Plyometric training can improve speed, strength and power. But can it also cause serious injury? Here's a review of the literature
An athlete's hunger for success is fuelled by a constant supply of new products and training principles. During the past decade plyometric training has increased in popularity and is now considered to be an essential training method for athletes competing in a wide variety of sports. Donald Chu, one of the most prolific writers in this area, considers plyometrics to be the 'icing-on-the-cake' that can enhance an athlete's ability, allowing him or her to remain at the cutting edge of their sport (1992). Donald Chu is not alone in this and many other respected coaches believe that, when performed correctly, plyometrics can improve speed, strength, acceleration and explosive power.
So should we all go out and start leaping from tall buildings in an attempt to improve our athletic performance? Well, the answer to that question once you've studied the scientific research is: look before you leap. Not everyone in the physical preparation industry is as enthusiastic as Chu. Just as steroids have now been proven to be detrimental to an athlete's health, there is growing evidence to suggest that gains made from practising plyometric drills may be outweighed by the risk of severe injuries attributed to this training method, if practised incorrectly (Horrigan & Shaw, 1989).
Just to remind you
Before we get down to the nitty gritty, here is a brief history followed by a basic guide to the physiology of plyometrics. Plyometrics is not a new concept (Gambetta, 1992). The term originated from the Latin words Plyo and Metrics and can be interpreted to mean measurable increases. Top-level athletes competing in various sports have used plyometrics for over 30 years. One of the first proponents of this practice was Yuri Verhoshanski (Radcliffe & Farentinos, 1985),
: the depth jump
although at this time it was better known as 'drop-jumping' (Bobbert, 1990). It was not until 1975 that Fred Wilt, an American track and field coach, first used the term plyometrics.
Events such as sprinting, gymnastics and basketball require muscles to reach maximal strength in as short a time as possible (Lundin & Berg, 1991). Current literature states that the elastic components of muscle and sensors within the muscle spindle are of primary importance (Thomas, 1996; La Chance, 1996). The two most important proprioceptors are the Golgi Tendon Organ (GTO) and the Muscle Spindle (MS) (Lundin & Berg, 1991). When stretched the MS sends a sensory message along the spinal column eliciting a motor response (Astrand & Rodahl, 1986). The rate of the stretch determines the rate of the MS firing; a large and fast stretch causes a high amount of firing and consequently a stronger muscle contraction (Lundin & Berg, 1991).
Dangerously high tension is inhibited by the GTO, ensuring against injury (Astrand & Rodahl, 1986). The elastic properties of muscle tissue contribute one-half to two-thirds of total positive work when performing repeated vertical jumps (Fukashiro et al 1983; cited in Lundin & Berg, 1991). Research indicates that greater results are achieved if concentric work is preceded by a period of eccentric work, and that to elicit the greatest gains the transfer from eccentric work to concentric work (amortisation phase) should be as short as possible (Aura & Komi, 1986; Wilson, Elliot & Wood, 1990). To summarise, improvements in power and jump performance could be attributed to the elastic nature of the muscle fibres and the effects of the stretch reflex. However, Lundin points out that the contribution each has on performance is difficult to estimate.
What about injuries?
Horrigan & Shaw (1989) are concerned that there are genuine factors for both athletes and coaches to consider before embarking on a plyometric training programme. Team orthopaedist for the Dallas Cowboys and Dallas Mavericks professional football team, J.P. Evans M.D., treats as many serious injuries caused by plyometrics as from any other training drill (Wikgren, 1988). Critics of plyometrics have focused their attention primarily on the most demanding of plyometric drills, depth jumps (DJ). The technique requires the participant to jump from a raised platform and, upon landing, instantly perform a vertical jump. High impact forces are associated with this advanced training technique. Studies by Boocock, Garbutt, Linge, Reilly & Troup (1990) and Fowler et al (1994) have assessed the risk of injury in terms of spinal compression following DJ, whilst studies by Pezullo, Irrgang & Whitney (1992) have linked high impact jumping activities with the onset of patellar tendinitis.
Sports participants taking part in repeated high impact activities have been identified as a high-risk group for back pain and injury (Alexander, 1985). The lumbar portion of the spine has been highlighted as one of the areas most susceptible to injury. Stress caused by accelerating body parts must be spread through the only support connecting the upper and lower portions of the body (Alexander, 1985). Shrinkage has been attributed to modifications in intervertebral discs due to compression (Fowler et al, 1994; Boocock et al, 1989). Studies by Markolf (1972), cited in Boocock et al (1990), indicate that the intervertebral discs dynamic characteristics change under compressive loading, culminating in a disc that is narrowed and stiffened and predisposed to injury. Causal links have been made between DJ and spinal shrinkage (Boocock et al, 1990, Fowler et al, 1994; Bryzcki, 1986).
Fowler et al (1994) report that high-impact repetitive exercise such as DJ has been shown to result in spinal shrinkage. It is this extreme level of intensity that produces excessively high ground reaction forces (Boocock et al, 1990). Studies indicate that the musculoskeletal system is subjected to impact forces between 3-5 times body weight as a result of landing from a plyometric activity (depth jumps). Radcliffe (1988) reported that the majority of plyometric training injuries occur during the landing phase of the jump. The findings of Boocock et al's study showed that a DJ session lasting just six minutes induced spinal loading, resulting in a mean loss of stature of 1.74mm. Boocock et al (1990) used a platform of 100cm, well in excess of the optimal jumping height of between 20-40cm as recommended by Bobbert, Huijing & Van Ingen Schenau (1987b) and Fowler et al (1995). His findings indicate the potential for damage if athletes practise depth jumps at incorrect heights.
Fowler et al (1994) used Boocock's measurement protocol, but the height of the platform in Fowler's study was just 26cm. Their experiment produced a mean loss of stature of 0.62mm. Fowler went on to add weight to the subjects when performing the DJ, producing a loss in stature of 2.14mm. Fowler's study clearly shows significantly higher levels of spinal shrinkage are experienced when additional loads are applied, even when the platform height is just 26cm.
The problem of added weight
Research indicates that the addition of weight and increased height causes alterations in technique. If weight is added, the body's true kinaesthetic sense is forfeited (Wikgren, 1988). Dr Westcott, a strength advisor to the YMCA, warns against the use of weights to increase the intensity of a plyometric drill (Wikgren, 1988), a view supported by Frank Costello (Costello, 1986; cited in Duda, 1988). To compensate for the increases in load, adaptations such as increased knee flexion occur (Bobbert et al, 1987b), resulting in a longer amortisation phase. Overloading the musculo-skeletal system reduces the ability of the body to exploit one of the key principles of plyometrics.
Deep angles of knee flexion have also been noted as one of the key factors predisposing an athlete to patellar tendinitis (Richards et al, 1996) -see later. Heights above the recommended level also cause alterations to landing strategies; if the drop height is too great, subjects are unable to stop their heels hitting the floor, predisposing the athlete to heel-pad bruising and Achilles tendinitis (Bobbert, 1990), a problem that will be discussed later. If landing technique is altered the mechanical output of the muscles can be affected (Bobbert, 1990; Bobbert, Huijing & Van Ingen Schenau, 1987a) and the chances of an off-balance landing and high-impact forces are increased (Bobbert et al, 1987b).
Coaches and athletes must seriously consider the contradic-tory advice of experts such as Chu, who recommends heights of up to 102cm in his text. Chu (1992) and La Chance (1996) further advocate the use of additional weights when performing DJ to increase the intensity, yet Yuri Verhoshanski, one of the original proponents of plyometrics, warns against the addition of weights to increase intensity (Lundin & Berg, 1991). Mike Gittleson, a strength and conditioning coach, agrees with Verhoshanski; because increased platform height corresponds with greater amounts of shock in the legs, he feels the risk of injury does not justify the payoff in terms of increased power (Wikgren, 1988). It is reasonable to postulate that spinal shrinkage occurs as a result of DJ due to the reduction in the dynamic characteristics of the intervertebral discs, therefore increasing the spine's suscepti-bility to injury. Lower platform heights reduce impact forces and the risk of injury is substantially reduced (Bobbert et al, 1987b). Studies have shown that variables such as platform height and weight can greatly affect the impact loading on the spine causing substantial spinal shrinkage and possibly other serious injuries through alteration in landing strategies.
Patellar tendinitis, or 'jumpers knee', is an inflammatory condi-tion involving the infrapatellar tendon (Pezullo et al, 1992). Strong eccentric contractions produce high levels of force, and repeated stretching of the extensor muscles results in damage to the tendon; loads of just 25 per cent max. generated by the quadriceps can create microscopic splitting of the tendon (Curwin & Stanish, 1984; cited in Pezullo et al, 1992).
The movement most fre-quently associated with patellar tendinitis is jumping, (Pezullo et al, 1992). The very nature of plyometrics, and DJ in particular, predisposes the athlete to injury. During DJ the knee, ankle and hip are inevitably exposed to high forces (Bobbert et al, 1987a). Injuries associated with fatigue, such as tendinitis and synovitis, can result from excessive plyometric training; the knee is an area of particular risk (Duda, 1988).
Bryzcki (1988) included patellar tendinitis in his list of potential injuries caused by plyometric training. The knee joint has been identified as a high-risk area when landing from a jump. Studies examining injuries caused during the landing phase found that 72 per cent of them affected the knee (Dufek & Bates, 1991). Richards et al (1996) reported that patellar tendinitis was just one injury that can result from high-impact, repetitive jumping. Humphries et al (1995) suggest that during landing the musculoskeletal system is placed under acute stress from impact forces, increasing the potential for injury. As soon as the foot comes into contact with the ground, the activity becomes a 'closed-kinetic chain' exercise; extraordinarily high forces cannot be dissipated, but must be absorbed in the musculoskeletal system.
Some specific instances
Anecdotal evidence from Lorna Booth, a former British Olympic hurdler, described acute knee pain as a result of plyometric training (Lundin & Berg, 1991). Chris Catanach, a U.S. volleyball coach, reported that one player required surgery for a knee injury attributed by the surgeon to damage caused by plyometrics (Robbie, 1988). One of the most severe cases of injury linked to plyometrics was that of U.S. shot putter, John Brenner. Mid-way through a depth jumping session his quadricep became detached, ending his professional career (Lundin & Berg, 1991). Merlene Ottey ceased plyometric training soon after knee pain developed (Lundin & Berg, 1991), one of the initial signs of patellar tendinitis (Torstenson, Bray & Wiley, 1994). One of the key concepts of plyometric training is the use of high-speed eccentric contractions to increase the effect of the stretch-shortening cycle. Research conducted by Pezullo et al (1992) stated that eccentric contractions place considerably more stress on the patellar tendon than concentric contractions, and movements such as landing from a jump are highly likely to produce microtrauma to the patellar tendon.