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Electro-Muscle Stimulation, Electro Myostimulation or EMS for short, is a technique to elicit muscle contraction by delivering electric impulses to the nervous fibers innervating the muscles. The electric impulses are generated by an electric device and delivered through terminals (i.e. electrode pads) to the skin in direct proximity to the muscles to be stimulated.

Because nervous fibers stimulated in this way cause a muscle contraction physiologically identical to what can be done with voluntary exercise, EMS can be used to complement traditional training. The impulses which mimic the action potential coming from the central nervous system trigger a muscle contraction. The electrodes generally are pads that are made to adhere to the skin.

EMS is mostly known and used for medical therapy and its uses are regulated in the USA by the FDA. However, EMS is also used as complementary technique for sport training. Numerous articles in research journals attest increased muscular performance by utilizing EMS; the Journal of Strength and Conditioning Research for instance published the following articles:

Effects of Electrostimulation Training on Muscle Strength and Power of Elite Rugby Players.
Effects of Electromyostimulation Training and Volleyball Practice on Jumping Ability.
Supplemental EMS and Dynamic Weight Training: Effects on Knee Extensor Strength and Vertical Jump of Female College Track & Field Athletes.
The Effects of Combined Electromyostimulation and Dynamic Muscular Contractions on the Strength of College Basketball Players.

History
Luigi Galvani in 1791 provided the first scientific evidence that current can activate muscle. During the 19th and 20th century researchers studied and documented the exact electrical properties that generate muscle movement. It was discovered that the body functions induced by electrical stimulation caused long-term changes in the muscles. Recent medical physiology research pinpointed the mechanisms by which electrical stimulation alters cells of muscles, blood vessels and nerves.

The reference section, at the end of this article lists a selection of titles showing that electro-stimulation research has continued through the centuries up to our days.

Theory
EMS training causes adaptation, i.e. training, of muscle fibers, like any type of muscle exercise. This is quoted from the University of California, San Diego, National Skeletal Muscle Research Center' Muscle Physiology - Electrical Stimulation. Because of the characteristics of skeletal muscle fibers, different types of fibers can be activated to differing degrees by different types of EMS, and the modifications induced depend on the pattern of EMS activity. These patterns, referred to as protocols or programs, will cause a different response from contraction of different fiber types. Some programs will improve fatigue resistance, i.e. endurance, others will increase force production.

A recent college-level coursework book, Application of Muscle/Nerve Stimulation in Health and Disease (G. Vrbova, O.Hudlicka, K.S. Centofanti – Springer, 2008), contains a good review of the science behind EMS, with pictures of muscle cell modifications before and after EMS.

Use
EMS can be used as a training tool by both elite athletes and by people who exercise as a habit or to keep in good shape. It can also be used to treat sport injuries and as a therapeutic tool .

In medicine EMS is used for rehabilitation purposes, for instance in the prevention of disuse muscle atrophy. However, this should not be confused with TENS (Transcutaneous Electrical Nerve Stimulator), which is the use of electric current in pain therapy. TENS devices, because of their purpose, deliver very weak impulses that barely twitch muscle and do not cause the vigorous contractions needed for muscle training.
Because of the effect that strengthened and toned muscles have on appearance (a stronger muscle has larger cross-section) EMS is also used by a niche of practitioners for aesthetics goals.

The FDA regularly reject certification of devices that make these claims. The rationale for this rejection is that many manufacturers make claims of fat burning and weight loss. However, EMS devices cause a calorie burning that is marginal at best: calories are burnt in significant amount only when most of the body is involved in physical exercise: several muscles, the heart and the respiratory system are all engaged. This is not the case with EMS in which training is targeted to few muscular groups at the same time, for specific training goals.

FDA Certification
Only FDA-certified devices can be lawfully sold in the US without medical prescription. These can be found at the corresponding FDA webpage for certified devices.

References

JSCR

Effects of Electrostimulation Training on Muscle Strength and Power of Elite Rugby Players.
Effects of Electromyostimulation Training and Volleyball Practice on Jumping Ability.
Supplemental EMS and Dynamic Weight Training: Effects on Knee Extensor Strength and Vertical Jump of Female College Track & Field Athletes.
The Effects of Combined Electromyostimulation and Dynamic Muscular Contractions on the Strength of College Basketball Players.

UC, San Diego

Muscle Physiology - Electrical Stimulation
Muscle Physiology - Skeletal Muscle Architecture

Skeletal muscle fibers - Wikipedia
S. Salmons and Gerta Vrbová, The influence of activity on some contractile characteristics of mammalian fast and slow muscles;; J Physiol. 1969 May; 201(3): 535–549
D. Pette, G. Vrbová; What does chronic electrical stimulation teach us about muscle plasticity?; Muscle & Nerve, May 1999, vol.22 666-677
P. Banerjee, B. Caulfield, L. Crowe, and A. Clark; Prolonged electrical muscle stimulation exercise improves strength and aerobic capacity in healthy sedentary adults. J. Appl. Physiol. 99:2307–2311 (2005)
J. Porcari, J. Miller, K. Cornwell, C. Foster, M. Gibson, K. McLean and T. Kernozek; The Effects Of neuromuscular Electrical Stimulation Training On Abdominal Strength, Endurance, And Selected Anthropometric Measures; J. of Sport Science and Medicine, 2005, 4, 66-75.
D. A. Lake; Neuromuscular electrical stimulation. An overview and its application in the treatment of sports injuries Sports Med. 13:320–336 (1992).
A. Delitto, S. J. Rose, J. M. McKowen, R. C. Lehman, J. A. Thomas, and R. A. Shively; Electrical stimulation versus voluntary exercise in strengthening thigh musculature after anterior cruciate ligament surgery; Phys. Ther. 68:660–663 (1988).

FDA

Guidance Document for Powered Muscle Stimulator
Certified Electrostimulators.

Selected Bibliography

L. Ranvier, De quelques faits relatifa à l’histologie et à la physiologie des muscles striés ; Arch. Physiol. Norm. Path. 6:1–15 (1874).
D. Denny-Brown, On the nature of postural reflexes, Proc. Roy. Soc. (Biol.) 104:252–301(1929).
A. J. Buller, J. C. Eccles, and R. M. Eccles, Interactions between motoneurones and muscles in respect of the characteristic speeds of their responses, J. Physiol. 150:417–439 (1960).
D. Pette, M. E. Smith, H. W. Staudte, and G. Vrbová, Effects of long-term electrical stimulation on some contractile and metabolic characteristics of fast rabbit muscle, Pflüger’s Arch. 338:257–272 (1973)..
C. G. Blomqvist, and B. Saltin, Cardiovascular adaptations to physical training, Annu. Rev. Physiol. 45:169–189 (1983).
M. Cabric, H. J. Appell, and A. Resic, Stereological analysis of capillaries in electrostimulated human muscles. Int. J. Sports. Med. 8:327–330 (1987).
G. Vrbová, T. Gordon, and R. Jones, Nerve-Muscle Interaction (Chapman & Hall, London, 1995)
B. A. Harris, The influence of endurance and resistance exercise in muscle capillarisation in the elderly: a review. Acta Physiol. Scand. 185:89–97 (2005).

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