Clients—sports-minded and otherwise—can reap the benefits of a plyometrics program. Plyometric exercise is one of the most efficient ways to train and has “arguably the greatest transfer to sport application” (Barnes 2003). The jumping, hopping and bounding movements involved in plyometrics have been part of the human motion landscape throughout the ages. However, it wasn’t until the mid-20th century that these activities were more formally applied to athletic performance enhancement (Gambetta 2007).
Researchers have attempted to verify the safety and effectiveness of “jump training,” with mixed results (Chu 1998). The challenge in coming to a finite conclusion is the variability of the parameters. For example, researchers have compared experienced athletes of different sports and levels of conditioning and ability with “untrained” athletes under even more variables and conditions. A main idea that has been overlooked is that “athletic development follows its own time curve” (Chu 1998). Before introducing clients to this “method of developing explosive power” (Radcliffe & Farentinos 2002), several considerations need to be made, including age, body weight, strength prerequisite, sports requirements, jumping surface and progressions, among other factors (Barnes 2003). A basic understanding of plyometrics is fundamental. This article explores plyometrics as a training modality, as well as the science behind its effectiveness in training for sports. Practical considerations for progressions, design and drills are also offered.
What Is Plyometric Exercise?
Plyometrics is defined as “exercises that enable a muscle to reach maximum strength in as short a time as possible” (Chu 1996). Plyometric exercise involves repeated, rapid, eccentric and concentric movements (via jumping and rebounding) to increase muscle power. In addition to the jumping, hopping and bounding movements for the lower body, plyometrics involves the upper body with quick-action push-offs, arm swinging, catching and throwing of weighted objects, and pulley throws (Ward, Dintiman & Ward 2003). Jumping drills are quantified in sets and reps (sometimes in distances and time elapsed) and are usually linear in nature and design. The key to plyometrics execution is the preparatory arm action at the shoulder, the posture of the pillar core and the preparatory dorsiflexion action of the ankle joint.
The goals of plyometric training are to raise explosive power; to learn to better attenuate ground reaction forces regardless of sport; and to learn to tolerate and use greater stretch loads (Gambetta 2007). Much of the training occurs during the stretch-shortening cycle, which is the time it takes to switch from the eccentric loading phase of the movement to the concentric power production phase. During an eccentric contraction, the muscle elongates during tension, as the opposing force is greater than the force being generated. A concentric contraction occurs when the muscle shortens during tension, as the force generated by the muscle is greater than the force opposing it. The time between the eccentric phase and the concentric phase is the amortization phase. The ability to shorten the amortization phase and also absorb greater forces during the eccentric contraction leads to greater force production in the concentric phase.
As the preparatory loads are absorbed, the inability to switch from the stretch to the shorten phase of the contraction results in greater ground contact times. The athlete looks less explosive and the training is more strength-speed in nature than speed-strength.
Impulse, or the rate of force development (RFD), is also critical to performance. The time it takes to generate peak force, which is RFD, is generally greater than the time it takes to contact the ground in sprinting and jumping. For example, in the high jump, ground contact time is less than 100 milliseconds and peak force is created at about 500 milliseconds.
What type of strength training is involved in plyometrics? Starting strength is the ability of a muscle system to overcome inertia and create force of movement, as in a sprint start or clean dead lift from the floor. This type of movement is concentric in nature. Stopping strength is the ability of the body to absorb force via the muscle-tendon system. This is an eccentric movement or contraction. Elastic strength is the ability of the muscle-tendon system to absorb force via the stretch-shortening cycle, to overcome the force in a relatively short amortization time and to move the body in the opposite direction.
The mechanical components of the tendon consist primarily of the series elastic tissue (connective tissue in series). When the eccentric contraction stretches the series elastic components (SEC) of the tendon, the kinetic energy is stored and the concentric contraction makes use of this stored energy (Baechle & Earle 2000). The muscle components consist of the actin-myosin SEC cross-bridges, the actin-myosin contractile components and the parallel elastic components (the epi-, peri- and endomysium as well as the sarcolemma), which enhance the release of the stored kinetic energy during the concentric contraction. If the eccentric or amortization phase is too long, much of the kinetic energy force is lost and dissipated as heat (Baechle & Earle 2000).
Is Plyometrics Appropriate?
Plyometrics works—there is no doubt. Is this type of training appropriate for everyone? One could argue that the very action of walking is somewhat plyometric in nature, as it utilizes the stretch reflex to a certain degree. Have you ever watched kids jump rope or play hopscotch? Both of these elementary-school games are plyometric in nature. Look at how children jump down from couches, stairs and bleachers. As humans we innately know we must develop the ability to absorb force before we ever get to the point of producing force. Therefore, it is imperative that we teach and reinforce landing mechanics and begin plyometric training with force absorption drills, such as downward jump and land-and-hold drills, in order to prepare the tendons for an increased ability to absorb force.
Flexibility, strength and proprioceptive elements are all prerequisites for plyometric training. Age, experience, past injuries, genetics and health status are also prime considerations (Radcliffe & Farentinos 2002). For lower-body plyometrics, the client’s one-repetition maximum (1RM) should be at least 1.5 times his or her body weight, although this criterion has been challenged. For any client weighing up to 220 pounds, the bench press 1RM should be at least as much as his or her body weight. The ability to perform five clap push-ups in a row is an alternate prerequisite test for upper-body plyometrics. Do not introduce plyometric drills to clients if they do not have adequate muscular strength (Baechle & Earle 2000). (See the sidebar “Position Statement: Plyometric Exercises.”)
Regarding training considerations, gender is not much of a factor. The greater concern is the ability of the client to absorb force in a timely manner—with great proprioceptive stability in the hip, knee and ankle—and to show proper core mechanics. The key to watch for with young women athletes is valgus, or internal collapse and rotation at the knee joint upon ground contact. This would signal a poor force absorption pattern, generally due to either lack of dorsiflexion at the ankle or poor lateral hip strength in the gluteus medius.
Age is not a big concern if it is qualified by the above parameters of mechanics, absorption pattern and pillar core posture. Sport is of some importance, especially as the athlete progresses in ability and skill. The more reps the athlete is doing in sport practice and competition, the fewer he will be able to handle in training. The body type of the athlete is critical to consider. An athlete who is overweight, whether from muscle or fat mass, will be absorbing greater forces upon landing and therefore will need to restrict the volume of jumps and landings. The surface the athlete competes on and the surface available for the athlete to execute plyometrics on are also important in terms of force absorption. Friction for no-slip change of direction is a prime consideration. Is the surface free of obstructions, traffic and holes? Are additional implements—such as boxes, hand weights, weighted vests, cones, hurdles and medicine balls—appropriate for the skill level of the client? Customize the program based on the client’s needs and the existing environment.
What Are the Categories of Plyometrics?
Over time, plyometric training will increase RFD and decrease ground contact time, as well as increase stopping strength and elastic strength. Couple this with the athlete’s increased technical skill mastery in the sport itself and performance will increase. However, this is a slow process that involves techniques applied in tenths and hundredths of seconds. The effect is truly enhancement of the neural system as it applies to the muscular system, and training must be applied consistently over time with an eye toward quality repetition. Quality rest between sets and sessions is also a factor.
Is plyometrics strength exercises or speed exercises, or a blend of speed-strength and/or strength-speed, and thus more about power development? It depends on the ground contact time of the respective drill. If the ground contact time is under 0.22 second, then the drill is truly a speed drill, by nature of the stimulus involved. If the contact time is between 0.22 and 0.30 second, then the drill is a valid plyometric drill. When the time goes much above 0.30, the stimulus to the nervous system becomes more of a fast strength rep than a “slower” speed repetition. If an athlete is oversized and does not possess much in terms of fast-twitch muscle response, then the principles are still applicable but the times are irrelevant. In terms of neural development, in true neuromuscular plyometric training the drills are high in quality and low in relative quantity and the client needs optimal rest for optimal results.
When training an athlete, the sport itself needs to be analyzed as to the following:
- the need for plyometrics
- the volume of jumps inherent to practice and competition in the sport itself
- the types of jumps needed in the sport, in terms of direction and height
Examples of plyometric categories and drills:
Upper Body. In the power drop, the ball is dropped down to the athlete and then the athlete “punches” the ball up into the partner’s hands. The chest pass is an example of a low-intensity drill (see the sidebar “Basic Drills”). Any type of rebound or clap push-up is also an application of the stretch reflex in order to train the tendon to accept load and produce force in a ballistic manner for greater power. “Pulling” plyometric exercises include tubing pulls for speed and any type of pull in which the athlete makes use of the stretch reflex and pulls against resistance in a forceful manner.
Core. Each vector in the core can and should be trained for a plyometric-type activity in order to ensure that the client can maintain stability, as these ballistic movements occur around the pillar core. The diagonal chop downward, the diagonal lift upward, the scoop action upward, the slam action downward and the twist action across the core are required for stabilization. Add or resist force for various movements in life and sport. It is crucial for any athlete who throws, swings or punches to train the core to translate the forces of the lower body optimally. The 45-degree sit-up is an example of an intermediate exercise for the trunk (see the sidebar “Basic Drills”).
Lower Body. Lower-body plyometrics is generally quantified by the number and type of foot contacts in the movements. Jumps are two-foot landings, hops are single-foot repetitive landings (R, R, R) and bounds are alternate-foot landings (R, L, R). Further quantification occurs with countermovement and noncountermovement jumps. A countermovement jump is one in which the client “cocks” with a preparatory eccentric stretch load of the musculature by lowering the center of mass. A noncountermovement jump is one in which the center of mass is lowered followed by an extended pause at the bottom of the movement in order to dampen the stretch reflex. In essence, the countermovement jump is elastic in nature and the noncountermovement jump is nonelastic and emphasizes starting strength. When a countermovement jump is used without any arm action, the load (due to stretch-reflex dampening) and ground contact time can be increased and strength emphasized. Execute beginning drills in place, and then as the technique is mastered, introduce moving sets. Linear drills are generally easier to master prior to lateral drills. The two-foot ankle hop and jumps in place are examples of beginning moves (see the sidebar “Basic Drills”).
With respect to stimulus response to the training effect, plyometrics is also categorized along the speed-strength continuum of training. Types include pure speed drills, in which the ground contact time is generally equal to that of sprinting; strength drills, in which ground contact time is greater than that of sprinting but the stretch reflex is still utilized; and the extreme assignment of “shock” plyometrics or depth jumps, in which the client steps off a box and then jumps up off the ground either into the air or onto another box.
How Should Progression and Design Be Approached?
Progression is important when teaching plyometrics, and should be intentional. Begin with basic movements (marching and skipping), and progress to lunging and footwork activities, alternate movement and lunging, and finally jumping drills (Kutz 2003). An example of a standard, safe progression of jumping drills would be stand-in-place jumps, standing jumps, multiple hops and jumps, bounding/cone drills and box/depth jumps (Kutz 2003).
The key is landing or force absorption. Prelanding, the client must cock the arms back at the shoulder, engage the core for posture and cock the foot up in dorsiflexion in order to prepare to absorb the forces with the tendons. However, avoid too much dorsiflexion or heel strike. Land mid-foot to reduce impact forces. Typical technical flaws include tipping the head down, which tends to round the upper back and collapse the core; overreaching or overstriding in the moving drills; having poor arm action/rhythm in the upper body; and plantar-flexing the foot instead of “cocking” the foot in dorsiflexion. Program design for volume (number of foot contacts) is pattern or technique dependent. Consider assigning a range of reps and sets so the client begins to get comfortable with the quality of the movement pattern rather than just getting it done. Another key is to assign 6–10 singles for 2–4 sets. Don’t assign 4 sets of 8 reps, because it becomes about getting 32 reps done in a hurry rather than doing one great rep followed by another. The key to performance and training is the quality of the repetition.
In order to add resistance to the program design in advanced stages, use a weighted vest, dumbbells, kettlebells, medicine balls or rubber bands. A weighted vest will add load to the system at the center of mass. A dumbbell or kettlebell is excellent for adding load diagonally for hops, as the implement is held at shoulder height lateral to the shoulder opposite the foot making ground contact. Medicine balls held in front during jumps will increase core load. Finally, rubber bands can be used for skipping, jumping, hopping and bounding drills. The resisting partner must keep a light hand so as not to degrade the technique being executed by the drill partner. In-place drills are easiest, followed by linear drills, lateral drills and multidirectional drills. Shock method drills are the most demanding on the nervous system, and technical proficiency is paramount.
Rest between sets is critical for optimal training stimulus for the neural and muscular systems. If the client is not rested from previous exercise bouts, then the application of plyometrics does not increase neural recruitment of the muscle bundles but rather maintains power production in a state of fatigue. True plyometric training enhances speed, power and RFD. Total optimal
recovery from bouts of plyometric training for speed can take up to 72 hours. For beginners or intermediate performers, recovery time could very well be much less, as they are not performing at high rates of skill and RFD. However, the better the athlete and the greater the training age, the more important recovery
between dosages and days becomes.
How Can Plyometrics
Be Integrated Safely?
Plyometrics is an integral part of our world and can play a key role in rehabilitation as well as performance enhancement training. The keys to proper integration of plyometric exercises are the same as they are with any training tool: proper pattern and technique execution; close monitoring of set and rep volume; proper dosage of plyometrics drills in relation to other training stimuli during the course of the training program; and sequential drill progressions in order to foster increased RFD without sacrificing ground contact time or technique.
Plyometrics can be a powerful aspect of training for a client with superior strength, cardiovascular conditioning, flexibility and general health and fitness. In terms of developing muscular power, this type of training can offer many benefits, especially as part of an integrated and safely designed program.
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Sidebar: Position Statement: Plyometric Exercises
It is the position of the National Strength and Conditioning Association that:
- The stretch-shortening cycle, characterized by a rapid
deceleration of a mass followed almost immediately by rapid acceleration of the mass in the opposite direction is essential in the performance of most competitive sports, particularly those involving running, jumping and rapid changes in direction.
- A plyometric exercise program that trains the muscles, connective tissue and nervous system to effectively carry out the stretch-shortening cycle can improve performance in most competitive sports.
- A plyometric training program for athletes should include sport-specific exercises.
- Carefully applied plyometric exercise programs are no more harmful than other forms of sports training and competition, and may be necessary for safe adaptation
to the rigors of explosive sports.