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plyometrics, sports injuries, spinal shrinkage patellar tendinitis, lower limb injuries, heel-pad bruising, shin splints, stress fractures

Plyometrics and sports injuries - spinal shrinkage, patellar tendinitis, lower limb injuries, heel-pad bruising, shin splints and stress fractures

Injuries that can be caused by ill-applied plyometric exercises

Scientific research has focused on causal links between plyometrics and two specific injuries, spinal shrinkage and patellar tendinitis. Critics would argue that the evidence is by no means conclusive; however, the limited research has provided a window for interested parties to look through. Increasing amounts of anecdotal evidence of other lower limb injuries are coming to the attention of coaches and doctors. Even Donald Chu, a vociferous proponent of plyometrics, admits that athletes training excessively are prone to overuse injuries (Duda, 1988). Overuse injuries such as heel-pad bruising, shin splints and stress fractures are commonly associated with plyometrics (Bryzcki, 1988; Bobbert, 1990). Overuse injuries arise when the musculoskeletal system is exposed to repetitive force beyond the capacity of that specific structure to withstand such a force (Stanish, 1984). Impact forces placed on the musculoskeletal system have the potential to cause fatigue fractures (Humpries et al, 1995).

'The bone attempts to adapt to changes in form and function, and severe overloads cause traumatic fractures'
Research by Martin & McCulloch (1987) highlighted repetitive activities and increases in strains upon the musculoskeletal system as contributing to an increase in fatigue fractures. Loading patterns of an impulsive nature such as those seen in plyometrics predispose bone to fractures (Radin et al, 1973). The bone attempts to adapt to changes in form and function (Hajek, Bates & Noble, 1982), and severe overloads cause traumatic fractures. However, moderate or even light loads applied in a repetitive cyclical nature such as plyometrics often results in fatigue or stress fractures (Martin & McCulloch, 1987). G. Dintiman, Ed.D, Professor of Health and Physical Education at Virginia Commonwealth University, is concerned at the lack of research into plyometrics, particularly surrounding links between intense forms of training and injuries such as shin splints (Duda, 1988).


And Achilles tendinitis
Heel-pad bruising is caused by repetitive heel strikes on hard surfaces. During jumping activity fatigue can set in causing the foot to weaken, resulting in an increased contact area between the heel and landing surface. Pain is felt in the plantar pad in the heel (Miller, 1982). When the unique shock-absorbing effect of the heel pad has degenerated, the amplitude of impulsive loading rises (Jorgensen, 1985). It is further suggested by Jorgensen that the reduced shock absorbing qualities of the heel pad could contribute to stress fractures and overuse injuries of the knee. Degeneration of the fat pad between the calcaneis is a predisposing factor to Achilles tendinitis; lessened impact absorption places excessive force on the Achilles tendon (Pecina & Bojanic, 1993). Achilles tendinitis is found in athletes where jumping is a major component of their sport. Excessive force placed on the Achilles tendon either by stretching (contraction of tricep surae); compression (reactive force of the surface); or torsion (uneven surface) can result in microtrauma of the tendon, and in extreme cases complete rupture.


Safety considerations
The previous sections have painted a pretty bleak picture, but let's not forget that every cloud has a silver lining. Although research would suggest that plyometric training is not without its risks, athletes throughout the world are using plyometrics in training without any detrimental effects. If practised correctly, plyometrics can be a safe and effective training technique. Proponents of plyometrics acknowledge that the potential for injury exists and many provide safety considerations in their literature. The majority of safety considerations are extrinsic factors such as landing surface, footwear and coaching, with relatively few intrinsic factors such as previous injuries. As with any training method, proper progression and evaluation of technique is of importance (Aller-heiligen & Rogers, 1996a). As well as monitoring the athletes' progress, coaches should be aware of additional factors that could increase the chance of injury during a plyometric session. So what are the top tips for a safe and effective plyometric training session?


Follow these tips to avoid injury
1. Make sure you have an adequate strength base before embarking on plyometrics, especially if you intend to carry out high- intensity drills. Early Eastern Bloc research suggests a strength ratio of 2.5 times body weight (Chu, 1992); however recent literature suggests that a strength ratio of 1.5-2.5 times body weight is sufficient (Gambetta, 1992; La Chance, 1996; Allerheiligen & Rogers, 1996a; Chu, 1992; Wikgren, 1988; Wathen, 1994). Wathen (1994) offers an alternative method using a free-weight squat of five reps at 60% in five seconds to establish the athlete's strength ratio. Strengthening the stabilising muscles is recommended in an attempt to reduce the occurrence of serious injuries (Gambetta, 1992).
Previous injuries must be assessed before embarking on a plyometric training programme (Sailer & Shot, 1993). The intense nature of plyometrics means that only athletes who are physically prepared should start a routine (Wathen, 1994). High-level plyometric training methods can place extreme amounts of stress on the lower limbs. Athletes suffering from injuries to the feet, ankles, shins, knees, hips and lower back should exercise caution before embarking on a demanding programme (Allerheiligen & Rogers, 1996a; La Chance, 1996). Athletes under 16 years of age are not advised to perform high-intensity exercises such as depth jumps, due to the risk of damaging growth plates (Chu, 1992), an approach that is supported by Sailer & Shot (1993) and La Chance (1996). Stanish (1984) clearly states that immature athletes are more likely to suffer from the type of overuse injuries associated with plyometrics due to the structural laxity and open epiphysial plates. Gambetta (1992) points out that research indicates that children are not capable of generating considerable amounts of eccentric force, making plyometrics a redundant training tool for young athletes. Athletes weighing more than 220lb. (98kg) should not perform depth jumps from platforms higher than 18 inches (Wathen, 1984; Chu, 1992; Allerheilegen & Rogers, 1996a; La Chance, 1996).

2. Choose an appropriate landing surface - solid concrete may not be the best choice! The frequency and severity of sports injuries can be affected by the landing surface (Nigg, Yeadon, & Herzog, 1988). Fukoda (1988); cited in Dufek & Bates (1991) reported that as surface hardness increased so did the peak energy absorption at the knee joint, suggesting that harder landing surfaces subjected athletes to increased possibility of injury. The shock-absorbing quality of landing surfaces is of paramount importance when performing plyometric drills (Wathen, 1994; Robbie, 1988). Spring loaded floors, or absorbent mats are best suited; synthetic tracks and grass are suitable but not ideal (Allerheiligen & Rogers, 1996a, Duda, 1988, Wikgren, 1988). Plyometrics should never be practised on concrete and asphalt. Allerheiligen & Rogers (1996a) also warn that plyometric drills incorporating horizon-tal movements should not be practised on artificial turf as horizontal friction influences translational or rotational movements (Nigg et al, 1988). The inability of the shoe to move upon landing increases the amount of stress placed on the lower limbs.

3. As with any training programme follow the basic training principles:

Warm up and cool down. Plyometrics necessitates flexibility and agility. Radcliffe & Farentinos (1985) note that warm up and cool down components are of particular importance due to plyometrics' use of eccentric muscle contractions which, as noted by (Lamb, 1978), contribute to delayed onset muscular soreness (DOMS).

Intensity. Several exercise variables can be manipulated. First, the intensity must be taken into account. Chu (1992) and Sailer & Shot (1993) consider intensity to relate to the type of exercise performed. Only when the subject has perfected low-intensity drills such as skipping and jumping on the spot can he or she progress on to the more advanced drills such as depth jumps.

Volume. Chu (1992) states that volume can be measured by the number of foot contacts with the floor or jumps performed. A study by Verhoshanski (1973) recommends that when performing depth jumps beginners limit them to between 20-30, whereas experienced performers can complete up to 40 jumps.

Frequency. Frequency and recovery can also be manipulated. Radcliffe & Farentinos (1985) recommend 48-72 hours rest between training sessions and that a work: rest ratio of 1:5-1:10 is observed, allowing full recovery between sets while maintaining an anaerobic workout. Specificity is a key component (Allerheilegen & Rogers, 1996a). Detailed sections devoted to sport-specific drills can be found within the texts of Chu (1992) and Radcliffe & Farentinos (1985).


Conclusions
Despite the growing number of publications expounding the virtues of plyometric training, an increasing amount of research and anecdotal evidence has questioned its safety. Evidence suggests that the gains made could be outweighed by the risks of injury. The papers cited in this article represent a comprehensive overview of the state of knowledge in the area of plyometrics and injuries. Substantial research of the underlying physiology of plyometrics and stretch-shortening cycle movements has contributed to a better understanding, and more effective application, of plyometrics training. However, a complete understanding of the complex interac-tions between the musculo-skeletal system and plyometrics has not yet been fully arrived at.
The type of injuries associated with plyometrics are categorised as overuse injuries. The knee and spine have been highlighted as two areas prone to injury. However, injuries caused by plyometrics are not limited to patellar tendinitis and spinal shrinkage. Evidence indicates that, as with jump sports, overuse injuries such as stress fractures, cartilage degeneration and heel-pad bruising can be caused by advanced plyometric training techniques. Specific biomechanical factors have been linked to the presence of overuse injuries in jump sports. Tentative links can be made between the incidence of injury in plyometrics and the biomechanical factors associated with injury in jump sports. Cyclic loading patterns, high impact forces, deep knee flexion and high rates of force development are contributory factors to injury in athletes participating in both activities.

'Continued research efforts must focus on understanding the processes during plyometrics which might predispose the athlete to injury'
Plyometrics have grown in popularity, despite the lack of studies into the biomechanics of the exercises and the potential risk of injury. Research in the area of plyometrics and injuries has been predominantly descriptive in nature. However, it has provided the foundations for further research. Continued research efforts must focus on understanding the processes during plyometrics which might predispose the athlete to injury. Do lower intensity drills such as bounding have the same effect on the musculoskeletal system? Further research must also determine the exact type of injuries caused by plyometrics, so that improved preventative strategies can be developed.


The bottom line
Plyometrics can be a useful training tool for athletes competing in explosive power events. Research must continue to examine links between plyometrics and injuries. Coaches and athletes should proceed with caution, taking into account safety precautions before carrying out a plyometric training programme. Co-operation between coaches and scientists will facilitate the development of safer plyometric training programmes, allowing the athlete to reap the benefits that plyometrics have to offer, without the worry of prematurely ending a sporting career.

Nick Grantham
Primary references

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Secondary

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plyometrics, sports injuries, spinal shrinkage patellar tendinitis, lower limb injuries, heel-pad bruising, shin splints, stress fractures