Eccentric (n)  ec·cen·tric is a term used to describe a type of movement or exercise.

Eccentric Definition:

The word concentric is most commonly used in sports science and medicine. In these settings eccentric defines a type of exercise or movement.

Translated literally eccentric means away from the centre. 

Whenever a muscle lengthens it generates a contractile force meaning all movements in the body occur with some muscular activity (this is absolute). The term lengthening is actually misleading as in most instances the muscle does not actually lengthen. In reality it returns from its shortened condition to its normal resting length. In most instances in which muscles contract eccentrically they actually act as a brake or resistive force against the moving force of gravity or other external force (like a weight). This work is often referred to as negative and can still be seen on many isokinetics machines as a -ve value rather than positive. In truth most eccentric contractions are more energetic and hence more positive than concnetric contractions. Eccentric actions produce greater loading of the elastic musculoskeletal components and are used during many dynamic movements (like walking down stairs or hitting a ball). As the majority of muscle tears are thought to occur during eccentric motions improvements in this performance may be beneficial for injury prevention (Bennett and Stauber 1986). However, eccentric motions produced by active dynamometers are not considered to be like those seen during functional activities (they are described by Chan and Maffulli 1996 as 'unnatural'). Short and rapid eccentric motions are normally produced during daily and sporting activities, isokinetically this is not seen as the movements are usually long and through full range of motion.

Eccentric actions are responsible for deep onset muscle soreness (the soreness after going to the gym or training).

In eccentric contractions the muscle lengthens using the sliding filament theory (shown below). Here the thick myosin filaments attach onto the thin actin filaments and are pulled apart. This has the effect of the Z disks moving further away which lengthens the muscle and will most often change the angle of a joint (although this not always true think of the eye muscles here there are no joints).

A good example is a eccentric contraction of the quadriceps muscles in the leg whilst walking down stairs. Here the eccentric contraction allows the knee to bend under control which allows a safe decent of the stairs. We can control the speed of the lengthening by using more or less muscular units.

Eccentric contractions tend to be the strongets contractions we can produce with isotonic eccentric contractions normally weaker (due to lever dis-advatage) than isokinetic eccentric contractions.