Background

Functional movements and athletic success depend on both the proper function of all active muscles and the speed at which these muscular forces are used (Potach & Chu, 2009). Plyometric training enables a muscle to reach maximal force in the shortest possible time and has consistently been shown to improve the production of muscle force and power. Plyometric training enhances force production through using a pre-stretch that influences the storage of elastic energy via the components of muscles and tendons and the stretch reflex. Basketball players have to make quick, powerful movements and changes of direction in all planes to compete successfully (Potach & Chu, 2009). A centre in basketball would especially find plyometric training beneficial as they need to repeatedly jump throughout the game for rebounds and have to out-jump the opposing centre in order to rebound more loose balls. Plyometric training of the lower body will help to improve performance as it will enable them to produce more force in the leg muscles that are used for jumping in a shorter amount of time, leading to a higher jump. 

Spurrs & Murphy (2003) found that 6-week plyometric training programme significantly improves counter-movement jump height. Fatouros et al (2000) also found that vertical jump height performance and maximal leg strength were improved over a 12-week programme.

This article will help you to understand the mechanical and physiological principles of plyometric training and to teach you how to use lower body plyometric exercises correctly, effectively and safely enabling you to increase your jump height.

Physiological Principles of Plyometric Training

The principle of plyometric exercise begins with the eccentric phase of a muscle. This involves a stretch and the pre-loading of the agonist muscle groups. This is followed by a concentric phase which involves the contraction of the muscle.  During the eccentric phase the series elastic component (SEC), which consists of the elastic tendon of the muscle (Khurana, 2006), acts as a spring and stores elastic energy that was produced during the stretch of the muscle. The stretch of the muscle during this phase is also detected by muscle spindles. These are intrafusal mechanoreceptors that are widely distributed within skeletal muscle fibres in the belly of the muscle. They are stimulated when a muscle is lengthened and send a signal to the ventral root of the spinal cord via the type Ia afferent nerve fibres, known as annulospiral endings (DiGiovanna, Schiowitz & Dowling, 2004). This generates an action potential to be conducted rapidly by the sensory nerves directly to the motor nerves of the same muscle to recruit motor units, initiating the agonist muscle to contract (Premkumar, 2004). The more force required results in more motor units recruited by the central nervous system. This is an involuntary response known as the stretch reflex. A slower eccentric movement prevents taking optimum advantage of the myotatic stretch reflex (Clark & Lucett, 2009) as the muscle spindles are sensitive to the rate and magnitude of a stretch, therefore the more rapid the pre-stretch then the more forceful the concentric muscle contraction (Bompa & Carrera, 2005).

During the concentric phase of a muscle the combination of the elastic energy stored in the SEC and reflex potentiation caused by alpha-motor neurons is used to produce maximal force in muscle contraction, which explains why plyometrics enhances performance in jump height (Bosco et al, 1982).

The phase in between the eccentric and concentric phases is the amortization phase. During this phase the muscle must switch from overcoming force to initiating force in the intended direction (Clark & Lucett, 2009). This is the most crucial phase in relation to plyometric training and greater power production. The timing of this phase has not been specified by researchers but it is emphasised that it must be kept as short as possible (Brummit, 2010) so contraction of the muscle is immediately after the eccentric phase. This allows the SEC to contribute to the total force of muscle contraction by naturally returning the muscles and tendons to their unstrecthed configuration (Potach & Chu, 2009). A prolonged amortization phase results in a less-than-optimum neuromuscular efficiency from a loss of elastic potential energy released through heat (Clark & Lucett, 2009).

Exercise Techniques

Using the following exercises appropriately and effectively and by patiently progressing from low intensity exercises to moderate to high intensity exercises will enable you to increase your vertical jump height and therefore enhancing performance when participating in basketball:

Donwnward Movement         Upward Movement

When you have performed the jump and reach for a sufficient amount of weeks and you are comfortable in performing a plyometric vertical jump easily and efficiently, you might want to make the exercise harder to improve leg strength even more to further increase jump height. The increased intensity of this exercise is described in the next exercise:

2)      Single-leg Vertical Jump

This exercise is of high intensity and is only suitable for advanced plyometric trainers with high levels of leg strength. Start in a comfortable upright position balancing on one foot, with the non-jumping leg held stationary with the knee flexed, keeping back straight throughout. The exercise begins with a countermovement by flexing the knee in the jumping leg (downward movement) which is immediately followed by the concentric contraction of the quadriceps to explosively drive upwards, using arms for assistance and push off toes to reach for a target (upward movement).  The landing phase should involve flexion at the knee joint in the jumping leg and another jump should be repeated immediately using the same leg following the same routine. You should then rest for 2-3 minutes and then carry out the same exercise using the opposite leg.

Downward Movement                            Upward Movement

3)      Lateral Barrier Hop

This exercise is of medium plyometric intensity and requires the use of a barrier. Start in an upright position with feet planted shoulder-width apart with the barrier to one side of you. Begin the exercise with a countermovement at the knee joint which will help to propel the body upwards and sidewards (downward movement). Using knee and hip flexion, jump sideways over the barrier keeping knees and feet together (upward movement). Land on the opposite side of the barrier in the same countermovement position you started the exercise in (landing phase) and immediately repeat the jump to return to the original side, minimising ground contact time. Repeat this process until you reach fatigue. You can increase the intensity of this exercise when you feel comfortable and strong enough to do so by increasing the height of the barrier or by doing the exercise on just one leg.

   Downward Movement           Upward Movement             Landing Phase

4)      Double Leg Depth Jump

This exercise is of low intensity and is suitable for beginners to plyometric training. To perform this exercise a box that can hold your body weight is required. Prepare by standing upright on the box with feet shoulder width apart and toes near the edge of the box. Begin the exercise by stepping off the box onto the ground – DO NOT jump off the box as this will affect the height that the exercise is performed from, just simply step off. Land on the ground in a pre-stretched position with feet together, then without hesitation immediately drive vertically upwards and push off toes getting as high as you can whilst keeping hands on hips. Repeat exercise by returning to the box and start again. The intensity can be increased by increasing the height of the box, however this should only be done when you have sufficient leg strength and are advanced in using this plyometric technique. Start with a box of approximately 15 inches in height if you are a beginner and progressively increase the height over a number of weeks to an advanced height of 40 inches. Intensity can also be increased by adding another box to the exercise which is described in the next exercise.

Starting Position- Landing Position - Upward Position

5)      Box to box depth jump

This exercise is of a high intensity and should only be carried out by advanced individuals with leg strength and plyometric experience. The exercise involves 2 boxes that can hold your body weight. Prepare by standing on the box facing the other box with feet shoulder width apart and toes near to the edge of the box. Start the exercise by stepping straight out off the box onto the ground – DO NOT jump off or lower your centre of gravity. Land in a pre-stretched position with feet together and immediately spring back up onto the next box, minimising contact time on the ground. Repeat exercise by turning round and following the same routine to return to the original box. The height of the box determines the intensity of this exercise so start with low boxes with heights of approximately 15 inches and slowly increase over number of weeks of training to a maximum of 40inches. The distance between the boxes also depends on experience and ability of this plyometric exercise. Start with the boxes 24 inches apart, as you get more advanced you can increase this distance.

Starting Position- Landing Position 1- Upward Position- Landing Position 2

Programme guidelines and exercise prescription

First of all before engaging in lower body plyometric training the individual has to have some level of functional strength. If the individual has no experience of plyometric training in the past then they should begin with the simplest of exercised such as the jump and reach and then slowly progress to harder exercises such as the lateral barrier hop and the single-leg vertical jump. It is recommended that an individual should be able to squat at least one and a half times their own body weight before progressing on to more advanced lower body plyometric exercises than the jump and reach and other low intensity exercises. An individual with a good strength training background will be able to progress quicker through the plyometric programme and could potentially start on harder exercises than total beginners, however this is not encouraged as preventing injury is essential. The key element to this training programme is to have patience and slowly progress onto higher intensities and advanced exercises such as the box to box depth drop which is of very high intensity and should only be carried out by top experienced athletes and after at least 8 weeks of plyometric training at lower intensities. If an individual tries to attempt exercises that are of higher intensity than their ability then injury is very likely to occur so therefore a professional  in plyometric training should be consulted for advice on which level of intensity to start with. Recovery is also important when preventing injury, 48 to 72 hours is sufficient recovery time between plyometric sessions. Suitable footwear that include appropriate shock absorption features should also be worn to prevent injury. The surface that the exercises are carried out on is important.  Beginners with low levels of strength should use soft surfaces such as grass or padded flooring; however this will inhibit the elements of the stretch reflex. Therefore a hard surface is advised to use when performing plyometric exercises to enhance the reactivity of the neuromuscular system. Like any other training programme a warm-up is essential before a plyometric session. This should consist of specific lower body exercises such as squats and lunges along with static and dynamic stretching of the muscles. 

The following table provides the guidelines of how a typical 12-week plyometric programme should be carried out. A variety of exercises should be used of the appropriate intensity. This is a guideline for athletes with previous background of leg strength training so the intensity and frequency should therefore be lowered for beginners to strength training:

	Weeks 1-4	Weeks 5-8	Weeks 9-12
No. of sessions a week	2-3	3-4	4+
Reps	20	25	25
Sets	5	5	6
Intensity	Low	Med	High

References

Bompa, T. Carrera, M. (2005) Periodization Training for Sports. 2^nd edn. Leeds: Human Kinetics. Pp. 198-205.

Bosco, C. Viitasalo, JT. Komi, PV. Luhtanen, P. (1982) ‘Combined effect of elastic energy and myoelectrical potentiation during stretch-shortening cycle exercise’, Acta Physiologica, 114(4), pp. 557-565.

Brummit, J. (2010) Core Assessment and Training. 1^st edn. Leeds: Human Kinetics. Pp. 119-121.

Clark, M. Lucett, S. (2009) Nasm Essentials of Sports Performance Training. 1^st edn. Philadelphia: Lippincott Williams & Wilkins. Pp. 209-212.

DiGiovanna, E. Schiowitz, S. Dowling, D. (2004) An Osteopathic Approach to Diagnosis and Treatment. 3^rd edn. Philadelphia: Lippincott Williams & Wilkins. Pp.42-43.

Fatouros, I. Jamurtas, A. Leontsini, D. Taxildaris, K. Aggelousis, N. Kostopoulos, N. Buckenmeyer, P. (2000) ‘Evaluation of Plyometric Exercise Training, Weight Training, and Their Combination on Vertical Jumping Performance and Leg Strength’, Journal of Strength and Conditioning Research, 14(4).

Khurana, I. (2006) Textbook of Medical Physiology. 1^st edn. India: Elsevier. Pp. 98-101.

Potach, D. Chu, D. (2009) ‘Plyometric Training’. Essentials of Strength training and Conditioning. 3^rd edn. Beachle, T. Earle, R. Leeds: Human Kinetics.

Premkumar, K. (2004) The Massage Connection: Anatomy and Physiology. 2^nd edn. Philadelphia: Lippincott Williams & Wilkins. Pp. 335-337.