Abstract
Plyometrics is not simple exercise though it may look simple. These exercises need to be sport specific exercises, proper footwear, surface types, proper equipment, and also training area. While there are many who have been using plyometrics to their advantage for increasing and improving strength and power, there are many who injure themselves during these exercises. It is important for all the coaches to take into account factors such as the age, strength, maturity, and weight of the young athlete in the planning of a plyometric program. Unless the coaches take initiative to teach the athletes each exercise and its significance, it is risky to move on to another set of exercises.
Introduction
The 2008 Olympics are on and every one of us knows the importance of these games to different countries. There are many who win and have memorable events of their life but there are also many who fail to achieve their goals. There are also many who are injured and return with disappointment. Sports have been a passion for many and are taken up seriously. Strength and conditioning training is now an integral part of athletic preparation for all serious athletes and sports teams.
However, the issue of how best to train to prepare for athletic competition is very controversial. Issues such as volume and frequency of training, choice of exercise and movement cadence are debated by athletes, coaches and exercise scientists. One of the most controversial issues in this field is the use of ‘explosive’ exercises to increase strength and power (Bruce-Low and Smith 2007).
In simple terms plyometrics can be defined as a set of exercises that with a main purpose of stretching the muscles of the human body like an elastic, then contracts them fast. In fact these are special exercises meant for Olympic competitors so as to add power and speed to their bodies and improve their performances in the chosen events. Scientifically these exercises are known to essentially reduce the time it takes for a muscle in the human body to contract which is a neuromuscular reaction. As a result of this the limited time it to contract the greater the muscle can exert power and force.
Although plyometrics is mostly used by Olympians for the past several years, the recent players are not very much used to do these workouts on a daily bases. In fact these are quite dangerous exercises and should be only performed in the presence of experts. Experts also have a few warning for the beginners. Primarily the persons should make sure that their body is physically fit for the work outs as these exercises tend to cause a high amount of stress on bones and joints, and also on muscles.
Besides, as in the case of other exercises the plyometric exercises also needs to be started with the basic warm ups everyday. In other words they should not start the complicated plyometric exercises or the advanced level plyometric exercises in the beginning. In general the most preferred warm up exercises are the jogging and stretching exercises. When stretching, it is important particularly to work on all of the joints and muscles that are going to concentrate in the exercise routine. And finally the exercises to cool down need to follow after each work out (essortment.com, 2002).
If we look at the history of plyometrics, the word “Plyometrics” initially appeared in Russian sports literature in 1966 and was used by V.M. Zarciorskij. However, even before the word originated the initial plyometric methods were used in as early as1950’s with the Eastern Bloc countries in performance with the former Soviet Union (plyopros.com, N.D.).
As a matter of fact the improvement of “speed” and “jump” training transformed into a very powerful plus point for these countries in their athletic and Olympic team performances. Subsequently their dominance from 60’s, to 90’s, brought severe attention amongst higher-level athletics and professional programs even in America. As a result of this in the mid-70’s ‘plyometrics’ was formally introduced to American track and field by Coach Fred Wilt. The varied customs and developments have turn out to be extremely effective in the course of scientific research and performance studies, particularly within the last few years (plyopros.com, N.D.).
There are studies that show that the purpose of plyometrics in not just to increase the power and efficiency but also to increase the agility or the alertness of a person (Stone and O’Bryant, 1984). Therefore, plyometric activities have been used in sports where in the player need to be alert such as football, tennis, soccer or other sporting events that agility may be useful for their athletes (Parsons and Jones, 1998; Renfro, 1999; Robinson and Owens, 2004; Roper, 1998; Yap and Brown, 2000). There is still some amount of doubt regarding this issue and studies are still on its way to find how plyometric training increase performance variables and its ability to enhances agility.
The Eastern Europeans first used this technique in 70s. Basically they used this to develop greater strength and power in their Olympic athletes. This set of exercise came up as a result of several research and hence it is based on scientific evidences that any stretching muscles prior to contracting them recruits the myotactic or stretch reflex of the muscle to enhance the power of contraction. In other words the pre-stretching of muscles helps in enhancing the power of second jump (American Council on Exercise, 2001).
Plyometric exercise is a popular set of exercises that mainly help the athletes who need to elevate themselves against the force of gravity (Jamurtas et.al. 2000). The purpose of vertical jump in a sport can be varied and is generally considered as a measure of power (Kowalski, 2003). The demand for learning and teaching athletes and trainers to use plyometric exercises has been increasing over the years. This was with the sole purpose to build power and speed, improve coordination and agility and effectively improve sports performance. However, there is not much of awareness about the risks involved in these activities. It is important to recognize that these are high risk exercises and if performed incorrectly or performed without a solid base of training, plyometrics can increase the risk of injury.
The main purpose of this study is to examine the effects of plyometric exercise on athletes, its effectiveness and also the training methods. Majority of coaches are progressing athlete’s way to fast. They do not even give the athlete time to master one set of movements before they move them on to harder ones. If a coach takes the time to teach the athlete how to master a set of movements first then progress them. Then they will see a decrease in injuries and an increase in explosiveness.
Review of Literature
Plyometrics word comes from the Greek word “pleythyein” which means to augment or increase. The actual word plyometrics was first coined in 1975 by American track coach, Fred Wilt (Santana, 2000). As mentioned in the earlier chapter, plyometrics can best be explained as “explosive-reactive” power training that involves powerful muscular contractions in response to a rapid stretching of the involved musculature.
If we look into the science of plyometrics, it can be said that it works on the principles of a rubber band. The powerful contractions are an extremely high degree of neuromuscular event and cannot be considered just the pure muscular event. To be more specific it can be said that this set of exercise is a combination of an involuntary reflex which is the neural event, which is then followed by a fast muscular contraction and it is a voluntary muscular event. In fact, as in the case of several common movements, it is also a common activity that takes place in our daily lives.
It is easy to understand the subject using simple example. When a person goes to an orthopedic surgeon with a problem of leg joints, the doctor taps at the joint using an instrument that causes the leg to jerk. This is because the taping has caused a sudden stretch of the tendon that connects to all of the quadriceps which is the muscle involved in extending the knee. In fact there are small receptors within the quadriceps that create a stretch reflex and makes the quadriceps responded by contracting explosively. The technical term used for the stretch reflex is called the “myotatic reflex” and is the main concept behind plyometric physiology. Therefore it can be said that the events such as running, throwing, swinging a golf club/bat, jumping and skipping is a part of plyometric event (Santana, 2000).
Secondly, the stretching of the muscles, just before the explosive contraction that follows, is often called “loading”. Studies have pointed out that the quicker and better the load, the more powerful will be the reflex and following contraction. For instance, when a basketball player jumps to put the ball into the basket, he/she first takes a few steps and then finally jumps higher to reach the basket. The explanation for this is that the initial few steps create the momentum which is then used to create a bigger and faster “load” on the leg before the final jumping. This is the same phenomenon exists with all explosive actions in a plyometric exercise.
There are several instances when some form of power training is confused with plyometrics. For instance, a simple jump from an athletic position on to a 24 inch box is only a power exercise and cannot be considered a plyometric exercise. A real plyometric exercise should include a very fast loading phase i.e. a “myotatic reflex” to produce a powerful contraction. Therefore athletes who jump off a 6-12 inches box and immediately jumps on to a 24 inches box are a plyometric exercise (Santana, 2000). The landing from smaller box loads the legs quick enough to create the stretch reflex needed in plyometric training. Studies have shown that these exercises need to be done under strict supervisions as it involves serious dangerous.
In fact, it is proven beyond doubt that a proper supervision and progression can participate in plyometric training for all age group i.e., from children to the senior people. The most essential aspect is the development of the proper strength base to support the amplified force production that consequences from the stretch reflex. It is important to understand that the reflex involved in plyometric training helps a person to contract the muscles with greater force then through an intentional contraction. Therefore, it is essential to ensure that the musculature can bear this amplified force production.
The supervisor or the coach must also ensure a higher degree of balance and stability for the quick loading phase. The reason behind this is that although a precise body part may seem wholly involved, the percussive shocks that bring about the myotatic reflex are felt all over the body and therefore all structures must have excellent integrity to support this training.
Finally, the success of the plyometric training is in the trainers hand and they should make sure that simpler skills must be mastered before moving ahead to more complicated exercises. Inapt use of plyometric training has been coupled with a variety of forms of “over-use” injuries, particularly in the lower extremities (e.g. patellar and Achilles tendinitis and plantar faciitis). This type of training, especially when done at a very high intensity, is a high-risk attempt. Therefore, like any other high-risk maneuver, high intensity plyometrics should not be performed without the supervision of a professional trainer (Santana, 2000).
Technically, it is proven that plyometrics makes use of the forces of gravity to accumulate potential energy in the muscles, and then rapidly convert the accumulated energy into kinetic energy. This can be explained using the box jump exercise. As the athlete steps off the box and lands, legs coiled that results in the storage of potential energy, then quickly leaps to the next box and utilizes the kinetic energy. The natural elastic properties of the muscle provide an exceptional store houses for the energy. One of the most important things to note here is that to gain the maximum benefits of plyometrics the stored energy must be instantly used in a reverse direction (Edell, 2006).
As it is well known that plyometric exercises are focused, high intensity training techniques used to develop athletic power especially the strength and speed. Plyometric training that involves high-intensity, explosive muscular contractions that invoke the stretch reflex, need a systematic training by professional coaches. Each and every exercise needs to be trained thoroughly before moving on to the next one. The most widespread plyometric exercises include hops, jumps and bounding movements. These exercises usually increase speed and strength and build power that is necessary for an athlete.
One of the most important aspects to remember is that in plyometric exercises, it is the fastness that produces result. A person who can execute a particular resistance movement, such as jumping, bench press etc., at a very fast phase would be said to have more power in that movement. Therefore, it is not just the contraction of the muscle, but how fast will it contract matters the most. Studies have shown that a muscle will contract the fastest when it has been loaded and is responsible for jumping higher. Hence, if this is being used to perform a powerful movement, practicing these movements is equally important. Plyometrics has been shown in study after study to decrease the time it takes for the muscles to contract, resulting in more power.
Several studies on this subject reveal the fact that plyometrics involves serious risks. For instance, scientific research has focused on links between plyometrics and the injuries of spinal shrinkage and patellar tendinitis. Though there are critics who argue that the evidence is not conclusive, further research is necessary to further prove this point (Grantham, N.D.). With the increasing use of unsupervised use of these exercises, increasing amounts of other lower limb injuries are seen by the coaches and doctors. According to Donald Chu who is a vociferous supporter of plyometrics “athletes training excessively are prone to overuse injuries” (Duda, 1986).
There are also studies that say that overuse injuries such as heel-pad bruising, shin splints and stress fractures are regularly coupled with plyometrics (Bobbert, 1990). According to Stanish (1984) these injuries are mainly due to the musculoskeletal system being exposed to repetitive force further than the ability of that particular structure to endure such a force. Besides, it is also evident that the impact forces placed on the musculoskeletal system have the potential to cause fatigue fractures (Humpries et al, 1995) and the bone attempts to acclimatize to changes in form and function, and harsh overloads causing traumatic fractures.
The results of research by Martin & McCulloch (1987) also highlighted similar facts that repetitive activities and increases in strains upon the musculoskeletal system and can contribute to an increase in fatigue fractures. Earlier studies by Radin et.al. (1973) point out that loading pattern of an impulsive nature in plyometric exercises influence bone to fractures. This mainly happens as the bone attempts to adapt to changes in form and function (Hajek, et.al, 1982), and when the load increases it cause traumatic fractures.
Moderate or light loads applied in a repetitive cyclical nature such as plyometrics frequently results in exhaustion or stress fractures (Martin & McCulloch, 1987). G. Dintiman, Ed.D, Professor of Health and Physical Education at Virginia Commonwealth University, is alarmed at the lack of research into plyometrics, mainly surrounding links between intense forms of training and injuries such as shin splints (Duda, 1986).
Achilles tendinitis is a common problem found in athletes where jumping is a major part of their sport. Disproportionate force placed on the Achilles tendon either by stretching, compression, or torsion can result in microtrauma of the tendon, and in extreme cases total rupture. Repetitive heel strikes on hard surfaces causes heel-pad bruising. Jumping activity causes fatigue and result in the foot to weaken. As a result of this there is an increased contact area between the heel and landing surface causing pain in the plantar pad in the heel (Miller, 1982). This results in the loss of the unique shock-absorbing effect of the heel pad and amplitude of impulsive loading rises.
The reduced shock absorbing qualities of the heel pad could add to stress fractures and overuse injuries of the knee (Jorgensen, 1985). Disintegration of the fat pad between the calcaneis is an influencing factor to Achilles tendonitis and this decreased impact absorption places unnecessary force on the Achilles tendon (Pecina & Bojanic, 1993).
In spite of the increasing number of publications illuminating the qualities and merits of plyometric training, an increasing amount of research and subjective evidence has questioned its safety. There are also studies that suggest that the gains made could be overshadowed by the risks of injury. Though, considerable research of the fundamental physiology of plyometrics and stretch-shortening cycle movements has contributed to a better understanding of this subject, further research can only bring about a complete understanding of the complex interactions between the musculo-skeletal system and plyometrics.
Researchers have linked the type of injuries with plyometrics are categorized as overuse injuries. This is especially true with the knee and spine and is highlighted as two areas very susceptible to injury but is not restricted to patellar tendinitis and spinal shrinkage. Studies conducted by various researchers suggest that as with jump sports, overuse injuries like stress fractures, cartilage degeneration and heel-pad bruising can be the result of advanced plyometric training techniques. Additionally there are specific biomechanical factors that are associated to the incidence of overuse injuries in jump sports. It is also reported that cyclic loading patterns, high impact forces, deep knee flexion and high rates of force development are contributory factors to injury in athletes (Grantham, N.D.).
Coaches and trainers who use the plyometric exercises need to be utterly careful while selecting specific exercise. Plyometrics used inappropriately can without doubt lead to injury or over- training. They must be principally thoughtful and careful to make sure that plyometrics are used correctly. For instance, in the case of youngsters, it is essential to train them correctly. Adolescents are at risk to a variety of injuries. Most of them are in their growth phase and are still growing. They have softer bone structure than adult athletes, and have not until now developed the complete strength to compete with others and handle the more highly developed and challenging plyometric drills.
In fact, it is often seen that coaches design programs for their best athletes, and not thinking that the capacity of other team members is much less. It should be noted that athletes will not gain from a schedule that they are not capable to handle. While it is most important to train the athletes of one set of exercise before moving on to another, it is also essential to consider their age, strength, maturity, and weight of the young athlete in the planning of a plyometric program.
Secondly, it is also important to develop a good technique and demonstrate and clearly explain the importance of appropriate implementation of an exercise before permitting the athlete to begin. Explaining the concept of plyometrics to the athletes will let them understand the impact of exercise on their bodies and how it is making them better athletes.
Thirdly, in order to reduce the risks of injuries, plyometric drills is often done on a soft level surface, such as grass or padded mats. It is especially dangerous to practice these exercise on concrete, asphalt, or the running track. Besides it is also important to use supportive shoes with good cushioning for drills.
As in the case of other form of exercises, extra weight, such as ankle weights or weighted vests, should never be used. While these can be injurious, it will never help to accomplish the goals of plyometrics to be more explosive. Gravity and velocity supply the indispensable resistance. Adding weight ruins the plyometric effect by causing the athlete to use up extra time on the ground and converts the exercise into a form of predictable strength training.
While some athletes will take a long time to move on to more advanced and specific plyometrics, some will never get beyond the most general stage because of its complexity and dangers. Any jumping done from boxes needs to be done only at low heights (12-18 inches) by stronger and mature athletes in presence of professionals (coachr.org, N.D.).
While general exercises such as push-ups, jogging, etc., can be done be any one and every one, plyometrics is not that simple. There are several factors that need to be measured when the decision has been made to begin plyometric training. These exercises need to be sport specific exercises, proper footwear, surface types, proper equipment, and also training area. Other most important areas of consideration are the rate, volume, intensity, progression, recovery, and the direction of motion recommendations for the exercises.
Safety includes several areas, together with proper footwear, flexible surface, appropriate equipment, and training area size. Footwear should make available adequate ankle and arch support to prevent injury. Running shoes need to be avoided due to their narrow sole and poor upper support; crosstraining shoes are the best for plyometrics.
Several researchers have suggested that in order to prevent injuries, the landing surface should possess good shock-absorbing properties that are best seen in a grass field. Another good substitute would be wrestling mats. While the size of the training area depends on the type of exercises being used, the height of the ceiling also makes a serious impact. For instance, long-response drills may require a straightaway of 100 meters and bounding drills needs at least 30 meters of straightaway on the other hand for box jumps, adequate ceiling height must be provided.
There are also several other factors that the coaches need to keep in mind while the safety is planned. It is essential that frequency, volume, intensity, progression, and recovery play important roles. Frequency is the number of workouts per week an athlete is engaged in. Volume refers to the number of foot contacts per workout. Intensity is the amount of stress placed on the muscle during the workout. Progression identifies with the change from low-intensity to medium-intensity to high-intensity levels as the athlete progresses. And last but not the least the recovery that is the rest which is permitted among the individual sets of the drills (Edell, 2006).
There are several researchers who have worked on various chemical reactions that occur in the muscles during these exercises. They have also linked this factor to different forms of injuries. For instance, though polymeric include both eccentric and concentric contractions, the link between these has been studied by Jamurtas et.al. (2000). They have found that if an initial exercise session is done, the muscle soreness and plasma creatine kinase levels are reduced when these exercise session is done with the same intensity. Besides, the results also suggest that delayed onset of muscle soreness (DOMS) after the intense exercise session seems to appear similar in plyometirc, concentric and eccentric exercises.
According to another study by Lees and Graham-Smith (1996) it is said that unfamiliar tiring exercise in general results in muscle damage to some degree. Delayed onset of muscle soreness (DOMS) is among the common problems. Eccentric exercises in general results in more muscle damage. In fact it is seen that soon after 12-24 hours of the post exercise period DOMS reaches peak. Besides they also pointed out that an excessive force applied to the skeletal system and the spinal column can result in spinal shrinkage. Excessive loads on the articular surfaces as during the work outs can cause degenerative joint diseases.
Coaches need to carefully plan the exercises as the intensity, volume and frequency of plyometric training is directly linked to the injury. The link between plyometric training and agility is studied by Miller et.al. (2006). The results of this study found profound benefit from this training for athletes. The strength and explosiveness in addition to agility is the gift of plyometric training.
Strength and conditioning exercise is presently a vital part of athletic preparation for all serious athletes and sports teams. However, the issue of how best to train to get ready for athletic competition is still very controversial. Issues such as volume and frequency of training, choice of exercise and movement tempo are debated by athletes, coaches and exercise scientists. One of the most controversial issues in this field is the use of ‘explosive’ exercises to increase strength and power. These resistance exercises are characterized by maximal or near-maximal rates of force development or by high acceleration. The selective conscription of muscle fiber types is impossible.
Studies have often claimed that such exercises convert superior into enhanced sporting performance compared to weight training with a slow tempo. However, there is little confirmation that these training techniques are effective in enhancing athletic performance, and no evidence that they are more effective than relatively safe, slow weight training. In fact, there are some studies that suggest slow weight training may be more successful in enhancing strength and power.
As mentioned earlier there is significant data that explosive exercises cause extensive injury risks that are ethically unacceptable. This study concluded by saying that “a training regime that encompasses slow, controlled weight training in combination with the sport specific training is all that is necessary to enhance both muscle strength and power and in turn progress actual sporting performance” (Bruce-Low and Smith, 2007).
Researchers have formerly examined only the effects of land plyometrics on power, force, and velocity. A study conducted by Robinson (2002) compare changes in performance outcome and muscle soreness between plyometric training on land and plyometric training conducted in an aquatic setting. The results showed that “aquatic plyometrics provides the same performance enhancement benefits when compared to land plyometrics with a reduced amount of muscle soreness”.
In general, coaches and trainers use maximal strength testing to determine the muscular fitness and track progress. They provide motivation and develop individual training program. According to Masamoto et.al. (2003) only high-intensity plyometircs can enhance neural stimulation to a level that is significantly increase maximal muscle strength. Athletes who are in practice of these explosive exercises may have improved intramuscular coordination.
Coaching, a one-on-one relationship-based service has roots in individual and consulting psychology, management consulting, leadership training, the human potential movement, and mentoring (Pinchot & Pinchot, 2000). Coaching is the art and practice of inspiring, energizing, and facilitating the performance, learning and development of the player. (Downey, 2003)
In conclusion, it can be said that coaches and athletes should go on with vigilance, taking into account safety precautions before carrying out a plyometric training programme. Collaboration between coaches and scientists will assist the development of safer plyometric training programmes, allowing the athlete to gather the benefits that plyometrics have to offer, without the worry of precipitately ending a sporting career.
Besides, each coach needs to take into account factors such as the age, strength, maturity, and weight of the young athlete in the planning of a plyometric program. Above all, as mentioned by several researchers it is essential to teach the athlete one set of exercise before moving on to the next. This will not only help to reduce the injury but also provide good practice of the exercise that can help improve the strength and power of the athlete.
References
American Council on Exercise, (2001) Plyometrics: Controlled Impact/Maximum Power. Fit Facts. Web.
Bobbert, M.F. (1990). Drop jumping as a training method for jumping ability. Sports Medicine. 9, 7-22.
Bruce-Low, S. and Smith, D. (2007) Explosive Exercises In Sports Training: A Critical Review, Journal of Exercise Physiology, Vol 10 (1) P 21-33.
coachr.org, (N.D.) Philosophy of Plyometric Training. Web.
Downey, M. (2003) Effective Coaching: Lessons from the Coach’s Coach. Texere; 3 edition.
Duda, M. (1986). Plyometrics: a legitimate form of power training? The Physician and Sports Medicine. 16, 213-218.
Edell, D. (2006) Introduction to Plyometrics. Web.
essortment. (2002) What are plyometrics? Web.
Grantham, N. (N.D.) Plyometrics and sports injuries – spinal shrinkage, patellar tendinitis, lower limb injuries, heel-pad bruising, shin splints and stress fractures. Web.
Humphries, B.J., Newton, R.U., & Wilson, G.J. (1995). The effect of a braking device in reducing the ground impact forces inherent in plyometric training. International Journal Sports Medicine, 16, 129-133.
Jamurtas, A.Z. et.al. (2000) Effects of Plyometric Exercise on Muscle Soreness and Plasma Creatine Kinase Levels and Its Comparison with Eccentric and Concentric Exercise, Journal of Strength and Conditioning Research, 2000, 14(1), 68–74.
Jorgensen, U. (1985). Achillodynia and loss of heel pad shock absorbency. The American Journal of Sports Medicine. 38, 41-48.
Kowalski, C.A. (2003) Correlation between time to peak torque and peak torque to vertical jump in college age athletes, The Graduate College of Marshall University.
Lees, A. and Graham-Smith, P. (1996) Plyometric training; a review of principles and practice. Sports Exercise and Injury. 2, 24-30.
Martin, A.D., & McCulloch, R.G. (1987). Bone dynamics: stress, strain and fracture. Journal of Sports Sciences. 5, 155-163.
Masamoto, N. et.al. (2003) Acute Effects of Plyometric Exercise on Maximum Squat Performance in Male Athletes, Journal of Strength and Conditioning Research, 2003, 17(1), 68–71.
Miller, M.G. et.al. (2006) The Effects Of A 6-Week Plyometric Training Program On Agility, Journal of Sports Science and Medicine (2006) 5, 459-465.
Miller, W.E. (1982). The heel pad. The American Journal of Sports Medicine. 10, 19-21.
Parsons, L.S. and Jones, M.T. (1998) Development of speed, agility and quickness for tennis athletes. Strength and Conditioning Vol 20(3), 14-19.
Pecina, M.M., & Bojanic, I. (1993). Overuse Injuries of the Musculoskeletal System (pp. 230-242). Bola Raton: CRC Press.
Pinchot, E., and Pinchot, G. (2000). Roots and boundaries of executive coaching. In M. Goldsmith, L. Lyons, & A. Freas, (Eds.), Coaching for leadership: How the world’s greatest coaches help leaders learn (pp. 43-64). San Francisco, CA: Jossey-Bass/Pfeiffer.
plyopros.com (N.D.) PlyoPros – Faster, Quicker, Higher, Better. Web.
Renfro, G. (1999) Summer plyometric training for football and its effect on speed and agility. Strength and Conditioning 21(3), 42-44.
Robinson, L.E. (2002) The Effects of Land verses Aquatic Plyometircs on Power, Torque, Velocity, and Muscle Soreness. Graduate School of the Ohio State University.
Robinson, B.M. and Owens, B. (2004) Five-week program to increase agility, speed, and power in the preparation phase of a yearly training plan. Strength and Conditioning 26(5), 30-35.
Roper, R.L. (1998) Incorporating agility training and backward movement into a plyometric program. Strength and Conditioning 20 (4), 60-63.
Santana, J.C. (2000) Plyometric Training – Part I. Web.
Stanish, W.D. (1984). Overuse injuries in athletes: a perspective. Medicine and Science in Sports Exercises, 16, 1-7.
Hajek, M.R., & Bates-Noble, H. (1982). Stress fractures of the femoral neck in joggers: case reports and review of literature. The American Journal of Sports Medicine. 10, 112-116.
Stone, M.H. and O’Bryant, H.S. (1984) Weight Training: A scientific approach. Minneapolis: Burgess.
Yap, C.W. and Brown, L.E. (2000) Development of speed, agility, and quickness for the female soccer athlete. Strength and Conditioning 22, 9-12.