Plyometrics are a commonly used means in many physical preparation programs. The true value in plyometrics and rationale for their use is in the name; the term is not, contrary to common practice, synonymous with jump training exercises. In 1978, plyometrics were defined by Fred Wilt as:
“Training drills designed to bring the gap between sheer strength and the power (rate of work or force x velocity) required in producing the explosive reactive movements so necessary to excellence in jumping, throwing and sprinting. ”
He continued with:
“To the best of my knowledge, there has been no previous reference made to plyometric exercises in American sports literature. This word has been used for a number of years by European coaches, especially those from Germany and Russia. The word plyometric is apparently derived from the Greek word plethyein, which means to increase and isometric. My present interpretation of plyometric is that it means the exercises or training drills used in producing an overload of isometric type muscle action which invokes the stretch reflex in muscles. I am not particularly happy with this interpretation, and it may alter when a precise definition evolves.”
The Greek word “plethyein” has a different meaning: “be or become full”. This Greek root is used, for example, in the English words “plethora” and “plenty”. The meaning of “Increase” or greater in size, extent, has another Greek root: “plio-“. In fact, the Russian, German, and other European coaches used and continue to use another term for Plyomentrics: “pliometric exercises”. To understand why they began to use this term, we need to take a quick trip to the early 20th century.
The term “pliometric” was introduced in 1938 by Hubbard and Stetson who “recognized that muscles underwent contractions during three different “conditions”: when they are shortening, are keeping the same length, or are lengthening. The three conditions were termed “miometric,” “isometric,” and “pliometric,” by coupling the Greek prefixes “mio” (shorter), “iso” (same), and “plio” (longer) to the noun “metric,” defined as “pertaining to measures or measurement, to differentiate among the three conditions under which the muscles ‘contracted’.”
The pliometric muscular action occurs when the external load actively extends (stretch) the muscles while the contraction is in progress. In Russian sport training literature it is usually termed as the yielding regime of muscular work. From a biomechanics point of view, during such “lengthening contraction”, the muscles do not produce any external positive work (i.e., mechanical work, equal to force generated x distance moved); all of the energy has been used to exert tension on the load.
The feature of such “negative” work was outlined in 1892 by A. Fick , who demonstrated that a muscle can exert greater force when stretched by an external force while contracting; the heat produced by actively stretched muscle was less than that measured during the active shortening. Using the terms introduced in 1938 by Hubbard and Stetson, it was shown, that the pliometric and miometric muscular contractions (with the same velocity) produce different forces and consume different amounts of energy. Pliometric muscular actions:
1) Produce a greater force
2) Consume less energy than miometric muscular actions
This phenomenon was confirmed and extended in 1923 by W. Fenn in his study on the quantitative relation between the heat production of muscles and the work that they perform . Fenn showed that “The work done in stretching the muscle does not therefore add itself to the ‘physiological’ heat but . . . replaced energy which would have been liberated by the muscle if it had not been stretched.”
Fenn’s study was performed in the laboratory of Archibald V. Hill, who termed the results as “Fenn’s Effect”. He summarized this effect in the following words: “ …Shortening during contraction, lengthening during relaxation, appears to require excess liberation of energy. Lengthening during contraction, shortening during relaxation, appear to cause an excess “absorption” of energy, i.e. to lead to a total energy liberation less than that of the isometric twitch…. If it be held fast and allowed to shorten only during relaxation, then again it will give out less heat. ” .
Numerous studies were carried out to discover the basic mechanisms of this phenomenon. In 1938 Hill theorized that Fenn’s effect could be related to a decrease in the rate of chemical transformation in the muscle. However, in 1950, Hill had also hypothesized that the mechanical energy produced by an external force, which causes a contracting muscle to stretch, could be stored in the series elastic components of muscle and reutilized in the subsequent shortening phase of movement: “An important factor of mechanical behavior of muscle is the passive elastic component in series with the active contractile one…. This acts as a buffer when a muscle passes abruptly from the resting to the active state, and it accumulates mechanical energy as the tension of the muscle rises. If a muscle is opposed, as in most ordinary movements, by the inertia of a limb or an external mass, this mechanical energy can be used in producing a final velocity greater than that at which the contractile component itself can shorten. This is important in such movements as jumping or throwing.”
In 1968, the research of G. Cavana, B. Dusman, and R. Margaria showed, in isolated frog muscle and in the muscle of working humans, that the work done by muscle shortening at a given velocity was greater if the shortening was preceded by a stretch during stimulation. This effect, they concluded, was partly due to an increase in the force of contraction of the contractile component. The force developed by contractile component, when the muscle shortens after being stretched, is greater than the force developed when the muscle shortens, at the same speed and length, but starting from a state of isometric contraction.
During the following two decades it was established that pliometric contraction:
− can maximize the force exerted and the work performed by muscle
− is associated with a greater mechanical efficiency
− can attenuate the mechanical effects of impact forces.
So, it was natural to suppose that the pliometric muscular action during the landing and take-off phases typical of jumping exercises stretches the activated muscles during the downward movement after touchdown, causing an increase in the force produced in the following take-off movement. In other words, the performance of the jump is enhanced.
This was likely the reason why high power jumps that involve repeated, rapid, and forceful shortening and lengthening actions during almost maximum activation of large muscle groups (as does similar forms of throwing) where the pliometric regime is emphasized, were termed “pliometric” exercises. This term was then converted to “plyometric” probably because both terms are pronounced similarly, though no one saw the written word. We know this because in 1953 E. Asmussen introduced another term for pliometric muscular actions: “eccentric”, which is also defined as whimsical as well as excentric , meaning to move away from the center of the muscle. In 1959, in Karpovich’s textbook “Physiology of Muscular Activity” , miometric actions were named “concentric” and pliometric were named “eccentric”.
“Presently, lengthening, miometric and pliometric, and concentric and eccentric are all in use in the physiological, biomechanics, sports medicine, and sports science literature. Despite their inappropriateness, the most commonly used expressions in the conditioning and sports exercise papers are concentric and eccentric contractions (Knuttgen HG and Kraemer WJ. Terminology and measurement in exercise performance. J Appl Sport Sci Res 1: 1–10,1987.)”
In the sport training literature the term “pliometric” became obsolete and was gradually replaced by the term “eccentric”, also thanks to the growth of popularity in “eccentric training”, which consists in using only the lowering phase of resistance exercise.
The term “pliometric” gradually lost its primary meaning and continued be used in Europe synonymously with plyometric. By the end of the 1980s, the new term came to be considered more appropriated for plyometrics: exercises that emphasize the Stretch-Shortening Cycle (SSC). However, the term Plyometrics is still more popular between the coaches, athletes and sport scientists. According to M. Siff, Plyometrics “consist of stimulating the muscles by means of a sudden stretch preceding any voluntary effort”.
Now, we know that under a sudden stretch, in this definition, the pliometric muscular action is implied, which increases the power output of the subsequent movement. We may thus apply a more suitable interpretation of the term Plyometrics: exercises in which the pliometric muscular action is applied as a means of intensifying the muscular activity. Essentially, in simplest terms, Plyometrics means “to apply pliometric” Fred Wilt. “Plyometrics – What is it and how it works”, Modern athlete and coach, 1978, n.16, pp.9-12.  Hubbard A.W. and Stetson R,H. An experimental analysis of human locomotion. J Physiol 124: 300–313, 1938.  Faulkner, John A. Terminology for contractions of muscles during shortening, while isometric, and during lengthening. J Appl Physiol, 95: 455–459, 2003  Fick A. Neue Beiträge zur Kenntniss von der Wärme-Entwicklung im Muskel. Pflügers Arch 51: 541–569, 1892.  Wallace Osgood Fenn, A Quantitative Comparison between the Energy Liberated and the Work Performed by the Isolated Sartorius Muscle of the Frog, Journal of Physiology, 58(1924): 175.  Fenn WO. The relationship between the work performed and the energy liberated in muscular contraction. J Physiol 58: 373–395, 1924.  Archibald V. Hill. The Mechanism of Muscular Contraction. Nobel Lecture, December 12, 1923.  Hill AV. Heat of shortening and the dynamic constants of muscle. Proc R Soc Lond B Biol Sci 126: 136–195, 1938.  Hill, A.V. (1950) The series elastic component of muscle. Proceedings of the Royal Society London Series B 137, 273–280.  G.Cavana, B.Dusman, R.Margaria. Positive work done by a previously stretched muscle . J Appl Physiol January 1, 1968.  “In 1962, during a discussion on muscle performance chaired by D. B. Dill (Rodahl K, Horvath SM, and Risch MPS. Muscle as a Tissue. New York: McGraw-Hill, 1962.) , Erling Asmussen used the terms concentric and eccentric and B. J. Ralston made the perceptive comment that these terms led to confusion and should be eliminated from the literature. Asmussen conceded that the terms miometric and pliometric might be better..” (Faulkner, John A. Terminology for contractions of muscles during shortening, while isometric, and during lengthening. J Appl Physiol, 95: 455–459, 2003). Karpovich PV. Physiology of Muscular Activity. Philadelphia,PA: Saunders, 1959.  Faulkner, John A. Terminology for contractions of muscles during shortening, while isometric, and during lengthening. J Appl Physiol, 95: 455–459, 2003.  Komi PV. Physiological and biomechanical correlates of muscle function: effects of muscle structure and stretch-shortening cycle on force and speed. In: Exercise and Sport Science Reviews, edited by Terjung RL. Lexington, MA: Collamore, 1984, p. 81–121.
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