What is their Origin?
In 1960's, Yuri Verkhoshansky, a Soviet coach, is credited with creating the principle which at the time was known as "shock training." While observing some Olympians, he realized that they had more strength and power coming out of a higher altitude landing when their muscles were stretched as opposed to a normal jump. He observed that jumpers with the shortest amount of ground contact time (amortization phase) displayed the greatest jumping performance. This led him to reason that maximal jumping performance requires muscles to be strong eccentrically so that they are able to withstand the high mechanical loading during the amortization phase. He believed that if the muscles are strong eccentrically, they will be able to quickly switch from overcoming the eccentric loading to immediately contracting concentrically to accelerate the body in the required direction. Again, this allows the athlete to take advantage of the tension created in the muscle during the eccentric stretch. Thus, improvements can be made in jumping performance by increasing the amount of tension the athlete can generate during the eccentric contraction and by improving the reactive ability of muscles in switching from eccentric to concentric work. Verkhoshansky’s findings are the basis of how plyometric training can exploit the stretch-shortening cycle to produce athletic improvement.
Benefits of Plyometric Exercise
According to the American Council on Exercise (1), research studies have shown that plyometric training can lead to huge improvements in:
- Vertical jump performance
- Leg strength
- Muscle power & explosiveness
- Acceleration
- Coordination
- Balance
- Overall agility
- Bone density (especially in younger participants)
- Decrease in fat mass
Plyometric exercises are great for challenging your fast-twitch muscle fibers, coordination and agility. All these things work together to help transform fat into lean muscle while elevating your heart rate and igniting a caloric burn.
The Stretch-Shortening Cycle
If a muscle is stretched, much of the energy required to stretch it is lost as heat, but some of this energy can be stored by the elastic components of the muscle. This stored energy is available to the muscle only during a subsequent contraction. It is important to realize that this energy boost is lost if the eccentric contraction is not followed immediately by a concentric contraction. To express this greater force the muscle must contract within the shortest time possible. This whole process is frequently called the Stretch-Shortening Cycle (SSC) and is the underlying mechanism of plyometric training.
The Three Phases of the Stretch-Shortening Cycle
Eccentric Phase (stretching)
The eccentric phase, involves the preloading of the agonist muscle group. During this phase, elastic energy is stored and muscle spindles are stimulated. An example of this is the portion of the drop jump from when the feet come into contact with the ground to the bottom of the movement (landing) (2).
Amortization Phase
The amortization phase, or transition phase, is the time between the concentric and eccentric phases. This phase of the SSC is perhaps the most crucial in production of power as the duration of amortization must be kept at a minimum. If the transition phase lasts too long, the energy stored during the eccentric phase dissipates, thereby negating the plyometric effect (2).
Concentric Phase (shortening)
The concentric phase is the response to the eccentric and amortization phases. During this phase, elastic energy is utilized to increase the force of the subsequent movement or is dissipated as heat. The force is increased beyond that of an isolated concentric muscular action (2).
In depth view of the mechanism behind the Stretch-Shortening Cycle:
Hill's muscle model:
The three-element Hill muscle model is a representation of the muscle mechanical response. The model is constituted by a contractile element (CE) and two on-linear spring elements, one in series (Series Elastic Component) (SEC) and another in parallel (Parallel Elastic Component) (PEC). The active force of the CE comes from the force generated by the actin and myosin cross-bridges at the sarcomere level. It is fully extensible when inactive but capable of shortening when activated (3)(4).
Parallel Elastic Component
The PEC represents the passive force of these connective tissues and has a soft tissue mechanical behavior. It consists of elastic elements (extra-cellular matrix, sarcolemma, fascia, epimysium, perimysium, endomysium), which are in parallel to the CE (5). It is responsible for the muscle passive behavior when it is stretched, even when the CE is not activated. It is the component of a muscle that provides resistive tension when a muscle is passively stretched. It is non-contractile and consists of the muscle membranes, which lie parallel to the muscle fibers. Along with the SEC, this component enables muscle to stretch and recoil in a time-dependent fashion. (3)(4).
Series Elastic Component
The series elastic component of a muscle is part of the mechanical model of plyometric exercise. The SEC represents all of the elastic elements in series with the CE, and this takes into consideration the elasticity of the contractile apparatus itself, tendon fibrils, and tendons themselves. It also has a soft tissue response and provides energy storing mechanism. The tendons attached to the muscle constitute the majority of the SEC. When the musculotendinous unit is stretched, as in eccentric muscle action, the SEC acts as a spring and is lengthened; as it lengthens, elastic energy is stored. If the muscle begins a concentric action immediately after the eccentric action, the stored energy is released, allowing the SEC to contribute to total force production by naturally returning the muscle and tendons to their preset configuration (5).
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| The three-element Hill muscle model as depicted by ACSM (5) |
The Neurological Model behind Plyometric Force
As said before, the key to utilizing stored elastic energy involved in the SSC is to minimize the conversion time between the eccentric and concentric phase of the movement. The attachment time between myosin and actin strands is very brief, usually 15 to 120 milliseconds (6). A long delay between the stretching and contracting phase of the movement results in increasing detachment of the myofilaments, which negates the potential to utilize the elastic energy stored in the muscle.
The elastic energy is recoiled (similar to a recoiled spring) and used to increase the efficiency of the concentric phase of the movement. The level of stored energy is proportional to the applied force and the speed of the stretch. The magnitude of the stretch is a function of muscle and tendon stiffness. The stiffness of a muscle is variable and depends on the forces applied, while tendon stiffness is constant. The higher the
tension in a muscle the harder it is to stretch. Studies show that elite athletes experience a higher level of stiffness in their muscles than in their tendons; thus, elastic energy in elite athletes is primarily stored in the tendons.
The elastic energy is recoiled (similar to a recoiled spring) and used to increase the efficiency of the concentric phase of the movement. The level of stored energy is proportional to the applied force and the speed of the stretch. The magnitude of the stretch is a function of muscle and tendon stiffness. The stiffness of a muscle is variable and depends on the forces applied, while tendon stiffness is constant. The higher the
tension in a muscle the harder it is to stretch. Studies show that elite athletes experience a higher level of stiffness in their muscles than in their tendons; thus, elastic energy in elite athletes is primarily stored in the tendons.
Myotatic reflex
The neural mechanisms most prominent in the SSC are the myotatic reflex (stretch reflex), and the Golgi tendon organ. The myotatic reflex receptors (muscle spindles) are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. The primary role of the muscle spindles is to set the muscle to a preset length. When the muscles are stretched the muscle spindles are also stretched. This causes muscle spindle discharge which results in alpha motoneuron activation. The reflex-evoked activity in the alpha motoneurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch.
Golgi tendon organ
The Golgi tendon organ, a proprioceptor, is located where muscle fibers of skeletal muscle meet tendons. Made up of strands of collagen, the organ also contains nerve tissue. The major function of the Golgi tendon organ is to sense muscle tension when a muscle is contracted, sending signals to the brain about how much force is being exerted and where. Unlike muscle spindles (which are located in parallel with muscle fibers), the Golgi tendon organs are in series with muscle fibers. The sensory dendrites of the Golgi tendon organ afferent are interwoven with collagen fibrils in the tendon. When the muscle contracts, the collagen fibrils are pulled tight, and this activates the Golgi tendon organ afferent. Because of their location in the tendon, these proprioceptors are well positioned to monitor tension developed by the whole muscle and not just individual fibers. The sensory neuron of each Golgi tendon organ travels to the spinal cord where it synapses with the alpha motor neurons of both the agonist and antagonist muscles. As an activated muscle develops force, the tension within the muscle’s tendon increases and is monitored by the Golgi tendon organs. If the tension becomes great enough to damage the muscle or tendon, the Golgi tendon organ inhibits the activated muscle. The tension within the muscle is alleviated so that damage to the muscle and/or tendon can be avoided (7).
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| The Golgi tendon organ |
One of the primary training goals for enhancing the SSC is to maximize the positive effects of the myotatic reflex while minimizing the negative effects of the Golgi tendon organ (8). This type of training could involve accelerative movements with or without weights. Accelerative training in this case would refer to very rapid stretching followed by accelerative contraction. Training with heavy weights at slow speeds can also be used to inhibit the Golgi tendon organ.
Are Plyometrics Safe?
Aside from professional and amateur athletes, everyone else can benefit. Small children perform plyometrics unknowingly on the playground every day. In turn they are more flexible, healthier and active. Senior citizens can greatly benefit by doing a slower, less intense form of the workout but still getting the good flexible and energetic effects.
Because plyometric training strengthens muscles and decreases impact forces on the joints, it may reduce the risk of injury in some people, especially in younger female basketball and soccer players who have a risk of anterior cruciate ligament (ACL) injury that's two to eight times higher than that of their male counterparts. ACL injury-prevention programs (such as plyometrics) are designed to enhance the proper nerve/muscle control of the knee, according to the American Orthopedic Society for Sports Medicine (9).
Some authors suggest that moderate jumps (low intensity) can be included in the athletic training of very young children. However, great care needs to be exerted when prescribing any training procedures for preadolescent children. Because of the relatively immature bone structure in preadolescent and adolescent children, the very great forces exerted during intensive drop jumps (high intensity) should be avoided.
Additionally, it is my opinion that if a correct periodization program is followed, a participant may begin plyometrics even in an untrained level, by executing plyometrics in low intensity and high volume. Less intensive plyometric exercises can be incorporated into general circuit and weight training during the early stages of training to progressively condition the athlete. Simple plyometric drills such as skipping, hopping and bounding should be introduced first. More demanding exercises such as flying start single-leg hops and drop jumps should be limited to thoroughly conditioned athletes. As time progresses there is a gradual shift from volume to intensity. This approach could impose an even faster adaptation, combined with numerous benefits, such as improvements in aerobic system and endurance.
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Combination of Plyometrics with resistance
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Safety Considerations for Plyometrics
Inappropriate use of plyometric training has been associated with various forms of "over-use" injuries, especially in the lower extremities (e.g. patellar and Achilles tendinitis and plantar fasciitis). This type of training, especially when done at a high intensity, is a high-risk endeavor (i.e. high returns but at high risk). Like any other high-risk maneuver, high intensity plyometrics should not designed or performed without the supervision of a professional overseeing the training, and response, to the exercise protocol. The forces sustained from these types of jumps onto hard surfaces can be as much as seven times one’s own body weight (1).
This intense nature of plyometrics created a necessity for safety precautions for the athletes. In literature there have been established some safety considerations that must be fulfilled in order to take part in plyometric training. Those considerations for lower-extremities concern Maximum repetition tests, leg power tests and balance tests.
This intense nature of plyometrics created a necessity for safety precautions for the athletes. In literature there have been established some safety considerations that must be fulfilled in order to take part in plyometric training. Those considerations for lower-extremities concern Maximum repetition tests, leg power tests and balance tests.
Maximum repetition
For the lower-extremity strength evaluation the athletes must undergo a maximum resistance test in squat. According to Wathen, an adequate percentage of performance is for the athlete to have 1 RM that equals 1,5 of his bodyweight for athletes under 100 kilos and at least 100% of the bodyweight if the athlete is over 100 kilos (10).
Power
The explosive nature of plyometrics creates the need for the ability to move rapidly. For the lower-extremities an athlete must be able to execute at least 5 repetitions in squats with 60% of his body weight as resistance, in 5 or less seconds (11).
Balance
Some exercises of plyometrics consist of frequent direction changes as well as changes of the movement pattern. According to «Plyometric Static Stability Testing» (12)(13), for an athlete to be able to participate in plyometrics of high intensity, he must be able to stand in one leg for at least 30 seconds in straight standing position, ¼ of squat and squat position. All the previous balance tests are executed initially with open and later with closed eyes. The ground in which the balance tests are executed must be the same as the one that will be used in plyometric training.
For upper body plyometrics the safety guidelines are exactly the same, with the difference that the athletes my perform the strength and speed tests in bench press rather than squat.
Drop Jump Guidelines
Some recommendations for the correct technique of the drop jump are:
- Minimize ground contact time (imagine the ground is a hot surface)
- Keep the legs stiff on landing
- Minimize the flexion at the knee and hip on landing
- Land on the mid-foot under the hips
- Maximize the height of jump (jump as high as possible after the drop
A video of NBA player Dwyane Wade performing drop jumps
The two key factors in drop jumping are a minimal contact time with the ground and the height achieved in the drive upwards. Schmidtbleicher says that a ground contact time of <0.25 seconds indicates a fast SSC and to adjust the drop height to achieve this contact time (14). If contact mats are not available to measure the contact time then observe the athlete's feet. On landing the athlete should stay on the balls of the feet (mid-foot). If the heels come into contact with the ground then the drop height needs to be reduced. Start at a drop height of 30cm and increment the drop height in 10cm steps.
References
1. American Council On Exercise [Internet]. Plyometrics: Controlled Impact/Maximum Power [cited 2013 Mar 19]. Available from: http://www.acefitness.org/acefit/fitness-fact-article/73/plyometrics-controlled-impact-maximum-power/
2. National Strength and Conditioning Association. Essentials of Strength Training and Conditioning - 3rd Edition. 3rd ed. Human Kinetics; 2008.
3. Martins JAC, Pires EB, Salvado R, Dinis PB. A numerical model of passive and active behavior of skeletal muscles. Computer Methods in Applied Mechanics and Engineering. 1998 Jan 20;151(3–4):419–33.
4. Fung YC. Biomechanics: Mechanical Properties of Living Tissues. Springer; 1993.
5. Medicine none] AC of S. ACSM’s Advanced Exercise Physiology. Second. FACSM PAFP, FACSM MJJM, FACSM VJCP, editors. Lippincott Williams & Wilkins; 2011.
6. Huxley AF, Simmons RM. Mechanical properties of the cross-bridges of frog striated muscle. J. Physiol. (Lond.). 1971 Oct;218 Suppl:59P–60P.
7. Kraemer WJ, Fleck SJ, Deschenes MR. Exercise Physiology: Integrating Theory and Application. Lippincott Williams & Wilkins; 2011.
8. Issurin VB. Vibrations and their applications in sport. A review J Sports Med Phys Fitness. 2005;45:324–36.
9. American Orthopaedic Society for Sports Medicine. Anterior Cruciate Ligament (ACL) Injury Prevention [Internet]. [cited 2013 Mar 19]. Available from: http://www.sportsmed.org/uploadedFiles/Content/Patient/Sports_Tips/ST%20ACL%20Injury%2008.pdf
10. Wathen D. Literature review: explosive/plyometric exercises. Training. 1993;25(2):122.
11. Chu DA. Jumping into plyometrics. Human Kinetics Publishers; 1998.
12. Voight ML, Draovitch P, Tippett S. Plyometrics. Eccentric Muscle Training in Sports and Orthopedics. New York, NY: Churchill Livingstone. 1991;45.
13. Voight M, Tippett S. Rehabilitation Techniques in Sports Medicine. 2nd ed. St Louis: Mosby; 1994. p. 88–97.
14. Schmidtbleicher D. Training for power event. Strength and power in sport. Komi PV. London: Blackwell Scientific; 1992. p. 381–95.
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