhancement and concentric strength
- Multiple hops and jumps (e.g. zigzag hops) – repeated jumps which emphasize on bilateral and horizontal locomotion, focus on amortization phase reduction and change of direction characteristics
- Bounds (e.g. alternate leg bounds) – exaggerated movements with greater horizontal speed, typically covers up to 30m, focus on explosive horizontal movement.
- Box drills (e.g. box jumps) – increase intensity of hops and jumps by jumping onto or off a box, can involve one or two legs, focus on concentric strength and/or reactive strength
- Depth jumps – gravity and athlete’s weight increases drill intensity, involves stepping off a box, landing and jumping immediately vertically, horizontally or onto another box, focus on reactive strength.
Integrating Plyometric Exercise
Plyometric exercise can also be integrated with other exercises to further athletic ability. One way is incorporating it with resistance training in a method known as augmented eccentric loading (AEL), which involves loading the eccentric phase of a plyometric exercise, theorized to enhance neuromuscular responses (Andersen, 2001). This includes a higher recruitment of motor units to counteract the augmented stretch imposed on the muscle, increased mechanical response to the deformation of the muscle, through enhanced levels of SEC elastic energy storage, and enhanced cross-bridge formation. AEL can be applied to target both the fast SSC and slow SSC; reactive strength drills such as depth jumps can be loaded using dumbbells or elastic bands for the pre-stretch phase, and drills that focus on concentric strength and cross-bridge formation (e.g. jump squats) can be loaded using specialist weight releasers (Ellenbecker, 2003).
The combination of plyometric training and weight training has also been found to be useful in the development of athletic power. One way of combining the two forms of training is complex training or the contrast method. Complex training alternates plyometric exercises with biomechanically similar high load-weight training exercises, in the same work out, set for set. An example of complex training would involve performing a set of squats followed by a set of jump squats (Cornie, 2011).
Summary
It is apparent that a focus on the plyometric training modality is beneficial to the body. This exercise has evolved today into a universally recognized and effective tool that can be used in improving power and agility. Plyometric training effectively bridges the gap between strength and power by utilising the SSC to provide an overload to the associated agonist muscles. The speed component in plyometrics acts as a transition between an athlete’s strength training adaptations and their athletic ability in sport. The term plyometrics refers to a training procedure based on the belief that pre-stretching a muscle before a concentric contraction can result in a more powerful concentric contraction. A plyometric training program can range from low to high intensity exercises like hopping, bounding, jumping, push-ups and medicine ball throw. Each of these exercises uses quick and powerful movements, which incorporate a muscular pre-stretch or counter movement, which in turn stores and uses energy like an elastic band. The SCC combines neurophysiological and mechanical mechanisms and forms the basis of plyometric exercise. A rapid eccentric muscle action stimulates the stretch reflex and storage of elastic energy, which in turn increase the force produced during the subsequent concentric action.
Three different modes of plyometric exercises have been pointed out: trunk, upper body and lower body plyometrics. The essay focused on lower body plyometrics since it is the most popular method of training within strength and conditioning research and can be applied by any athlete. Lower body plyometrics are suitable for training maximal power production for athletic requirements, such as horizontal and lateral locomotion, change of direction, jumping and sprinting. Training drills like box drills, depth jumps, bounds, and standing jumps can promote these movements. More importantly, the results from this investigation can give a strength and conditioning coach a practical and theoretical framework for which to develop and implement lower body plyometric programs effective for relevant technique.
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