PLYOMETRIC TRAINING FOR THE LOWER LIMB
Matt Woronczak 1998
Although the actual term “plyometric” is new, this particular form of training has been in existence for quite some time. The term was first applied in 1975 by American track and field coach Fred Wilt to describe the training methods of the Eastern European athletes at the time, which were the supposed difference between the eastern and western athletes. Translated from its Latin origins, it literally means “measurable increases” , although there is some speculation as to the effectiveness of this particular form of training. It is thought of as the missing link between weight training (strength) and athletic performance (power), with particular emphasis on the speed of activity.
On observation of sports involving jumping at the elite level, particularly basketball, volleyball and the jumping field events, it is obvious that these athletes have the ability to spring very high indeed. Perhaps what is most striking is not how high they can rise, but how quickly they can elevate. Athletes such as Michael Jordan are able to translate from changing direction to releasing a jump shot extremely quickly. This has obvious advantages, especially in team sports such as basketball or volleyball.
Plyometric training is designed to enable muscles to reach their maximum strength in as short a time as possible. Focus is placed on improving eccentric strength, achieving a rapid eccentric/concentric transition (termed amortization phase) and decreasing ground contact time using high force jumping exercises (for the lower limb). It has been shown that elite high jumpers spend only 0.12 seconds on the ground before a jump (Chu, 1992), indicating the need for minimal contact time.
Jumping itself is a complex, multi joint and muscle skill, which must be learned. This form of training serves two main purposes – to improve existing jumping ability to enhance sporting performance and in recovery from injury. The eccentric exercise program as described by Stanish et al (1986) incorporates rapid eccentric exercises into the rehabilitation program, but does actually include sport specific or jumping exercises, which an athlete requires to return to their optimal level of functioning post injury. Thus, these techniques may be utilised as an extension to the program and to ensure that adequate rehabilitation has been performed prior to a return to sport.
Unfortunately, there is a distinct lack of reliable scientific research conducted in this area, with little information available regarding the physiological basis, training effectiveness, or even the establishment of a standard protocol evident. Most information provided in books is written by physical educators with little knowledge or understanding of the human musculoskeletal system, or the principles of conducting proper scientific research. Even less information is found regarding the use of plyometric type exercises in a rehabilitation program.
The effectiveness of this form of training is thought to rely on two main principles – the utilisation of stored elastic energy and the stretch reflex. Elastic energy is produced when the muscle filaments within muscles are put on stretch, and may be described as the tendency for them to return to a normal resting length. This aids muscular force production and hence jumping performance. The stretch reflex is reliant on the rate at which a muscle is stretched. Although reflexes cannot be invoked voluntarily and the response time of the stretch reflex cannot be altered by training, training will increase the magnitude of the muscular response, thereby increasing concentric force (Chu, 1992; Voight and Dracovitch, 1991).
Repetitive explosive training has been shown to desensitise the inhibitory effect of the golgi tendon organs, and as with strength training, there is recruitment of additional motor units and improved neuromuscular coordination or skill learning (Voight and Dracovitch, 1991; Hakkinen and Komi, 1985; Hakkinen et al, 1981).
This particular area is one which especially requires more scientific research.
COMPONENTS OF A PLYOMETRIC PROGRAM
As with any exercise program, there are many different training principles utilised. Progressive overload is maintained such that once the athlete can complete the desired task with relative ease, the degree of difficulty of the exercises is increased. Brief high intensity (90-100% of maximal effort) bursts of exercises anaerobic in nature are used with adequate rest between sets (90-180 seconds). It is recommended that no more than 5-10 bounds per set, reaching a maximal total of 50-75 jumps per session, be performed (Verhoshansky, 1969; Bisetto, 1997). A frequency greater that two times a week is not encouraged, with a minimum of 48 hours between sessions to ensure adequate recovery (Jacoby and Fraley, 1995; Bisetto, 1997). Specificity of performance is highly stressed, with emphasis on single or double leg, jumping for height or length and incorporating many sport specific exercises into the program (Bisetto, 1997; Kraemar and Newton, 1994). Given the intense, high impact nature of these exercises, it is important that flexibility is addressed to avoid biomechanical problems and overuse type injuries. As with high intensity weight training, periodization is encouraged to maintain interest and ensure adequate recovery. Using this principle, as the intensity increases, the volume decreases, and conversely, as the volume increases, the intensity decreases. Although no long term studies have confirmed the hypothesis, periodization of training is used by nearly all elite athletes in one form or another. The principles of this theory appear to be sound in nature despite this lack of scientific data (Kraemar and Newton, 1994; Voight and Dracovitch, 1991). The importance of skills training highly stressed, since plyometric exercises should be considered as an adjunct to sport training and not the basis of it.
Some authors maintain that subjects be able to half squat double their body weight or perform a single leg squat before commencing on a plyometric exercise program (Jacoby and Fraley, 1995; Voight and Dracovitch, 1991), but this has been refuted (Kraemar and Newton, 1994; Stone and Kroll, 1991). Other authors suggest the need for balance assessment prior to commencement, but this too is unsubstantiated (Voight and Dracovitch, 1991). Obviously, it depends very much on the individual situation as to whether an individual is ready to begin training, and it is not universally recognised that these conditions mentioned above be met before commencement. Perhaps most important is that the participants are injury and pain free, and have performed an adequate warm up, in additional to being minimally fatigued. For rehabilitation prospects, it can be assumed that once the subject has reached a level where he is completing his exercises in his eccentric program with ease that he is ready to move to plyometric exercises. Indeed eccentric exercises such as quadriceps setting following a hop is a component of hopping or bounding, two commonly used plyometric exercises. This is not substantiated in the literature, but can be assumed to be sound in principle.
Appropriate equipment is important to ensure safety of the participant. This includes good shoes with adequate support and shock absorption, an appropriate, non slip surface (parquetry or grass are recommended, however mats on other surfaces to minimise shock), stable steps or boxes for depth jumps (those used in step classes are useful here), and collapsible items for jumping over to minimise injury.
Prior to beginning a training program, vertical jump should be measured for future reference. This can be achieved through the standing jump test as described by Chu (1992). Lower limb strength may also be assessed, with the 1RM in the squat or leg press. While there is no question that strength contributes to jump performance, there is debate as to whether isolated strength increases will result in improved jumping ability. In subjects with prior weight training experience, there was an insignificant change in jumping ability (Hakkinen and Komi, 1985) , whereas in subjects with no prior weight training experience, jumping height increased significantly. Indeed, squat training with heavy loads may decrease the ability to develop force rapidly (Hakkinen, 1981) presumably by alteration in innervation.
Strength training also increases lean body mass, and although some feel that this would hamper athletic performance, this increase in mass is accompanied by an increase in strength and power to weight ratio (Kraemar and Newton, 1994). Wilson et al (1995) found that in a ten week training program, those trained with loaded jump squats improved significantly, as did those training with plyometrics, while those who used weight training alone did not. The jump squats improved 18%, the plyometrics group 10% and the weight training group 5%. Depth jump training has been shown to increase vertical jump height significantly (Adams et al, 1992; Blakey and Southard, 1987; Clutch et al, 1983). A combination of weight training and plyometric exercises was shown to be more effective than either alternative alone (Bauer et al, 1990; Blakey and Southard, 1987; Clutch et al, 1983). As such, it is recommended that participants follow a weight training regime, with particular emphasis on exercises such as the Olympic lifts, which are relatively specific to the task (Bisetto, 1997; Kraemar and Newton, 1994).
Several aspects of the jumping technique are stressed for each exercise. Heel contact is to be avoided, since this increases contact forces and shock, thus increasing the potential for injury (Radcliffe and Farentinos, 1985). This also aids in minimising ground contact time. Subjects are instructed to think of jumping onto a hot plate. This is necessary since with a slow amortization phase, the elastic energy is lost as heat and the stretch reflex is not activated. Knee flexion should be limited to 120 degrees, since greater flexion results in a slower movement and loss of power from the eccentric phase to the concentric phase. The trunk is maintained in a relatively upright position and the chest held high. Arm movement should be utilised to drive the movement. With this form of training, QUALITY not QUANTITY is stressed and the subject is advised to stop if fatigued (Jacoby and Fraley, 1995).
The exercises utilised are limitless. Basic exercises are often incorporated into training programs without a second thought as to what or why they are being done. Perhaps the most often associated exercise with the term plyometric training is the depth jump with its many variations. These involve the participant jumping from a predetermined height onto the ground and back up as quickly as possible. Varieties may include uni or bilateral, two footed or stride landing, leaping for length or height, and the incorporation of functional tasks into the jump (eg with a slam dunk). The exercises should be kept as close to the original skill as possible with, for example, training for sports such as basketball utilising a combination of double and single leg patterns, jumping for both length and height, and also adding tasks such as shooting or dunking to depth jumps. For rehabilitation purposes, an initial program may be predominantly unilateral, progressing to bilateral exercises once the function of the affected side is adequate.
Optimal box height for professional athletes ranges from 75-110cm, with up to 3.2 metres used for setting exercises to increase eccentric strength (Verhoshansky and Chormonson, 1967). Shorter boxes (less than 60cm) are used for athletes learning the techniques (Voight and Dracovitch, 1991).
Hopping, bounding for height or length (bounding can be thought of as exaggerated running), two legged broad jumping, tuck jumps, jumping items of varying heights and lengths in an obstacle course, jumping over object laterally or forwards and backwards are other commonly used exercises. Slopes may be utilised for bounding and broad jumping drills, with uphill jumping used in preseason (harder) and downhill jumping during the season, which is easier (Jacoby and Fraley, 1995; Verhoshansky, 1969).
Other useful exercises include jumps from height with holds and hop holds to improve eccentric strength, and weight training using explosive movements such as power cleans and snatches.
There is much speculation regarding the effectiveness of these techniques and also the potential for injury, however this is unsubstantiated by statistical proof. Poor interpretation of the work done by Verhoshansky resulted in subjects being asked to attempt depth jumps from heights utilised by Verhoshansky for drop holds (exercises where the subject is asked from a height or length and land with the leg or legs in a relatively rigid position and hence “hold”) and consequently many of the participants were sceptical and cautious, and injuries probably occurred at a higher than expected level since this is not how the training regime was intended to be employed. Bearing this in mind, given the nature of the exercises there is a high potential risk for injury, just as there is in any jumping sport.
Children and adolescents have been discussed in regards to this form of training and although no formal conclusions have been made, it should be noted that people in this age group normally complete plyometric type exercises anyway – skipping, hopping, hop scotch, long jumps etc. Consensus is that if depth jumps are to be utilised, very small boxes are used (ie less than 15cm) due to the high forces created, and if this form of training was considered, it should be fun and consist of more child activity related than a formalised program (Radcliffe and Farentinos, 1985).
A sample beginner’s plyometric program for basketball players in the off season could be as follows. This regime is to be used twice per week, with at least two days between sessions. Two minutes rest is taken between sets. All additional principles outlined previously should be followed regarding jumping technique.
* Light jog, progressing to wind sprints. Alternatively, skipping may be used. Either followed by low to moderate intensity jumping exercises such as star jumps and jumps on the spot (10 mins)
* Stretches for gluteals, quadriceps, hamstrings, hip flexors and calves (5 mins)
* Depth jumps, bilateral for height, using 40cm box to begin with. 3 sets of 6
* Witches hat lateral jumping. 2 sets of 8
* Hopping length of court. 2 laps each leg
* Brisk walk, reducing to slow walk (5 mins)
* Stretches for gluteals, quadriceps, hamstrings, hip flexors and calves (5 mins)
Illustrations for these exercises and stretches can be found in Woronczak (1997).
Plyometric training principles, despite a lack of scientific data, are used by most elite athletes in some form, and indeed by almost all athletes involved in the Olympic jumps, with good effectiveness. The keys to minimising the risk of injuries during these particular exercise sessions are to use appropriate equipment, choose suitable surfaces, ensure that the participant is well rested, uses correct form in all exercises and only performs those exercises that are within his or her level. Given the nature of the exercises, there is a high potential for injury, however if these principles are adhered to, these should be kept to a minimum. Given the widespread usage of such training methods, plyometric exercise demands more research, particularly in the areas of physiology and methodology.
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