Before one can fully understand the mechanisms involved with plyometrics one must first understand the physiology of muscle. Every muscle fiber has a tiny cellular body known as a muscle spindle and every muscle tendon a Golgi tendon organ. These cellular mechanisms are critical in controlling muscle nerve activity. The muscle spindle is usually located near the middle of the muscle fiber and monitors its static and dynamic length. The spindle is responsible for muscle activation to generate force and tension. The classic knee tap test performed by a doctor demonstrates the working of the muscle spindle. The doctor taps the patellar tendon (tendon joining the kneecap to the tibia) and the impact of the mallet transfers tension to the quadriceps (thigh muscles), very slightly but quickly stretching the fibers. The muscle spindles being extremely sensitive to the amount and rate of stretch cause the muscle fibers to contract and shorten in proportion to the magnitude of these two factors. This concentric contraction results in extension of the knee or the familiar kicking motion. This is what is called the stretch (or monosynaptic) reflex and has the effect of increasing the force of a muscle contraction. Once the fibers have shortened and the tension relieved, the activity of the muscle spindle is reduced and the muscle returns to its original resting length. The Golgi Tendon Organ (GTO) essentially performs the opposite function of the muscle spindle and is responsible for muscle contraction inhibition. This small cellular body is located in the muscle-tendon junction and is capable of sensing increasing tension, thus protecting the muscle from generating so much force as to cause injury. Unlike the muscle spindle, the GTO activates inhibitory muscle nerves in direct proportion to the amount of muscle-tendon tension sensed. Therefore, at low tension/load levels the inhibition is low and vice versa for high tension/load levels.

Another piece of information one must also understand is that muscle has the ability perform three types of contractions. Most people falsely assume that the word contraction automatically implies a shortening of muscle however, this is not always the case. A concentric contraction occurs where the muscle actually does shorten when generating tension and usually involves the acceleration of a mass against the force of gravity. This occurs because the force/tension generated in the muscle exceeds the resistive force. An example of a concentric contraction is the upward movement of a bicep curl. A muscle has the ability to perform isometric contractions as well. This type of contraction occurs when the resistive force balances the muscle force and there “appears” to be no movement. An example of multiple isometric muscle contractions is when an individual attempts to push against a very heavy or immovable object such as a brick wall or semi truck. The object resists with a force equal to the push being applied to it. Finally, a muscle can also perform an eccentric contraction, that is the generation of muscle tension to slow down/brake a mass against the force of gravity. Eccentric contractions are the result of the resistive force exceeding the amount of generated force. Thus, during an eccentric contraction, the muscle may actually lengthen and stretch even though tension is being generated, in other words the muscle is attempting to shorten but cannot since the force generated is less than the resistance. An example of an eccentric contraction is the slow downward movement of a bicep curl. The stretching of the muscle is possible due to its own inherent elastic qualities and especially that of its tendon. Microscopically, in its relaxed/non stretched state muscle tendon structure appears wavy and crimped and also contains a special stretchable protein known as elastin. Therefore, when one combines the qualities of muscle with those of tendon tissue, it produces a fair degree of muscle elasticity. (Interestingly, a muscle will perform all three types of contractions at different times during an exercise involving the lifting and lowering of a weight).

There are two models used to explain the mechanism behind plyometric training. The first model is known as the mechanical model. This model holds that during an eccentric contraction, a stretched muscle is able to store elastic energy, and is able to recoil and increase the magnitude of a subsequent concentric contraction. An example of this is to consider what happens when stretched a rubber band is suddenly released and flies through the air – the potential (stored) energy is converted to kinetic (movement) energy. Another model that is used to help explain the increased power gained from plyometric training, is the neurophysiologic model. This model holds that if following an eccentric contraction, a concentric contraction of the same muscle is performed; the force of the concentric muscular contraction will be increased, thus manipulating the previously mentioned stretch reflex.

The Stretch Shortening Cycle (SSC) employs both models to explain how plyometrics help recruit maximal muscle force in minimal time. The SSC is broken into three phases: Phase I - eccentric contraction, Phase II - amortization or transition stage and Phase III – concentric contraction. The SSC occurs automatically during most functional (closed chain) movements such as walking, running or jumping since a concentric contraction is nearly always preceded by an eccentric contraction. A good example is to consider at the calf muscles (which is often the target muscle of plyometric training anyway) during a jump or hop. As one descends quickly into a semi-squat, the calf muscles have to slow down ankle dorsiflexion (extension) and therefore eccentrically contract, stretching (storing elastic energy) and sensitizing the calf muscle spindle. As the knees and hips begin to extend (straighten), the ankle moves into flexion and when the tibia/shin is near vertical, the muscle is at its resting length. This is the amortization or transition phase between the concentric and eccentric contractions and it is crucial that this stage be kept as short as possible, otherwise the elastic energy will be converted to heat and the sensitization of the muscle spindle lost. As the ankle joint moves further into flexion (plantar flexion), the concentric contraction of the calf muscles are increased by both the elastic recoil and the stretch reflex generating more power and a higher jump height. Therefore, plyometric training should be performed quickly and in a ballistic manner to fully take advantage of the stretch reflex and elastic energy. Exercises for the lower body involve jumps (e.g. jumps in place, countermovement jumps, drop jumps) and hopping and bounding drills. Upper body plyometric training includes clap push-ups and medicine ball work.

Plyometrics has many applications in the exercise and conditioning. Since it involves training the muscles to generate maximal work in the shortest time, it has a high carry over to sporting performance in particular. This form of functional training is highly beneficial in improving the vertical jump of a basketball players and increasing the agility and quickness of any athlete participating in a sport requiring sudden starts and direction changes. Agility in this sense refers to the ability of an athlete in both accelerating and decelerating his or her body against external forces such as gravity, friction and opponent resistance. Technically, plyometric training may also have a strong applicability to sports not traditionally considered power sports such as long distance running. When one considers the thousands of concentric and eccentric contractions occurring in the legs of a marathoner, any conserved or stored energy within muscles and tendons that have been plyometrically trained to generate more power will most definitely increase running economy. Running economy is defined as the metabolic cost of running and may be as Dr Tim Noakes says one of the primary factors affecting running performance.

It is important that an individual have the necessary strength and technique instruction to participate in high intensity training like plyometrics. This form of training as stated earlier is a form of power training and should not be attempted by those individuals who have not experienced the physiological adaptations that occur with strength training such as increased tendon and bone strength. Some sources suggest that an athlete be able to squat twice his body weight before participating in lower body plyometric training. Unfortunately this requirement would eliminate a large majority of the number of individuals looking to increase their power and agility. Since it is true that the precursor to power is strength, the NSCA (National Strength and Conditioning Association) recommends that an athlete’s 1 RM (repetition max- the maximum amount of weight that can be lifted with good technique for one repetition) squat be at least 1.5 times the body weight. For upper body plyometric training, the NSCA suggests that the 1RM bench press be at least 1.0 times the body weight for heavy individuals (> 220 lbs) and at least 1.5 times the body weight for lighter athletes (< 220 lbs). Because plyometric training is performed quickly and ballistically, the NSCA recommends that individuals be able to squat 5 repetitions using 60% of their body weight within 5 seconds for lower body training and perform using the same requirements in the bench press for upper body plyometric training.

In conclusion it has been demonstrated in this article that plyometrics is a highly effective form of power training that effectively manipulates the physiology of the Stretch Shortening Cycle to the performers advantage. In addition, since plyometrics uses functional closed chain movements in a ballistic manner, the benefits of being able to generate high work rates very quickly, transfer directly to increased athlete power. Depending on the specificity of plyometric training this power can in turn, transfer to increased speed, agility and even vertical jump height. With experienced instruction and pre-conditioning this form of training is a safe method of increasing performance in almost any sport.

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Copyright 2005 David Petersen BS, CSCS & B.O.S.S. Fitness