Plyometric Exercise in Rehabilitation
Michael L. Voight, DHSc, PT, SCS, OCS, ATC, CSCS, FAPTA
Steven R. Tippett, PhD, PT, SCS, ATC
After reading this chapter,
the athletic training student should be able to:
- Define plyometric exercise and identify its function in a rehabilitation program.
- Describe the mechanical, neurophysiologic, and neuromuscular control mechanisms involved in plyometric training.
- Discuss how biomechanical evaluation, stability, dynamic movement, and flexibility should be assessed before beginning a plyometric program.
- Explain how a plyometric program can be modified by changing intensity, volume, frequency, and recovery.
- Discuss how plyometrics can be integrated into a rehabilitation program.
- Recognize the value of different plyometric exercises in rehabilitation.
WHAT IS PLYOMETRIC EXERCISE?
In sports training and rehabilitation of athletic injuries, the concept of specificity has emerged as an important parameter in determining the proper choice and sequence of exercise in a training program. The jumping movement is inherent in numerous sport activities such as basketball, volleyball, gymnastics, and aerobic dancing. Even running is a repeated series of jump-landing cycles. Consequently, jump training should be used in the design and implementation of the overall training program.
Peak performance in sport requires technical skill and power. Skill in most activities combines natural athletic ability and learned specialized proficiency in an activity. Success in most activities is dependent upon the speed at which muscular force or power can be generated. Strength and conditioning programs throughout the years have attempted to augment the force production system to maximize the power generation. Because power combines strength and speed, it can be increased by increasing the amount of work or force that is produced by the muscles or by decreasing the amount of time required to produce the force. Although weight training can produce increased gains in strength, the speed of movement is limited. The amount of time required to produce muscular force is an important variable for increasing the power output. Plyometrics is a form of training that attempts to combine speed of movement with strength. There is considerable research evidence that plyometric training is well-suited to enhance muscular power.24,55 Plyometric training should be incorporated with other types of strength, conditioning, and sport-specific practice into a well-rounded program.32
Plyometric training is a very popular form of physical conditioning of healthy individuals that has been extensively studied over the last 3 decades.59 The roots of plyometric training can be traced to Eastern Europe, where it was known simply as jump training.26,27,66–68 Jump training exercises have been demonstrated to be an effective means of physical conditioning for the promotion of health, injury prevention, and skill-related measures of athletic performance.22 The term plyometrics was coined by an American track and field coach, Fred Wilt.73 The development of the term is confusing. Plyocomes from the Greek word plythein, which means “to increase.” Plio is the Greek word for “ore,” and metric literally means “to measure.” Practically, plyometrics is defined as a series of quick, powerful body weight resistance exercises involving prestretching the muscle and activating the stretch-shortening cycle of the muscle fiber to produce a subsequently stronger concentric contraction. It takes advantage of the length-shortening cycle to increase muscular speed, strength, and power.7,17 Efficient use of the stretch-shortening cycle is a key factor in jump performance, with the accumulation of elastic energy in a muscle action facilitating greater mechanical work in subsequent actions.
In the late 1960s and early 1970s, when the Eastern Bloc countries began to dominate sports requiring power, their training methods became the focus of attention. After the 1972 Olympics, articles began to appear in coaching magazines outlining a strange new system of jumps and bounds that had been used by the Soviets to increase speed. Valery Borzov, the 100-meter gold medalist, credited plyometric exercise for his success. As it turns out, the Eastern Bloc countries were not the originators of plyometrics, just the organizers. This system of hops and jumps has been used by American coaches for years as a method of conditioning. Both rope jumping and bench hops have been used to improve quickness and reaction times. The organization of this training method has been credited to the legendary Soviet jump coach Yuri Verhoshanski, who, during the late 1960s, began to tie this method of miscellaneous hops and jumps into an organized training plan.66–68 The main purpose of plyometric training is to heighten the excitability of the nervous system for improved reactive ability of the neuromuscular system.70 Therefore, any type of exercise that uses the myotatic stretch reflex to produce a more powerful response of the contracting muscle is plyometric in nature. All movement patterns in both athletes and activities of daily living involve repeated stretch-shortening cycles.
Picture a jumping athlete preparing to transfer horizontal energy to vertical energy. As the final step is taken before jumping, the loaded leg must stop the forward momentum and change it into an upward direction. As this happens, the muscle undergoes a lengthening eccentric contraction to decelerate the movement and prestretch the muscle. This prestretch energy is then immediately released in an equal and opposite reaction, thereby producing kinetic energy. The neuromuscular system must react quickly to produce the concentric shortening contraction to prevent falling and produce the upward change in direction. Most elite athletes will naturally exhibit with great ease this ability to maximally use stored kinetic energy in the shortest amount of time to improve power output and speed.44. Less-gifted athletes can train this ability and enhance their production of power. Consequently, specific functional exercise to emphasize this rapid change of direction must be used to prepare patients and athletes for return to activity.23 Because plyometric exercises train specific movements in a biomechanically accurate manner, the muscles, tendons, and ligaments are all strengthened in a functional manner.
Much of the earlier literature on plyometric training has focused on the lower quarter.1 Because all movements in athletics involve a repeated series of stretch-shortening cycles, adaptation of the plyometric principles can be used to enhance the specificity of training in other sports or activities that require a maximum amount of muscular force in a minimal amount of time. Whether the athlete is jumping or throwing, the musculature around the involved joints must first stretch and then contract to produce the explosive movement. Because of the muscular demands during the overhead throw, plyometrics have been advocated as a form of conditioning for the overhead throwing athlete.69,72 Although the principles are similar, different forms of plyometric exercises should be applied to the upper extremity to train the stretch-shortening cycle. It has been shown that the use of a combination of upper extremity plyometrics and strength training can not only improve upper extremity performance, but also reduce commonly identified upper extremity injury risk factors.61 Additionally, the intensity of the upper extremity plyometric program is usually less than that of the lower extremity as a result of the smaller muscle mass and type of muscle function of the upper extremity compared to the lower extremity.
The role of the core muscles of the abdominal region and the lumbar spine in providing a vital link for stability and power cannot be overlooked. Plyometric training for these muscles can be incorporated in isolated drills as well as functional activities.
Plyometrics in the Athletic Population
In the athletic population, increased power ultimately translates into improved athletic performance.7 For developing muscular power, combining resistance training and plyometric exercises that involve eccentric, concentric, and isometric contractions should provide the largest magnitude of change.6,32 It has been demonstrated with plyometric training in many sport activities, athletes perform explosive dynamic movements such as jumping,60 turning, sprinting,14,33 stopping, starting, and changing pace and direction.2,32,59 Vertical jump, sprint performance, and agility tests are commonly used in both research and applied settings to investigate the effects of plyometric training on physical fitness of team sport athletes.56,59 Research has shown that plyometric training can improve speed,25,35,48 agility,3 balance,56 and muscle strength and power.45 Multiple studies have shown plyometric training to be effective in improving jumping ability in team sports including soccer,55,56 basketball,3,47 volleyball,46,49 and handball.34
BIOMECHANICAL AND PHYSIOLOGIC PRINCIPLES OF PLYOMETRIC TRAINING
The goal of plyometric training is to decrease the amount of time required between the yielding eccentric muscle contraction and the initiation of the overcoming concentric contraction.54 Normal physiologic movement rarely begins from a static starting position, but rather is preceded by an eccentric prestretch that loads the muscle and prepares it for the ensuing concentric contraction.16 The coupling of this eccentric-concentric muscle contraction is known as the stretch-shortening cycle. The stretch-shortening cycle develops during the transition from a rapid eccentric muscle contraction (deceleration phase) to a rapid concentric muscle contraction (acceleration phase).5,43,48 The stretch-shortening cycle uses the elastic properties of muscle fibers and connective tissues by allowing the muscle to store elastic energy through the deceleration phase and release it later during the acceleration phase to enhance that muscle’s power output.51 The physiology of this stretch-shortening cycle can be broken down into 2 components: proprioceptive reflexes and the elastic properties of muscle fibers.70 These components work together to produce a response but are discussed separately to aid understanding.
The mechanical characteristics of a muscle can best be represented by a 3-component model (Figure 11-1). A contractile component, a series elastic component (SEC), and a parallel elastic component all interact to produce a force output. Although the contractile component is usually the focal point of motor control, the SEC and parallel elastic component also play an important role in providing stability and integrity to the individual fibers when a muscle is lengthened.70 During this lengthening process, energy is stored within the musculature in the form of kinetic energy.
When a muscle contracts in a concentric fashion, most of the force that is produced comes from the muscle fiber filaments sliding past one another. Force is registered externally by being transferred through the SEC. When eccentric contraction occurs, the muscle lengthens like a spring. With this lengthening, the SEC is also stretched and allowed to contribute to the overall force production. Therefore, the total force production is the sum of the force produced by the contractile component and the stretching of the SEC.
An analogy would be the stretching of a rubber band. When a stretch is applied, potential energy is stored and applied as it returns to its original length when the stretch is released. Significant increases in concentric muscle force production have been documented when immediately preceded by an eccentric contraction.4,8,13 This increase might partly be a result of the storage of elastic energy because the muscles are able to use the force produced by the SEC. When the muscle contracts in a concentric manner, the elastic energy that is stored in the SEC can be recovered and used to augment the shortening contraction.41 The ability to use this stored elastic energy is affected by 3 variables: time, magnitude of stretch, and velocity of stretch.30
The concentric contraction can be magnified only if the preceding eccentric contraction is of short range and performed quickly without delay.4,8,13 Bosco and Komi proved this concept experimentally when they compared damped vs undamped jumps.8 Undamped jumps produced minimal knee flexion upon landing and were followed by an immediate rebound jump. With damped jumps, the knee flexion angle increased significantly. The power output was much higher with the undamped jumps. The increased knee flexion seen in the damped jumps decreased elastic behavior of the muscle, and the potential elastic energy stored in the SEC was lost as heat. Similar investigations produced greater vertical jump height when the movement was preceded by a countermovement compared to a static jump.4,9,10,40
The type of muscle fiber involved in the contraction can also affect storage of elastic energy. Bosco et al noted a difference in the recoil of elastic energy in slow- vs fast-twitch muscle fibers.11 This study indicates that fast-twitch muscle fibers respond to a high-speed, small-amplitude prestretch. The amount of elastic energy used was proportional to the amount stored. When a long, slow stretch is applied to muscle, slow- and fast-twitch fibers exhibit a similar amount of stored elastic energy; however, this stored energy is used to a greater extent with the slow-twitch fibers. This trend would suggest that slow-twitch muscle fibers might be able to use elastic energy more efficiently in ballistic movement characterized by long and slow prestretching in the stretch-shortening cycle.
The proprioceptive stretch reflex is the other mechanism by which force can be produced during the stretch-shortening cycle.15 Mechanoreceptors located within the muscle provide information about the degree of muscular stretch. This information is transmitted to the central nervous system and becomes capable of influencing muscle tone, motor execution programs, and kinesthetic awareness.70 The mechanoreceptors that are primarily responsible for the stretch reflex are the Golgi tendon organs and muscle spindles.42 The muscle spindle is a complex stretch receptor that is located in parallel within the muscle fibers. Sensory information regarding the length of the muscle spindle and the rate of the applied stretch is transmitted to the central nervous system. If the length of the surrounding muscle fibers is less than that of the spindle, the frequency of the nerve impulses from the spindle is reduced. When the muscle spindle becomes stretched, an afferent sensory response is produced and transmitted to the central nervous system.
Neurologic impulses from cortical and sub-cortical levels are, in turn, sent back to the muscle, causing a motor response.62 As the muscle contracts, the stretch on the muscle spindle is relieved, thereby removing the original stimulus. The strength of the muscle spindle response is determined by the rate of stretch.42 The more rapidly the load is applied to the muscle, the greater the firing frequency of the spindle and resultant reflexive muscle contraction.
The Golgi tendon organ lies within the muscle tendon near the point of attachment of the muscle fiber to the tendon. Unlike the facilitatory action of the muscle spindle, the Golgi tendon organ has an inhibitory effect on the muscle by contributing to a tension-limiting reflex. Because the Golgi tendon organs are in series alignment with the contracting muscle fibers, they become activated with tension or stretch within the muscle. Upon activation, sensory impulses are transmitted to the central nervous system. These sensory impulses cause an inhibition of the alpha motor neurons of the contracting muscle and its synergists, thereby limiting the amount of force produced. With a concentric muscle contraction, the activity of the muscle spindle is reduced because the surrounding muscle fibers are shortening. During an eccentric muscle contraction, the muscle stretch reflex generates more tension in the lengthening muscle. When the tension within the muscle reaches a potentially harmful level, the Golgi tendon organ fires, thereby reducing the excitation of the muscle. The muscle spindle and Golgi tendon organ systems oppose each other, and increasing force is produced. The descending neural pathways from the brain help to balance these forces and ultimately control which reflex will dominate.57
The degree of muscle fiber elongation is dependent upon 3 physiologic factors. Fiber length is proportional to the amount of stretching force applied to the muscle. The ultimate elongation or deformation is also dependent upon the absolute strength of the individual muscle fibers. The stronger the tensile strength, the less elongation that will occur. The last factor for elongation is the ability of the muscle spindle to produce a neurophysiologic response. A muscle spindle with a low sensitivity level will result in a difficulty in overcoming the rapid elongation and, therefore, produce a less powerful response. Plyometric training will assist in enhancing muscular control within the neurologic system.15
The increased force production seen during the stretch-shortening cycle is a result of the combined effects of the storage of elastic energy and the myotatic reflex activation of the muscle.4,8,9,12,13,31,66 The percentage of contribution from each component is unknown.9 The increased amount of force production is dependent upon the time frame between the eccentric and concentric contractions.13 This time frame can be defined as the amortization phase.20 The amortization phase is the electromechanical delay between eccentric and concentric contraction during which time the muscle must switch from overcoming work to acceleration in the opposite direction. Komi found that the greatest amount of tension developed within the muscle during the stretch-shortening cycle occurred during the phase of muscle lengthening just before the concentric contraction.39 The conclusion from this study was that an increased time in the amortization phase would lead to a decrease in force production.
Physiologic performance can be improved by several mechanisms with plyometric training. Although there has been documented evidence of increased speed of the stretch reflex, the increased intensity of the subsequent muscle contraction might be best attributed to better recruitment of additional motor units.17,28 The force-velocity relationship states that the faster a muscle is loaded or lengthened eccentrically, the greater the resultant force output. Eccentric lengthening will also place a load on the elastic components of the muscle fibers. The stretch reflex might also increase the stiffness of the muscular spring by recruiting additional muscle fibers.17,28 This additional stiffness might allow the muscular system to use more external stress in the form of elastic recoil.17
Another possible mechanism by which plyometric training can increase the force or power output involves the inhibitory effect of the Golgi tendon organs on force production. Because the Golgi tendon organ serves as a tension-limiting reflex, restricting the amount of force that can be produced, the stimulation threshold for the Golgi tendon organ becomes a limiting factor. Bosco and Komi have suggested that plyometric training can desensitize the Golgi tendon organ, thereby raising the level of inhibition.8 If the level of inhibition is raised, a greater amount of force production and load can be applied to the musculoskeletal system.
The last mechanism in which plyometric training might improve muscular performance centers around neuromuscular coordination. The speed of muscular contraction can be limited by neuromuscular coordination. In other words, the body can move only within a set speed range, no matter how strong the muscles are. Training with an explosive prestretch of the muscle can improve the neural efficiency, thereby increasing neuromuscular performance. Plyometric training can promote changes within the neuromuscular system that allow the individual to have better control of the contracting muscle and its synergists, yielding a greater net force even in the absence of morphologic adaptation of the muscle.22 This neural adaptation can increase performance by enhancing the nervous system to become more automatic.
In summary, effective plyometric training relies more on the rate of stretch than on the length of stretch. Emphasis should center on decreasing the time of the amortization phase. If the amortization phase is slow, the elastic energy is lost as heat and the stretch reflex is not activated. Conversely, the quicker the individual is able to switch from yielding eccentric work to overcoming concentric work, the more powerful the response.
Clinical Decision-Making Exercise 11-1
A female high school basketball player is engaged in an off-season conditioning program that involves box jumps and depth jumps. As a result of these activities in conjunction with a running program to enhance cardiovascular fitness, she now complains of unilateral parapatellar pain. The knee pain is significant enough that she cannot take part in the plyometric program. The coach feels the athlete needs to address both her conditioning and power training, and wants to know what can be done to improve the athlete’s performance but not increase the knee pain. How can you help?
Specificity is the key concept in any training program. Sport-specific activities should be analyzed and broken down into basic movement patterns. These specific movement patterns should then be stressed in a gradual fashion, based on individual tolerance to these activities. Development of a plyometric program should begin by establishing an adequate strength base that will allow the body to withstand the large stress that will be placed on it. A greater strength base will allow for greater force production because of increased muscular cross-sectional area. Additionally, a larger cross-sectional area can contribute to the SEC and subsequently store a greater amount of elastic energy.
Plyometric exercises can be characterized as rapid eccentric loading of the musculoskeletal complex.17 This type of exercise trains the neuromuscular system by teaching it to more readily accept the increased strength loads.17 Also, the nervous system is more readily able to react with maximal speed to the lengthening muscle by exploiting the stretch reflex. Plyometric training attempts to fine-tune the neuromuscular system, so all training programs should be designed with specificity in mind.53 This goal will help to ensure that the body is prepared to accept the stress that will be placed on it during return to function.
Before beginning a plyometric training program, a cursory biomechanical examination and a battery of functional tests should be performed to identify potential contraindications or precautions. Lower-quarter biomechanics should be sound to help ensure a stable base of support and normal force transmission. Biomechanical abnormalities of the lower quarter are not contraindications for plyometrics but can contribute to stress failure-overuse injury if not addressed. Before initiating plyometric training, an adequate strength base of the stabilizing musculature must be present.
Functional tests are very effective to screen for an adequate strength base before initiating plyometrics. Poor strength in the lower extremities will result in a loss of stability when landing and also increase the amount of stress that is absorbed by the weightbearing tissues with high-impact forces, which will reduce performance and increase the risk of injury. The Eastern Bloc countries arbitrarily placed a one-repetition maximum in the squat at 1.5 to 2 times the individual’s body weight before initiating lower-quarter plyometrics.17 If this were to hold true, a 200-lb individual would have to squat 300 to 400 lbs before beginning plyometrics. Unfortunately, not many individuals would meet this minimal criteria. Clinical and practical experience has demonstrated that plyometrics can be started without that kind of leg strength.17 A simple functional parameter to use in determining whether an individual is strong enough to initiate a plyometric training program has been advocated by Chu.18,19 Power squat testing with a weight equal to 60% of the individual’s body weight is used. The individual is asked to perform 5 squat repetitions in 5 seconds. If the individual cannot perform this task, emphasis in the training program should again center on the strength-training program to develop an adequate base.
Clinical Decision-Making Exercise 11-2
A college track sprinter has been instructed by her strength coach to begin off-season plyometrics consisting of box jumps and high-stepping drills. The patient suffered a second-degree upper hamstring strain during the last half of track season and is reluctant to start the plyometric program. What can be done to prevent this injury from recurring?
Because eccentric muscle strength is an important component to plyometric training, it is especially important to ensure an adequate eccentric strength base is present. Before an individual is allowed to begin a plyometric regimen, a program of closed chain stability training that focuses on eccentric lower-quarter strength should be initiated. In addition to strengthening in a functional manner, closed chain weightbearing exercises also allow the individual to use functional movement patterns. The same holds true for adequate upper extremity strength prior to initiating an upper extremity plyometric program. Closed chain activities, such as wall push-ups, traditional push-ups, and their modification, as well as functional tests, can be used to ascertain readiness for upper extremity plyometrics.31,64,65 Once cleared to participate in the plyometric program, precautionary safety tips should be adhered to.
Stability testing before initiating plyometric training can be divided into 2 subcategories: static stability and dynamic movement testing. Static stability testing determines the individual’s ability to stabilize and control the body. The muscles of postural support must be strong enough to withstand the stress of explosive training. Static stability testing should begin with simple movements of low motor complexity and progress to more difficult high motor skills. The basis for lower-quarter stability centers on single-leg strength. Difficulty can be increased by having the individual close his or her eyes. The basic static tests are one-leg standing and single-leg quarter squats that are held for 30 seconds (Table 11-1). An individual should be able to perform one-leg standing for 30 seconds with eyes open and closed before the initiation of plyometric training. The individual should be observed for shaking or wobbling of the extremity joints. If there is more movement of a weightbearing joint in one direction than the other, the musculature producing the movement in the opposite direction needs to be assessed for specific weakness. If weakness is determined, the individual’s program should be limited and emphasis placed on isolated strengthening of the weak muscles. For dynamic jump exercises to be initiated, there should be no wobbling of the support leg during the quarter knee squats. After an individual has satisfactorily demonstrated both single-leg static stance and a single-leg quarter squat, more dynamic tests of eccentric capabilities can be initiated.
Once an individual has stabilization strength, the concern shifts toward developing and evaluating eccentric strength. The limiting factor in high-intensity, high-volume plyometrics is eccentric capabilities. Eccentric strength can be assessed with stabilization jump tests. If an individual has an excessively long amortization phase or a slow switching from eccentric to concentric contractions, the eccentric strength levels are insufficient.
Dynamic Movement Testing
Dynamic movement testing assesses the individual’s ability to produce explosive, coordinated movement. Vertical or single-leg jumping for distance can be used for the lower quarter (see Figures 16-21B and 16-22). Researchers have investigated the use of single-leg hop for distance and a determinant for return to play after knee injury. A passing score on the patient’s test is 85% regarding symmetry. The involved leg is tested twice, and the average between the 2 trials is recorded. The noninvolved leg is tested in the same fashion, and then the scores of the noninvolved leg are divided by the scores of the involved leg and multiplied by 100. This provides the symmetry index score. Another functional test that can be used to determine whether an individual is ready for plyometric training is the ability to long jump a distance equal to the individual’s height.
In the upper quarter, the medicine ball toss is used as a functional assessment. The single-arm seated shotput test is used as a measure of upper body power (see Figure 16-4). To perform this test, the patient sits tall with his or her back against the backrest of a chair. While holding onto a medicine ball (4 kg for men; 2 kg for women and juniors), the patient tries to chest pass the ball as far as possible, keeping his or her back in contact with the chair. This should be repeated until the longest pass has been measured. Use the distance from where the ball bounces to the patient’s chest as the distance. As can be seen in Table 11-2, less than 17 feet for men and 15 feet for women is an indicator of power weakness.
The sit-up and throw test is a great test to assess abdominal and latissimus dorsi power. The sit-up evaluates core power and the overhead throw evaluates latissimus dorsi and trunk power. To perform this test, the patient lies supine with knees bent and feet flat on the ground, while holding onto a medicine ball with both hands (4 kg for men; 2 kg for women and juniors) with the ball directly over the patient’s head like a soccer throw-in. Next, have the patient try to sit up and throw the ball as far as possible. This should be repeated until the longest pass has been measured. Use the distance from where the ball bounces to the patient’s chest as the distance (Table 11-3).
Multiple-response jumps and hops
In-depth jumping and box drills
High-stress sport-specific drills