What type of movement happens during the concentric phase of plyometric exercise?

HISTORY AND DEFINITION

Plyometrics was initially used for lower extremity training for several years. After numerous years of success, several clinicians, trainers, and conditioning experts advocated using plyometrics to train the upper extremity.3 Since then, many clinicians have advocated plyometrics and have discussed using these drills clinically, with specific recommendations for programs and movements.3,6–8 Investigators have documented the efficiency of plyometrics for the upper extremity. Carter and colleagues have documented that after an 8-week plyometric training program, strength and power improved.9 Schulte-Edelmann and colleagues reported similar results.10 Swanik and colleagues stated that after an 8-week plyometric training program, proprioception and kinesthesia also were improved; plyometrics might therefore have neuromuscular benefits as well.11

Although the term plyometrics is relatively new, the basic concepts are old. The roots of plyometric training can be traced to Eastern Europe, where it was simply known as jump training. The actual roots of the word plyometric are a little confusing. “Plyo” comes from the Greek word plythein, which means to increase. Plio is the Greek word for “more,” and metric literally means “measure.” The practical definition of plyometrics is a quick powerful movement involving a prestretching of the muscle, thereby activating the stretch-shortening cycle. As the Eastern European 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 an unusual program of leaps and bounds used to increase speed. As it turns out, the Eastern European nations were not the originators of plyometrics, they were the ones who developed organized programs.

This system of hops and jumps has been used by American coaches for years as a form of conditioning. Rope jumping and bench hops have been used for years as a method to increase reaction time and quickness. The organization of this method of conditioning has been credited to the legendary Soviet jump coach Yuri Verkhoshanski, who, during the late 1960s, began to incorporate this miscellaneous program of hops and jumps into an organized plan of training.12 The actual term plyometrics was first introduced in 1975 by American track coach Fred Wilt.13 A literature review shows that since 1969, many trainers have used variances of Verkhoshanski's method in an attempt to establish the best plyometrics technique and training program.14–18 Although there is agreement on the benefits of basic plyometric principles, there is controversy regarding an optimal training routine19–22 Today, the chief proponents of plyometrics are still in the track and field community, and they continue to use Verkhoshanski's “reactive neuromuscular apparatus” for reproducing and enhancing the reactive properties of the lower extremity musculature.17,18,23 Numerous articles have been published discussing upper extremity plyometric training techniques. The main purpose of plyometric training is to increase the excitability of the neurologic receptors for improved reactivity of the neuromuscular system.

Most of the literature to date on plyometric training has been focused on the lower quarter. Adaption of 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. All movements in competitive athletics involve a repeated series of stretch-shortening cycles. The musculature surrounding the shoulder girdle possesses the same physiologic characteristics as the musculature of the lower extremity. Therefore, different forms of plyometric exercises can be applied to the upper quarter to exploit the stretch-shortening cycle. The intensity of the upper-quarter plyometrics program is much less due to the small muscle mass as compared with the program for the lower quarter, but the basic concepts and principles are the same.

Perhaps in no one single athletic endeavor is the use of elastic loading to produce a maximal explosive concentric contraction and the rapid decelerative eccentric contraction seen more than in the violent activity of throwing a baseball. Similar stretch-shortening movements can be seen in such sports as tennis, swimming, and golf. To replicate these forces during rehabilitation is beyond the scope of every traditional exercise tool. For example, the isokinetic dynamometer that reaches maximal velocities of 450 to 500 deg/sec is not specific to the velocities greater than 7000 deg/sec seen during a baseball pitch.24 Consequently, to exercise with the philosophy of specificity, drills such as plyometrics should be an intrinsic part of every upper extremity training program to facilitate a complete return to athletic participation.

Plyometric Muscle Strength Training.

Plyometrics are high intensity, high velocity exercises, such as jumping and bounding in the lower extremities and ballistic push-ups off a wall (Fig. 5-10) in the upper extremities, intended to develop muscular power and coordination.175,176 This type of training is based on the series elastic and stretch reflex properties of the neuromuscular unit.175,176Plyometric exercise employs high velocity eccentric and concentric muscle loading, reflexive reactions, and functional movement patterns. Because this type of training places high mechanical demands on the body, it should only be introduced when the patient has good strength and endurance.177,178 Plyometrics should therefore be reserved for later, higher levels of rehabilitation and focus on functional activities.

It has been proposed that plyometrics should be included in all resistance training programs for both athletes and other patients because plyometric type movements are used in basic activities such as walking and running.179 However, high velocity, low resistance (approximately 30% of maximum) dynamic weight training has been found to increase vertical jumping and isokinetic tested leg extension strength at high speeds more than plyometric training, suggesting that as long as the movement is fast plyometrics may not be necessary to optimize functional outcome.180

Plyometric drills should be preceded by a warm-up period to prepare the patient's cardiovascular and musculoskeletal system for the demands of this type of exercise. With lower extremity plyometrics, bilateral activities should precede unilateral activities, and low intensity jumps should precede higher level jumps. As with other forms of resistance training, there should be a systematic progression to more advanced drills and the patient should not be moved forward until they have mastered the previous level.

Plyometric training is proposed to reduce the risk of future injury by training muscle coactivation through neuromuscular adaptation.181 This is particularly relevant for reducing the risk of anterior cruciate ligament (ACL) tears by training quadriceps and hamstring muscle coactivation. It has been suggested that quadriceps training and hypertrophy alone may predispose patients to ACL injuries because the hypertrophied quadriceps muscles may reduce coactivation of the hamstrings by reciprocal inhibition.182-184 Advantages and disadvantages of plyometric strengthening are listed in Table 5-6.

Plyometric exercises

Plyometric exercises incorporate an eccentric preload (a quick eccentric stretch) followed by a forceful concentric contraction. This exercise technique is thought to enhance reflex joint stabilization and may increase muscle stiffness. It has become increasingly popular as an example of neuromuscular control exercise that integrate spinal and brain stem levels and has been effective addition to upper and lower extremity conditioning and rehabilitation programs.26 As with the ankle and knee, plyometric exercises are added after near-normal strength in all targeted muscles has been achieved. In the shoulder, plyometrics are performed using the balls with known weight, thrown or bounced and caught at various angles using stable and unstable rebounding boards or tossed at varying speeds by a physiotherapist.28

Aquatic plyometrics

Aquatic plyometric exercises represent an interesting option in rehabilitation, strengthening, and conditioning. As opposed to an explosive workout for power that involves high levels of impact force, plyometrics in water allows increasing intensity with low impact and low risk for injury to healing tissues. In addition, there may be the opportunity for plyometric workouts of longer duration and greater frequency. Several studies have observed increased VJH, muscle power, and torque with aquatic plyometrics programming.80,81 Clearly, this is a specialized area, and there are safety issues related to slipping on the bottom of the pool's surface and stumbling on any boxes that are placed in the water. Water depth is probably the most important consideration since buoyancy is the key element separating aquatic from land-based plyometrics. For example, studies have shown approximately 50% of a person's body weight is supported by water when submerged to the waist.80,81

In addition to the buoyancy factor, plyometrics in water also provides different sensory input for proprioception and kinesthetic awareness because of the hydrostatic pressure. As discussed in the next section, intensity can be manipulated in various ways. The key advantage of aquatic plyometrics is probably that an athlete might be able to perform a typically high-intensity plyometric drill in a low-intensity manner.

Plyometrics

Plyometric exercises emphasize explosive motions (Figure 33.4). This type of exercise takes advantage of the elastic properties of connective tissue coupled with the force generated by muscle itself. A rapid prestretch (eccentric load) is followed immediately by a forceful concentric contraction.17,18 For example, high jumpers first lower their bodies toward the ground, placing a prestretch on the leg muscles; this is followed by the forceful contraction of the same muscles, which propels the athlete over the bar.19 Plyometric exercises are introduced during the functional phase of rehabilitation and are among the most sport-specific exercises performed during this stage.

Plyometrics

Rapid eccentric contraction used immediately before an explosive concentric action (stretch–shorten cycle) forms the basis of plyometric training. This type of training was first used in Eastern Bloc countries in the development of speed (Verhoshanski and Chornonson, 1967). The movements involve a pre-stretch of a muscle, followed by a rapid contraction, causing the athlete to move in the opposite direction. Effects are achieved in both the contractile and inert structures of the muscle.

The rapid stretch of the muscle stimulates a stretch reflex, which in turn generates greater tension within the lengthening muscle fibres. In addition to increased tension, the release of stored energy within the elastic components of the muscle makes the concentric contraction greater than it would be in isolation. Increased tension will in turn stimulate Golgi tendon organ (GTO) activity, inhibiting excitation of the contracting muscle. Desensitization of the GTO has been suggested as a possible mechanism by which plyometrics allows greater force production (Bosco and Komi, 1979).

The use of muscle contraction involving acceleration in the concentric phase and deceleration in the eccentric phase more closely matches the normal function seen in sport, and therefore has advantages in terms of training specificity. However, the rapid movements involved are not suitable in early stage training as they can be relatively uncontrolled.

Several neuromuscular adaptations have been proposed for the effect of plyometric exercise (Table 4.12), and exercise of this type has been shown to significantly increase peak power output (Potteiger, 1999). Comparing plyometric exercises with their non-power equivalents demonstrates the advantages of this training. A plyometric jump compared to a deep knee bend action used 22% less energy, produced 9% more work and was 40% more efficient (Lees and Graham-Smith, 1996), while a rebound bench press compared to a standard lift gives 30% more work, allowing the athlete to lift 5.4% greater weight.

CONCLUSION

Plyometric training can be a wonderful adjunct to the athlete who is rehabilitating from injury and attempting to return to play. The expected demands on high-end athletes today are exceptional. Reduced down time from injury, early return to play, minimizing opportunities for reinjury, and performance at a consistent level equal to or higher than before the onset of the injury are all expectations that are placed on today's athletes. When properly introduced to younger athletes, plyometrics can be the tool that leads to increased confidence and satisfaction with sport activity through the positive feedback that occurs with accomplishing new tasks, setting higher scores for oneself, and correcting imbalances within the musculoskeletal system. All of these can have a motivating effect on young athletes attempting to engage in competitive sports for the first time.

A general strength base can and should be developed through exercises, such as step-ups and different types of squats, in which practicing landing strategies and dynamic postural control exercises can develop greater body control. The balance between qualitative and quantitative measures of performance is important. Using the staggered-ladder formation or maintaining a relatively centered COG as the feet move quickly from box to box is a “qualitative” measure in low-level plyometrics (i.e., inverted funnel principle). Quantitative measures with low-level plyometrics include monitoring work volume per session and following a logical and strategic progression throughout a training period. Work volume can be measured through actual “foot contacts,” and the number of foot contacts prescribed can be based on the athlete's skill level, the intensities of combined plyometrics in one workout (low, moderate, high), and seasonal period (in season vs. off season).

Foam ladders and blocks also contribute to a safe environment, should an athlete land on the blocks/ladders. Adequate space and a yielding surface are additional safety points to consider when performing jump patterns.

Finally, the effectiveness of a plyometric workout should not be measured by “how tired” an athlete feels. The “quality” of the movement is still the most crucial factor in development of the neuromuscular system as a whole. The “quantity” above “quality” approach may lead to overtraining, exercise-related pain, and even overuse injuries. Structure and accountability are necessary when including plyometric exercises in rehabilitation and strength and conditioning programs, but the overall impression should be the satisfaction of the client/athlete as the end result.

General Principles

Plyometric exercises use the stretch-shortening cycle to store potential energy. The stretch-shortening cycle consists of the eccentric phase where preloading occurs as the muscle is placed on stretch, thus storing potential energy. The amortization phase refers to the time period between the eccentric and concentric contraction. The amortization phase must be kept short; the longer it is, the greater the loss of stored energy. The final phase is the concentric phase, in which the stored energy is used in an opposite reaction (i.e., the muscle contracts concentrically to provide the force necessary for the required movement).19

When designing a plyometric training program, consideration must be given to age, body weight, current strength and conditioning level, experience, previous injury, and demands of the sport (Boxes 33-3 through 33-5).19,20

Plyometrics

Plyometric exercise is an attempt to use the stretch reflex of the muscle spindle and the elastic energy that is stored in a stretched muscle to enhance an immediate reciprocal contraction in that muscle. Plyometrics usually consists of an eccentric (lengthening) contraction followed by a concentric (shortening) contraction of the same muscles. For example, a patient would rapidly squat and then follow that by a ballistic contraction to achieve a jumping motion. In this example, energy is stored in the gastrocnemius as the ankle dorsiflexes and in the quadriceps as the knee flexes. As the person begins to jump, a strong and rapid contraction of the gastrocnemius and quadriceps propels the patient into a jumping motion.

Plyometric Training

Plyometric training is begun upon successful completion of the running program to minimize bilateral alterations in neuromuscular function and proprioception. Plyometric training in combination with resistance training is recommended by the ACSM to improve vertical jump height.2 Important parameters to consider when performing plyometric exercises include surface, footwear, and warmup. The jump training should be done on a firm yet forgiving surface such as a wooden gym floor. Very hard surfaces such as concrete should be avoided. A cross-training or running shoe should be worn to provide adequate shock absorption as well as adequate stability to the foot. Checking wear patterns and outer sole wear will help avoid overuse injuries. Beginning plyometric training is done in individual sessions performed in a manner similar to interval training. Initially, a rest period is incorporated that lasts two to three times the length of the exercise period; this is gradually decreased to one to two times the length of the exercise period. As well, plyometrics are performed two to three times each week, along with strength and cardiovascular endurance exercises.

During the various jumps, the patient is instructed to keep the body weight on the balls of the feet and to jump and land with the knees flexed and shoulder-width apart to avoid knee hyperextension and an overall valgus lower limb position (Fig. 11-25). The patient should understand that the exercises are reaction and agility drills and that, though speed is emphasized, correct body posture must be maintained throughout the drills.

The first exercise is level-surface box-hopping using both legs. A four-square grid of four equally sized boxes is created with tape on the floor. The patient is instructed to first hop from box 1 to box 3 (front to back) and then from box 1 box 2 (side to side) (Fig. 11-26). The second level incorporates both of these directions into one sequence and also includes hopping in both right and left directions (e.g., box 1 to box 2 to box 4 to box 2 to box 1). Level three progresses to diagonal hops (Fig. 11-27), and level four includes pivot hops in 90- and 180-degree directions (Fig. 11-28). Once the patient can perform level four double-leg hops, the same exercises are done on a single leg. The next phase incorporates vertical box hops.

Improvement is measured by counting the number of hops in a defined time period. The initial exercise time period is 15 seconds. The patient is asked to complete as many hops between the squares as possible in 15 seconds. Three sets are performed for both directions, and the number of hops is recorded. The program is progressed as the number of hops increases, as does the patient's confidence.

We recommend that patients complete a course of Sportsmetrics training to reduce the risk of a noncontact reinjury before return to strenuous athletics.53 This training program teaches athletes to control the upper body, trunk, and lower body position; lower the center of gravity by increasing hip and knee flexion during activities; and develop muscular strength and techniques to land with decreased ground reaction forces. In addition, athletes are instructed to preposition the body and lower extremity before initial ground contact to obtain the position of greatest knee joint stability and stiffness. The program consists of a dynamic warmup, plyometric jump training, strengthening exercises, aerobic conditioning, agility, and risk awareness training. The training is conducted three times per week on alternating days for 6 weeks. It is described in detail in Chapter 14.