Plyometric training: improving lower extremity power in track and field athletes: sprinters

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Plyometric training has become a very popular form of physical conditioning used by athletes and healthy individuals due to its strong affiliation with increasing power (Chu, 1983). Plyometric training has been extensively studied over the past three decades (Markovic & Mikulic, 2010) and is used in an array of sporting fields by varied experience levelled sports coaches. The popularity developed from the mid to late 70’s where the technique of plyometric training became increasingly popular amongst elite track and field coaches (Chu, 1983).

Picture Adapted from: http:// http://nikeinc.com/ Nike (2011).

Plyometric exercises have been defined as being eccentric loading followed by a concentric contraction of the targeted muscle area (Anderson & Pandy, 1993; Wilt, 1975). Essentially the term relates to specific exercises which stretch the targeted muscle areas in a rapid manner, eccentric stress of the muscle is followed by a rapid concentric contraction effectively resulting in a short forceful movement (Chu, 1983). In scientific terms the muscle contraction sequence involved in a plyometric type exercise is referred to as the ‘stretch shortening cycle’. Plyometric exercises are strongly associated with the term ‘power’ which co-insides a combined output of strength and speed, plyometrics are said to be aimed at developing the speed component of power through the use of muscle elasticity and the SSC (McNeely, 2007).

For track and field athletes who compete in sprinting short powerful movements are required throughout their event. The outcome of the event is determined by which athlete can exert higher, more powerful movements repetitively, producing short continuous sequences of the SSC. Through training the SSC sequences enhancements to the concentric muscle action have been shown, due to recovery of stored elastic energy and increased agonist muscle innervation as a result of the stretch reflex (Bosco & Komi 1979). A Sprinters ability to change rapidly from the eccentric to concentric contraction has been defined as their ‘reactive ability’, and comes of high importance in relation to sprint performance (Young, 1992). Sprinters should be well acquainted with the SSC and its importance to their type of exercise, in relevance to sprinters it has been found that in elite athletes sprinting at top speeds produce support times (SSC times) of 80-100 milliseconds (Young, 1992), which should be replicated in training exercises looking to specifically improve an athlete’s SSC and overall power output.  

Physiological Rationale -

So, as mentioned for track and field athletes involved in sprint events, plyometric training is strongly linked with the increase of power, and SSC muscle function but what is the SSC? And what adaptations to an athlete’s physiology are caused through plyometric training?

The SSC increases the ability and function of the neural and musculo-tendinous systems to produce maximal force in short bursts, which highlights why the use of plyometric exercise, as a bridge between strength and speed, has become so popular (Chmielewski, et al. 2006).

Plyometric exercises look to improve strength, power and SSC muscle function through periods of training. Furthermore, Sustained plyometric exercises are linked with changes in neuromuscular function which look to produce positive adaptations such as: increased neural drive to the agonist muscles; muscle activation strategies’ (intramuscular co-ordination); mechanical characteristics of the muscle-tendon complex of the plantar flexors; muscular hypertrophy/architecture; single-fibre mechanics (Markovic and Mikulic, 2010). In relation to a sprinter these adaptations are important aspects of their physiological performance and will be explained in later literature.

Plyometric activities have been divided into three parts:

·         eccentric loading

·          amortization time

·          concentric contraction

-     (Swanik, et al. 2002)

Picture Adapted from: http://feldmanperformance.com/ (Feldman performance, 2010)

From a sprinters perspective you are looking to reduce the time spent in the amortization phase, not only decreasing time spent in ground contact but also generating maximal force when the muscle shortens. A study has shown that sprint based plyometric drills can improve performance over a 40m distance, reducing ground contact time by 4.4%, exercising the relevance plyometrics has in sprint-based training (Rimmer & Sleivert, 2000).

It is believed that muscle force and power generation are enhanced during the final phase of the SSC, the concentric contraction (Komi, 2000; Cavagna, et al. 1965). It has been suggested that the enhancement found from plyometric training comes as a result of the elastic energy stored in the tensile and contractile elements of the SSC, during the eccentric phase, effectively increasing the muscular force and power in the final concentric phase (Swanik, et al. 2002).

Other mechanisms have been proposed to explain why plyometric exercises increase force development:

·         Neural stimulation, more motor units are recruited due to the increased load which triggers a counteraction to the exaggerated stretch during the initial eccentric loading phase.

·         Greater extension is produced as a result of the alterations in contractile properties as additional force is applied to the tendon during the concentric phase.

·         Agonist muscle action, this comes as a response to the pre-load allowing more cross-bridge attachments in the muscles before the concentric contraction occurs

-          Moore and Schilling, (2005)

Adaptive changes which occur as a result of Plyometric training?-

The physiological adaptations associated with resistance and plyometric training can potentially affect sports performance positively or negatively depending on the needs of the athlete’s sport (Carrol, et al. 2001; Young, 1992). Power based training is associated with muscular hypertrophy which effectively increases body mass, this in-turn could hinder the ‘power: weight ratio’ of the athlete which may not enhance performance. Sprinters require unilateral contractions of the leg extensors to produce powerful movements in a horizontal fashion. Studies have shown that plyometric squat exercises for experienced sprinters improve sprint performance insignificantly (Wilson, et al. 1993), the transfer of power from training based exercises to performance could be due to the lack of specificity undermining the training effect. More influential findings were produced when more specific unilateral type plyometric exercises over a 9-week period improved 10m sprint performances which effectively lead to increase 100m performance (Delecluse, 1995).

Transferring optimum muscle-activation patterns from plyometric training to performance comes through neural adaptations to intramuscular co-ordination, involving muscles in the lower extremities in terms of sprint performance. Further adaptations could come in the form of:

·         motor-unit recruitment

·         firing rates

·         synchronization

·         reflex potentiation

-   Behm, (1995); Scmidtbleicher, (1992)

·         Decreased inhibition from eccentric loads during SSC contractions to optimize musculo-tendinous stiffness

-   Gollhofer, et al (1988)

Studies have shown that plyometric exercises could in-fact induce muscular fibre type transition in trained muscles. Many of the studies are animal based but in human based studies a significant increase in the transition to ‘type 2a’ fibres in the vastus lateralis muscle as a result of plyometric type training (Malisoux, et al. 2006), although other studies have found contrasting results with no significant changes found(Kyrolainen, et al. 2005; Potteiger, et al. 1999).

Exercise Techniques-

As stated plyometric use is popular amongst athletes. In relation to elite sprint training an article reveals Bolt and Green perform sprint specific exercises three times a day (O’Merara, 2011): including-

·         bounding on one leg without bending it;

·         bounding on each leg separately;

·         bounding while skipping as high as possible;

·         bounding while skipping as far as possible;

·         standing long jump;

·         bunny hops

Bounding Exercises-

Bounding exercises are hugely involved in an elite sprinters exercise regime programmes. Plyometric bounding exercises involve over exaggerated strides, increasing the time which the athlete spends off the ground and possessing movements which involve the SSC. Bounding exercises have been found to possess ground-contact times very similar to those of the acceleration phase of sprinting (Rimmer & Sleivert, 2000; Young, 1992), which will optimize the performance specificity to sprint performance, essentially replicating aspects of sprinting through ground-contact times involved during the exercise. In-turn this could be a good exercise to mirror the acceleration phase of a sprint performance over the initial 30m distance, also transferring similar muscular-activation patterns from bounding exercises to performance.

Bounding exercises aim to reduce ground contact time; along with external pull support which facilitates greater contraction in the concentric phase. This results in an improvement in explosive power, which is largely associated with the outcome of sprint performance.

Bounding exercises should be performed over a 30-40 metre distance, replicating the acceleration phase of a sprint. Recovery between each set is important as the fast twitch fibres involved in the exercise are more subjected to the likelihood of fatigue which will deter the quality of the exercise performance (Sale, 2002).

How to perform the exercise-

Bounding can be described as an over-exaggerated running movement alternating the foot between each landing, each landing being rapidly counteracted with a powerful foot push. The action should be initiated with a short jog to provide momentum, the movements involved look to increase the amount of ground force applied in each stride essentially increasing speed. It’s vital each stride landing is on the balls of the feet to produce a spring into the next step, implementing the SSC within the muscles involved. Arms should drive through as they would when performing a sprint, making sure each stride is as long as possible increasing airborne activity and decreasing the time spent on the ground. Throughout the exercise the athletes head should remain upright with their back in an upright position which is initially presented horizontally to provide initial momentum.

Contrasting forms of bounding exercises exist-

Picture Adapted from: http://www.volleyballmag.com/  Sudicky, A.(2012)

·         Double leg Bound (provides more stability and is less intense)

-          Same procedure but performing with both legs, similar to ‘bunny hopping’ over a 30-40m distance, more suited to a beginner in plyometric exercises

Picture Adapted from: http://ultimatehockeyconditioning.com/ Janicas, A (2011)

·         Alternate leg bound (Less stability, but intensifies the exercise)-

-          Same procedure the explanation is given on

·         Single leg bound (difficult action to perform, increases the intensity to a high level)

-          Same procedure but using only one leg

This exercise can also be alternated into a combined skipping action, this involves the same concept but propels the body upwards for more height rather than forwards for distance.

Squat Jumps-

Jumping provides a powerful burst of movements from the lower section of the body, the squat jump itself is a low intensity plyometric exercise.

How to perform the exercise-

Feet should be set shoulder-width apart, with your back kept in a straight position and the torso angled slightly forward to allow extended hip and lower extremity action. Placing your hands on your hips should set your elbows at an angle close to 90 degrees. The next phase involves lowering your body mass by flexing the knee angle to its optimal position, keeping the rest of your stature intact. Once in the position a rapid contraction, extension of the knee angle should be followed, providing a powerful extension which implements the SSC. The final phase should be landing on the balls of your feet which then allowing a rapid proceeding into reproducing the same sequence for the next repetitions.

The exercise can intensify through implementing the jump into a forward motion over a piece of apparatus such as a hurdle. This could possibly increase the muscular-activation patterns formed from the exercise motion, although plyometric squat exercises for experienced sprinters have been shown to improve sprint performance insignificantly (Wilson, et al. 1993). This finding could be counteracted by the intensification of the exercise as the motion would be horizontal in order to propel the body in a forward and upwards motion replicating the initial starting phase of a sprint. When landing the athlete should land on the balls of their feet initiating the replica sequence to jump over another hurdle and repeating this action until all the apparatus’ have been completed. This action would implement the SSC, and could also improve muscular co-ordination and muscle-activation patterns.

Picture Adapted from http://dunkculture.com/ Dunkculture, 2012

Programme Guidelines and Exercise Prescription -

For an experienced athlete a weekly prescription of plyometric exercises should be as follows;

Table 1- A medium intensity levelled plyometric exercise prescription for an experienced sprinter (sub-elite)-

Day	Plyometric session
Monday	Lower Body ( Low Intensity)
Tuesday	Rest
Wednesday	Lower Body (High Intensity)
Thursday	Rest
Friday	Rest
Saturday	Lower Body (Low Intensity)
Sunday	Rest

The prescribed plyometric exercise programme would be opposing strength type intensity levels, for example:

On Monday a low intensity plyometric session would be matched with a higher intensity strength or sprint specific session and vice versa

This would counteract the likelihood of fatigue caused by frequent and combined high intensity sessions. Muscular fatigue in sprinters is strongly associated with injury and induced performance which would decrease the progression of the athlete (Worrell, 1994). If injury were to occur during rehabilitation and the initial recovery stage the exercises could be regressed and reduced in intensity. For example:
The athlete who was performing one-legged bounding movements would be regressed to the double legged movement applying a more stable base and prevention of reoccurrence. The regression would occur until the coach providing the training perceives the athlete to be fit enough to undergo more sessions at higher intensities.

The exercise prescription would be performed in sporting complex’s providing ample amounts of space for the plyometric exercises to be completed; open fields; sports halls; gym area; track and field complex. Each exercise would have to be explained and demonstrated and no progressions made until the initial exercises could be completed by the athlete. Demonstrations would highlight correct postures throughout an exercise and the intensities should be suited to the athletes’ experience and performance level.  

References-

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Markovic, G. Mikulic, P. (2010). Neuro-Musculoskeletal and Performance Adaptations to Lower-Extremity Plyometric Training. Journal of Sports Medicine, 40 (10): 859-895

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