With reasonable confidence it can be said the application of plyometrics permits effective use of strength to produce explosiveness in events demanding speed and quickness. The overall role of plyometrics is to facilitate the neuromuscular system into making a more rapid transition from an eccentric to a concentric contraction, whereby maximal ballistic force is generated. This is similar to the theories of (Gambetta) (Bosco et al., 1982) and (Yessis and Hartfield) who believe that plyometric training is the link between strength and speed.
With reasonable confidence it can be said the application of plyometrics permits effective use of strength to produce explosiveness in events demanding speed and quickness. The overall role of plyometrics is to facilitate the neuromuscular system into making a more rapid transition from an eccentric to a concentric contraction, whereby maximal ballistic force is generated. This is similar to the theories of (Gambetta) (Bosco et al., 1982) and (Yessis and Hartfield) who believe that plyometric training is the link between strength and speed.
The increased production of muscular power is best explained by two models;
Mechanical Model
In the mechanical model elastic energy in the musculotendinous components is increased with a rapid stretch and then stored (Cavagna et al., 1965). When immediately followed by a concentric muscle action, this stored elastic energy is then released, increasing the total force production. To support this (Hill 1970) provides a description of the mechanical model.
Figure1 . Mechanical model of skeletal muscle function. The stretch elastic component when stretched stores elastic energy that increases the force produced. The contractile component is the primary source of muscle force during the concentric muscle action. The parallel elastic component exerts passive force with unstimulated muscle stretch.
Neurophysiological Model
The neurophysiological model involves the potentiation of the concentric muscle action by use of the stretch reflex (Figure 2) (Bosco et al., 1979, 1981, 1982). The stretch reflex is the body’s involuntary response to an external stimulus that stretches the muscles (Guyton and Hall 1995; Matthews 1990).The reflexive component of plyometric exercise is comprised of muscle spindle activity. Muscle spindles are proprioceptive organs that are sensitive to the rate and magnitude of a stretch, when a quick stretch is detected, muscular activity reflexively increases.
During plyometric exercises, the muscle spindles are stimulated by a rapid stretch, causing a reflective muscle action. This increases the force the muscle produces (Kilani et al., 1989.)
Just like the mechanical model, if a concentric action does not immediately follow a stretch, the stretch reflex is negated.
Figure 2. A diagram of the stretch reflex. Once the stretch reflex is enthused it sends input to the spinal cord via the nerve fibres. After connecting with the alpha motor neurons, impulses travel to the agonist fibres causing a muscle action.
Stretch-Shortening Cycle
The Stretch Shortening Cycle employs the energy storage capabilities of the series elastic component and stimulation of the stretch reflex to facilitate a maximal increase in muscle recruitment over a minimal amount of time.
3 phases-
1) Eccentric Phase – preloading the agonist muscle groups. The stretch elastic component stores elastic energy and the muscle spindles are stimulated. As the muscle spindles are stretched they send a signal to the ventral root of the spinal cord.
2) Amortization Phase – time from the end of the eccentric phase to the initiation of the concentric muscle action.
This phase of the SSC is the most crucial in allowing greater power production; the duration must be kept short. If the phase lasts too long, the energy stored dissipates as heat, and the stretch reflex will not increase muscle activity during the concentric phase (Cavagna et al., 1977).
3) Concentric Phase – body’s response to the eccentric and amortization phase. The energy that has been stored in this phase increases the force of the subsequent movement. (Svantesson et al., 1994) As soon as movement begins in an upward direction, the concentric phase of the stretch shortening cycle begins.
Example long jump;
Agonist muscle is the gastrocnemius –
Gastrocnemius undergoes a rapid stretch (eccentric phase), there is a delay in the movement (amortization phase), the muscle then concentrically plantar flexes the ankle allowing the athlete to push of the ground (concentric phase).
The rate of musculotendinous stretch is vital to plyometric exercise (Kilani et al., 1989). A high stretch rate results in greater muscle recruitment and activity during the SSC concentric phase. Below are some lower body plyometric drills that help to increase muscular force resulting in more power for explosive events, in particular sprinting.
Exercise 1- Squat Jump, low intensity
Direction of Jump – Vertical
Throughout this jump there is no arm movement and no preparatory movement.
Starting position – Feet should be shoulder-width apart, get into the squat position (thighs should be parallel with the ground). Place hands behind the head and interlock fingers.
Upward Movement – Explosively jump up to maximum height, pushing up from the ground with maximum strength whilst keeping as vertical as possible. Here the muscle will be lengthened (eccentric phase). Downward Movement – Land in the squat position the same as which you started the jump. It is essential the jump is immediately repeated. Here the muscle is rapidly shortened (concentric phase).
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| Downward Movement Upward Movement
Exercise 2 – Cycled Split Squat Jump, High intensity
Direction of Jump - Vertical
Starting Position – Put one leg forward stretched out in front of you and assume a lunge position. (Hip and knee joints should be flexed to approximately 90⁰). The other leg should be placed behind the midline of the body. The arm action used when performing this squat jump is a double arm action, the exercise begins with a countermovement. Upward Movement – Explosively jump up, producing as great of force possible from the feet reaching up for the ceiling with your arms, (Eccentric Phase). While off the ground, switch the position of the legs so they are the opposite way round. Throughout this point it is necessary to generate the greatest height possible in order to have time to switch the position of your legs, (Amortization Phase).
Downward Movement – When landing, maintain the lunge position but with the opposite leg put forward, (Concentric Phase). It is crucial the jump is instantly repeated. It is important to make sure that the lunge is not too deep (picture 3), as the SSC may not be able to contribute effectively to consecutive jumps.
Starting Position - Upward Movement - Picture 3
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Exercise 3 – Double Leg Vertical Jump, Low intensity
Assuming a relaxed, upright position with feet shoulder with apart, using a double arm action begin the exercise with a countermovement. Upward Movement – Explosively jump up, toes plantar flexed in order to produce as much force as possible from the ground. Use both arms to assist during the jump reaching up as high as possible, whilst keeping as straight as possible, (Eccentric Phase). Downward Movement – Land in the original starting position and suddenly repeat the jump, (Concentric Phase).
Start/Downward Movement Upward Movement
Exercise 4 – Single Leg Vertical Jump, High Intensity
This exercise is similar to the previous one, apart from the fact it is at a more advanced level. Again, you assume a comfortable, upright position but this time on one foot. The arm action is the same as the double leg vertical jump, using both arms you begin the exercise with a countermovement. Upward Movement – Jump up powerfully, generating as much force possible from the foot on the ground, using both arms to reach up high towards the ceiling, (Eccentric Phase). Downward Movement – Land in the original starting position and repeat the jump using the same leg, (Concentric Phase). Remember to allow recovery time between each jump. After a number of reps on your right leg, repeat this exercise with the left leg completing the same number of reps.
Downward Movement - Upward Movement
Exercise 5 – Depth Jump, High Intensity
In order to complete this exercise a plyometric box is needed preferably 12-42 inches high. Assume a comfortable upright stance on the box with feet shoulder width apart and toes near the edge of the box. Downward Movement - Using a double arm action step down from the box and land on the floor with both feet, (Concentric Phase).Upward Movement – Upon landing, immediately jump up as high and as vertical as possible, (Eccentric Phase). Time on the ground when you’ve stepped down from the box should be kept to a minimum, increasing the height of the box will also increase the intensity.
Starting Position Downward Movement Upward Movement
Exercise 6 – Single Leg Depth Jump, High Intensity
Similar to exercise 5, using a plyometric box anything from 12-42 inches high, on top of the box stand in a relaxed position feet shoulder width apart with toes on the edge of the box. Downward Movement - Using a double arm action, go to step down from the box, land on the floor with your chosen leg, (Concentric Phase). Upward Movement – Upon landing immediately jump up as high as possible again landing on one foot, (Eccentric Phase). After a number of reps repeat the exercise with the opposite leg. It is important to note this form of depth jump is very advanced and should only be performed by those with sufficient experience.
Start Position Upward Movement Downward Movement
Plyometric Exercise Prescription
As with any training programme, the plyometric exercise session must begin with a general warm-up, stretching and a specific warm up towards plyometrics for example low intensity dynamic movements for example jogging/skipping.
Intensity
Plyometric intensity refers to the amount of stress placed on the active muscles and joints, this is controlled primarily by the type of exercise performed. Exercises can vary from a low to a high intensity (See Exercises).
Frequency and Recovery
The frequency of a plyometric training program will vary considerably from athlete to athlete, sport to sport and point of the training cycle. It can range from 1 – 3 sessions per week depending on the sport and the time of year. Because plyometrics focus on improving anaerobic power, using maximal efforts, adequate recovery is required between drills. A typical example would be 10 seconds between reps and 3 minutes between sets, 48 – 72 hours of recovery is required between each plyometric session.
Volume
Progression
Plyometrics is a form of resistance training; therefore it follows the principles of progressive overload. As intensity increases volume decreases. In a plyometric session, ways in which the intensity can be increased to achieve this overload include:
- Increased speed of performance
- Higher box jumps, depth jumps and bounds
- Performing single leg drills
- Adding weights to the athlete (weight vests/ankle weights etc) to increase their body weight
Safety Considerations
It is important to note, although plyometric exercise is not inherently dangerous, like all models of exercise the risk of injury is present. In order to prevent the risk of an injury occurring the intensity of the exercise must be taking into account specifically for the person taking part in the training, for lower body plyometrics, the athletes 1Rm squat should be at least 1.5 times his/her body weight (14, 18)
Requirements
Speed- As plyometrics involves quick movements, the ability to move rapidly is essential. For lower body plyometrics the athlete should be able to perform 5 repetitions of a squat with 60% body weight in 5 seconds or less.
Age- Nobody under the age of 14 should participate in plyometrics training due to the fact in children growth plates are open so high intense activity and injury from plyometrics may cause them to prematurely close, resulting in limb length discrepancies (30).
Footwear- Footwear with good ankle and arch support with a wide non-slip sole are vital.
Equipment- Boxes used for drills must be sturdy; heights should range from 6-42 inches, with landing surfaces of at least 18 by 24 inches.
References
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Arazi, M. Asadi, A. (2011) The effect of plyometric training on strength, sprint and balance in young sprinters. The Journal Of Strength and Conditioning pp317-329.
Asmussen, E. Bonde-Peterson, F. (1974) Storage of elastic energy in skeletal muscles Acta Physiological. Scand 91: 385-392.
Baechle, T.R. Earle, R.W (1994) Essentials of Strength Training and Conditioning National Strength and Conditioning Association 2nd edition.
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Cavagna, G.A. Dusman, B. Margaria, R. (1968) Positive work done by a previously stretched muscle Journal of Applied Physiology 24: 21-32.
Fowler, N.E. Lees, A. Reilly, T. (1994) Spinal shrinkage in unloaded and loaded drop-jumping Ergonomics 37: 133-139.
Gambetta, V. (1978) Plyometric Training, Track Field Q 4: 225-239. Cambridge University Press.
Guyton, A.C. Hall, J.E. (1995) Textbook of Medical Physiology, 9th edition Philadelphia: Saunders.
Hill, A.V. (1970) First and last experiments in muscle mechanics
Hewett, T. E. Stroupe, A.L. Nance, T.A. Noyes, F.R. (1996) Plyometric training in female athletes, American Journal of Sports Medicine 24: 765-773.
Kilani, H.A. Palmer, S.S. Adrian, M.J. Gapsis, J.J (1989) Block of the stretch reflex of vastus lateralis during vertical jump Human Movement Science 8:247-269.
Matthews, P.B.C. (1990) The knee jerk: Still an enigma? Journal of Physiological Pharmacal 68:347-354.
Svantesson, U. Grimby, G. Thomee, R. (1994) Potentiation of concentric plantar flexion torque following eccentric and isometric muscle actions. Acta Physiological. Scand. 152:287-293
Turner, A.M. Owings, M. Schwane, J.A (2004) Improvement in Sprinting After 6 Weeks of Plyometric Training The Journal Of Applied Research and Design Methods pp552-564.
Wilk, K.E. Voight, M.L. Keirns, M.A. Gambetta, V. Andrews, J.R. Dillman, C.J (1993) Stretch-shortening drills for the upper extremities. Theory and Clinical Application Journal of Orthopedix Sports Physiotheraphy 17: 225-239.
