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Eccentric Contraction
A type of muscle contraction in which the resistance (the weight it is carrying) is greater than the force applied by the muscle so that the muscle lengthens as it contracts. Eccentric contractions also occur when the muscular force is used to brake or slow down the opening of a joint.
Muscle contraction
An eccentric muscular contraction is a contraction of a muscle through a lengthening of the fibers. Most individuals are familiar with concentric contractions, where the muscle shortens. In terms of a bicep curl, during a complete concentric contraction the elbow goes from fully straight (hand at mid-thigh) to completely bent, hand at the shoulder, with the elbow at approximately a 30° angle. During this type of contraction the muscle shortens, with actin and myosin filaments sliding across each other to pull the joint closed. With an eccentric contraction, the muscle moves in the opposite direction, with the actin and myosin filaments lengthening as the joint opens, the hand moving from shoulder to thigh and the elbow straightening. In essence, rather than the muscle acting as an active force to move a weight, instead the muscle works to 'brake' the motion, slowing down the opening of the joint. Eccentric contractions are usually used to slow down or control the lowering of a load.
Because the contraction works in the opposite direction the muscles are generally supposed to move (i.e. muscles can only shorten, never lengthen), muscles undergoing heavy eccentric loading suffer greater damage when overloaded (such as during muscle building or strength training exercise) as compared to concentric loading. Normally muscle fibers move across each other akin to rowers in a boat - arms along the filaments pull in a single direction simultaneously (i.e. when the oars would be in the water). During an eccentric contraction, the 'oars' try to pull (controlling the movement), but instead the water reacts to pull back, 'breaking' some of the oars. This also occurs during regular contractions, but as a result of more of the individual 'oars' being pulled in the wrong direction, more are broken. As a result, exercise featuring a heavy eccentric load can actually support a greater weight (muscles are approximately 10% stronger during eccentric contractions than during concentric contractions) and also results in greater muscular damage and delayed-onset muscle soreness one to two days after training. Exercise that incorporates both eccentric and concentric muscular contractions (i.e. involving a strong contraction and a controlled lowering of the weight) can produce greater gains in strength and toughness rather than concentric contractions alone.
References
Brooks, G.A, Fahey, T.D. & White, T.P. (1996). Exercise Physiology: Human Bioenergetics and Its Applications. (2nd ed.). Mountain View, California: Mayfield Publishing Co.
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