Plyometrics - a review

[ryancostill]

There has been much discussion on this website regarding the principle of specificity and in particular the principle of specificity pertaining to plyometrics and their implementation in sports conditioning programs.

We’ve all heard the claims: performing plyometrics will make you jump higher, run faster and a better cook. Fact.

Or maybe not.

The principle of specificity is not one which I intend to discuss here in detail as many more learned members on the site have discussed it in sufficient detail before me. I will not comment on it further than to say that as a single mode of practice I agree that practicing jumping, will improve your vertical jump best, practicing sprinting will improve your sprinting best and practicing pitching will improve you pitching best. This principle I agree completely with, however it is my opinion that plyometrics are not without their place in sports conditioning. The following post will outline their effect and relevance within strength and conditioning relating to muscle/leg spring stiffness, injury prevention and injury rehabilitation.

[ryancostill]

<font color="red"> Introduction</font>

The term plyometrics is based on latin origins and plyo + metrics is interpreted to mean “measurable increases”. (Chu 1998)

Modern strength and conditioning training programs incorporating stretching, plyometric exercise and weight-lifting have been proposed as measures to increase performance and decrease injury risk in athletes competing in sports with “jumping” components, for example Soccer, Football, Volleyball, Basketball, Gaelic Football (Ireland’s in the house) and Australian Rules Football. Accordingly many scholastic, collegiate and Olympic level teams have developed such plyometric-based programs. (Hewett 1999, Hedrick 2003, Hewett 1996 citing Dunnam 1988, Marsit 1992, McGown 1990).

There appears however, to be a misunderstanding amongst coaches and athletes alike as to exactly why plyometrics are incorporated into strength and conditioning programs for sports. Increased jump height, increased foot speed and increased “power” are oft-cited benefits. However, such claims appear to be in direct violation of the principle of specificity. To jump higher on rebounds in basketball, you must practice rebounds. Practicing pitching and working on your pitching mechanics will improve your pitching better than any plyometric program and to bring down that 100m time you have to hit the track not the gym.

However, it is the contention of the current article that plyometrics may aid in improving task specific performance (vertical jump, sprinting, pitching etc.) in conjunction with task specific practice and may also benefit the athlete through injury prevention and during injury rehabilitation.

<font color="red">
What are plyometrics?</font>

In modern exercise science plyometrics can be defined as rapid powerful movements which are preceded by a preloading countermovement that creates a stretch-shortened cycle of a muscle (Wilk 1993, Kisner, 1996). Plyometrics train the muscle to reach maximal force in as short amount of time as possible (Kutz, 2003). Plyometrics, therefore, are dependent on the eccentric loading and timing with the concentric phase of the muscle contraction (Wilk, 1993).

Essentially, if the time between the eccentric and concentric phase of movement is kept minimal, muscular contraction will be more powerful. However eccentric loading remains crucial. If there is not adequate eccentric loading of the muscle (pre-stretch), then the following concentric contraction will not be optimal (Wilk, 1993; Kisner, 1996).

The importance of utilising the stretch-shortening cycle effectively can be evidenced in exercise performance. Counter-movement jumps, in which an active pre-stretch is utilised, produce greater forces at the point of take-off and achieve greater jump heights than those of jumps from the squat position (Bobbert 1996).

Three phases take place in the course of a plyometric exercise. Lets consider them in the context of the most common form of plyometric exercise, the drop jump. Here the athlete steps of a platform of a given height (lets say approx. 18inches) and on foot contact with the ground explosively jumps upward. The key to appropriate execution of the plyometric exercise is to minimise ground contact time and maximise jump height.

So to re-state, Three phases take place in the course of a plyometric exercise. The first phase is the eccentric phase. Here the pre-loading countermovement takes place. The agonistic muscle is pre-stretched to induce an optimal post stretch contraction. In the drop jump this phase is where the athlete has landed on the ground and begins to flex the knee to absorb their downward momentum. The next phase is the amortization phase. In this phase the eccentric contraction converts to concentric contraction. This reversal phase is performed quickly to increase the elastic energy contribution to an optimal concentric response from the stretch reflex. In the drop jump example the athlete begins to halt his downward momentum and reverse it. The last stage is the concentric phase. Here, the concentric contraction takes place to execute the movement (Wilk, 1993). In our example the athlete contracts the appropriate muscles rapidly to execute the “jump”.

Plyometric exercise causes an increased excitability of proprioceptors for an optimal reaction by the neuromuscular system. Two proprioceptors are of most relevance in plyometrics. The first is the golgi-tendon organ (GTO) located in the extrafusal fibers and innervated by alpha motor neurons (Riemann 2002a). The second is the muscle spindle located in the intrafusal fibers and innervated by g-motor neurons (Riemann 2002a). There are different types of intrafusal muscle fibers, some are predominantly responsive to the magnitude of changes in muscle length while others are mainly responsive to the rate of changes in muscle length.

With plyometric exercise, the muscle spindle reacts to the quick pre-stretch to reflexively produce an opposite contraction of the agonist. The rate at which the spindle will react is strongly dependent on the rate of the stretch.

Golgi-tendon organs (GTOs) respond to changes in tension (Riemann 2002a) rather than of those in length. They exert inhibitory effects on agonist muscles and facillitatory effects on antagonist muscles (Brooks 2000). When muscle contractile forces reach a point at which damage to the muscle muscle-tendon complex may occur GTOs cause inhibitory post-synaptic potentials on the cell body of the agonist motor units (Brooks, 2000, Riemann 2002a).

The inhibitory action of GTOs can be minimised however. Their inhibitory action can be counteracted by additional excitary post-synaptic potentials from higher centres (Brooks, 2000).

Pertaining to plyometrics, the facillitatory action of the spindle caused by a pre-stretch can cause the golgi-tendon pathway to be overpowered. As the GTO has an interneuron at the synapse in the spinal cord conduction times are slower than that of the spindle which operates through g-motor neurons. With g-motor neurons firing at a faster rate than normal the muscle can fully pre-stretch and maximally contract in the concentric phase of plyometric exercise (Wilk, 1993).


<font color="red"> Possible benefits of Plyometrics </font>

Plyometrics may have a role as a preventative measure against injury. Hewett 1996 showed that a training program, which incorporated plyometrics, decreased peak landing forces by decreasing abduction/adduction moments of the knee (Hewett 1996).

Previous research has shown the incidence of knee injury to be six times higher in female athletes than in male counterparts (Lindenfield 1994). There is debate, however, as to whether such higher incidence in knee injury in females is due to anatomical and physiological differences or training status differences.

Hewett 1996, in a study using young, relatively untrained subjects, implemented a plyometric program designed to decrease landing forces by improving neuromuscular control of the lower limb during landing and to increase the strength of the musculature at the knee to increase joint stability.

Ten of the eleven female subjects, administered with the intervention training program decreased their peak landing forces. Previous research by Markolf 1978 has shown that forces greater than 29Nm can put the collateral ligaments at greater risk of injury-causing stress. Importantly the training intervention used by Hewett 1996 reduced abduction and adduction moments at the knee in the frontal plane from 42 and 36 Nm respectively to 20 and 22 Nm post intervention. Such decreases in landing forces directly translate to a decrease in forces experienced at the joints of the lower extremity. The authors postulate that the decreases in these moments may reduce the risk of medial or lateral joint lift-off and associated ligamentous injury.

Landing forces were only decreased through reduction in abduction and adduction moments at the knee. No change was observed in knee flexion and extension angles or moments. This data suggests that knee flexion angle may not be the most important factor causing decreases in peak landing force after training as previous research had suggested. Ankle dorsiflexion and hip flexion also did not change significantly as a result of the training intervention. This evidence suggests that in the Hewett study abduction and adduction moments at the knee were the dominant predictors of peak landing forces.

Maximal vertical jump height was tested using the Vertec apparatus pre-intervention, at the beginning of each intervention week and post-intervention. Mean maximal vertical jump increased (significantly, p&lt;0.05) by 1.5 ± 0.5 inches for the intervention group after training. This suggests that a plyometric training program can increase vertical jump height. Doesn’t that completely contradict the aforementioned principle of specificity!!?? Afraid not. While the portion of the study reporting peak landing forces is very strongly and strictly controlled the validity of the vertical jumping results is highly questionable. It is possible that the observed increases may have been due to a training effect using the Vertec apparatus. In a sport specific jumping task (a volleyball “jump block”) measured in a more controlled manner (on a force plate), recorded pre- and post- intervention, no difference in jumping height was recorded.

The technique applied in the present study using the Vertec apparatus is not ideal for monitoring changes in vertical jump performance. Using this method, the “height of jump” is partly dependent on an arm swinging or reaching ability. This arm swinging or reaching ability may have improved from week to week due to practicing this particular style of jumping. Resultantly increases in “height of jump” measured in this manner may be due to a more effective arm swing rather than an actual improvement in vertical jump height or changes in the neuromuscular aspects of the lower limbs.

The study provides some important information regarding plyometric training as a possible knee-injury risk-reducer, however results cannot be taken to demonstrate a plyometric training program as a means of increasing vertical jump height.

Like any good researcher should, Hewett followed up his findings with another study. This time a longitudinal intervention study reporting the effects of implementing a plyometric training program on injury incidence.

A plyometric training intervention was used in Hewett 1999 to the examine the effects it had on knee injury incidence in female athletes (female athletes are used as, if you remember, females suffer a higher incidence of serious knee injury than males). Subject base comprised of untrained female athletes (n=463), trained female athletes (n=366) and a control group of untrained male athletes (n=434). Athletes chosen for participation in the study were from the sports of soccer, volleyball, or basketball. These were chosen due to the high degree of jumping and cutting components in these particular sports. The plyometric training protocol was implemented over a full season for 60-90 min/day, 3 days/week. Serious knee injury was defined as a knee ligament sprain or rupture that caused a player to seek care by an athletic trainer and that led to at least 5 consecutive days of lost time from practice and games.

The untrained female athletes had a 3.6 times higher incidence of knee injury than the trained female athletes. The authors suggest that the implemented plyometric training program may be effective in reducing the incidence of knee injury for female athletes. However the study cannot be taken as a definitive result. The study is greatly limited by the unequal numbers of each type of athlete in each sporting group. For example, in the trained group there was a larger number of volleyball players in comparison to the untrained and control groups. This may have bias results toward a lower incidence of injuries for the trained females. The authors state that in past research volleyball players have been shown to have a decreased incidence of overall knee injuries compared to their soccer and basketball playing peers. The results of the study must also be considered in light of the low number of observed injuries. With such low incidence of injury, recruiting a large enough subject base for achieving high statistical power is difficult.

So while the study’s results do not provide definitive evidence for plyometrics’ role as a preventative measure for serious knee injury, they do strongly suggest that plyometric training may reduce knee injury risk. The authors postulate that the reduction in knee injury incidence rate is due to plyometric training’s increase in neuromuscular control and strength in the knee musculature causing reduction in knee abduction and adduction forces as described in Hewett 1996.

<font color="red"> Plyometrics and Muscle Stiffness </font>

Joint stability is the state of a joint remaining or promptly returning to proper alignment through an equalisation of forces (Riemann 2002a). The body uses a reflexive response mechanism to maintain or restore joint stability after an imposed joint perturbation (Riemann 2002a). With such a mechanism there are several processes that contribute to the net effect. In the case of a joint perturbation the processes are mechanoreceptor stimulation, neural transmission, integration of the signals by the central nervous system, transmission of an efferent signal, muscle activation and force production (Riemann 2002a).

Note: it is important to state that the concept of muscle stiffness discussed here is not the concept of “stiffness” that bodybuilders may be most au-fait with. Us in the bodybuilding community think of stiffness as “I tore my biceps to shreds in the gym yesterday and now they are so stiff!!”. When mentioning muscle stiffness here I am not discussing DOMS. To continue:

Muscle stiffness is defined as the ratio of change in force per change in muscle length (Riemann 2002b). Increased muscle stiffness (and therefore improved joint stiffness) appears to enhance functional joint stability (Riemann 2002b). Hamstring muscle stiffness has been demonstrated to have an important role in the functional ability of individuals with anterior cruciate ligament deficiency (McNair 1991). One reason for this is that stiffer muscles resist sudden joint displacements quicker and more effectively. In a study on the effect of eccentric muscle soreness on leg stiffness, it was found that subjects have the ability to control the stiffness and to increase it, in order to control acceleration and reduce injury risk (Dutto and Braun).

Stiffness can also be considered as leg stiffness and is represented as an integration of all the musculoskeletal structures during locomotion (Ferris and Farley, 1997). Leg stiffness describes all such structures’ ability to work in unison as a spring.

Joint, muscle and leg stiffness are all hugely important from a sports and exercise perspective because of their effect in increasing joint stability and also, for reasons relating to increased performance. Improved running performance has been established to correlate with greater stiffness of the ankle joint (Stefanyshyn 1998). Kerdok 2002 showed that the metabolic costs of running were at a minimum when leg stiffness is at its greatest. Similarly Turner 2003 showed an improvement in running economy after 6 weeks of plyometric training and Increases in knee spring stiffness have been shown to result in increases in leg spring stiffness, which accordingly increases speed of running (Arampatzis 1998).

The re-training of muscle stiffness, following serious knee injury is therefore vital for athletes in two regards. Firstly to optimise subsequent sporting performance and secondly to increase knee joint stability to reduce re-injury likelihood.

Plyometrics are a training protocol which are being increasingly associated with increasing athletes’ ability to demonstrate muscle stiffness. Remember, muscle stiffness is the ratio of change in force per change in muscle length. To conceptualise this with an example consider the following. A sprinter would be able to generate much more force during foot contact with the ground than an endurance runner. They would also be able to generate this force without bending their leg (lengthening the muscle in the pre-stretch) as much as the endurance runner. The sprinter, in essence, has a “stiffer” muscle or a stiffer leg.

Plyometrics as a training intervention can increase the rate at which athletes can generate force (Vossen 2000, Walsh 2004, Turner 2003) therefore increasing leg stiffness. It is postulated that that this increase in leg stiffness will translate to improved joint stability, decreasing the risk of injury to the joints protected by the muscles specifically targeted by the plyometrics. It also suggests that stiffening the joints involved in particular task through plyometric training in conjunction with task specific practice may improve the rate of task specific improvement.

For increased benefit, it makes sense that the plyometric task in question should as closely mimic the sport specific task in question. So for example a football player whose primary game-time responsibility is tackling, perhaps should not perform drop jumps. After all they are rarely required in the game to jump up in the air. Instead they could perform a tackle into a tackle bag immediately and explosively following a drop from a box to more specifically target and increase the stiffness of the muscles involved in tackling.

This, however, is currently only educated speculation. Much more empirical evidence is needed regarding plyometrics’ specific effect on muscle stiffness. Also, study investigating plyometrics effect in conjunction with task specific training is necessary before definitive conclusions can be made.

<font color="red"> Conclusions
</font>
Many modern scholastic, collegiate and Olympic level strength and conditioning training programs incorporate plyometric exercises. Plyometrics may have a role as a preventative measure against injury in sports requiring a large jumping component (volleyball, basketball etc.) by decreasing abduction/adduction moments of the knee (Hewett 1996, 1999). Further benefits of plyometrics, on injury prevention and rehabilitation, maybe due to a plyometric-induced increase in muscle stiffness which would add stability to joints. Finally, increased muscle stiffness induced by plyometrics may benefit task specific practice, but more research is needed in this area.



References

Arampatzis, A., Bruggemann, G.P., and Metzler.V (1999) The effect of speed on leg stiffness and joint kinematics on human running. J. Biomechanics, 32: 1349-1353

Bobbert, M.F., K.G.M. Gerritsen, M.C.A. Litjens, Van Soest, A.J.V., (1996), Why is Countermovement Jump Height Greater than Squat Jump Height? Medicine and Science in Sport and Exercise, Vol. 28 (11) pp. 1402-1412

Brooks, G.A., Fahey, T.D., White, T.P., Baldwin, K.M., (2000), Exercise Physiology: Human Bioenergetics and its Applications, Mayfield Publishing, California

Chu, D., (1998), Jumping into Plyometric, Human Kinetics, Champaign, IL.

Hedrick, A., Dahoda, J., Rogers, R., Bennett, S., (2003), Learning From Each Other: Plyometric Training, Strength and Conditioning Journal, Vol. 25 (6), pp. 53-54

Hewett, T. E. (1996), Plyometric Training in Female Athletes: Decreased Impact Forces and Increased Hamstring Torques, The American Journal of Sports Medicine, Vol. 24 (6), pp. 765-773.

Hewett, T. E., Lindenfeld, T.N., Riccobene, J.V., Noyes, F.R., (1999), The Effect of Neuromusclear Training on the Incidence of Knee Injury in Female Athletes, The American Journal of Sports Medicine, Vol. 27 (6), pp. 699-705

Kisner, C., Colby, L.A. (1996). Therapeutic Exercise Foundations and Technique, Philidelphia: F.A. Davis Company.

Kutz, M.R., (2003), Theoretical and Practical Applications for Plyometrics Training, NSCA Performance Training Journal, Vol. 2 (2), pp. 10-13

McNair, P.J., Wood, G.A., Marshall, R,N., (1991), Stiffness of the Hamstring Muscles and its Relationship to Function in Anterior Cruciate Ligament Deficient Individuals, Clinical Biomechanics, Vol. 7, pp. 131-137

Riemann, B., Lephart, S, (2002a) The Sensorimotor System, Part I:The Physiologic Basis of Functional Joint Stability, Journal of Athletic Training, Vol. 37 (1), pp. 71-79.

Riemann, B., Lephart, S, (2002b) The Sensorimotor System, Part II: The Role of Proprioception in Motor Control and Functional Joint Stability, Journal of Athletic Training, Vol. 37 (1), pp. 80-84.

Stefanyshyn DJ and Nigg BM (1998) Dynamic Angular Stiffness of the Ankle Joint During Running and Sprinting, Journal of Applied Biomechanics, Vol. 14 (3), pp. 292-299

Turner, A., (2003) Improvement in Running Economy After 6 Weeks of Plyometric Training, Journal of Strength and Conditioning Research, 2003, 17(1), 60–67

Vossen, J.F., (2000), Comparison of Dynamic Push-Up Training and Plyometric Push-Up Training on Upper-Body Power and Strength Journal of Strength and Conditioning Research, 2000, 14(3), 248–253

Walsh M., Arampatzis, A., (2004), The Effect of drop jump starting height and Contact time on power, work performed, and Moment of force, Journal of Strength and Conditioning Research, 2004, 18(3), 561–566

Wilk, K. E. (1993). Stretch-shortening drills for the upper extremities: theory and clinical application. Journal of Orthopedic and Sports Physical Therapy, Vol. 17(5), pp. 225-234.

[NJIron]

very interesting, ryan....thanks

[Ultra Man]

plyometrics has really helped my explosiveness for kick boxing, all i do is jump on and off of a box as quickly as i can

[ryancostill]

[ QUOTE ]
plyometrics has really helped my explosiveness for kick boxing, all i do is jump on and off of a box as quickly as i can

[/ QUOTE ]

Remember the specific task your plyometrics training (jumping on and off a box) will improve the most is jumping on and off a box! Maybe alter your plyometrics so that you are dropping off a box a performing an explosive lower limb action specific to your sport os kick boxing. Always consider the principle of specificity in your training.

[ryancostill]

Also please note that the review I wrote and posted above is by no means definitive in its conclusions! There is still alot of necessary research to be done in this area especially relating to using plyometrics to increase muscle stiffness.

Although here is another reference from the NSCA's journal JSCR backing up the findings I referenced of Hewett 1996


Researchers concluded that a low volume and intensity plyometric training program can improve landing mechanics in a manner conducive to injury prevention. This study examined peak vertical impact forces and rate of force development following a 9-week, low intensity (simple jump-landing-jump tasks) and volume plyometric based knee ligament injury prevention program. Fourteen women were assigned to perform the plyometric program while the control group received no intervention. Significant reduction in peak vertical impact forces and rate of force development in the treatment group were observed after the 9-weeks of training. These changes are considered conducive to a reduced risk of knee injury while landing.

Irmischer B, Harris C, Pfeiffer R, DeBeliso M, Adams K, Shea K. (2004). Effects of a Knee Ligament Injury Prevention Exercise Program on Impact Forces in Women. Journal of Strength and Conditioning Research, 18 (4): 703 – 707.