Mar 22, 2012 - 10:33 AM
When a muscle contracts eccentrically, it is exerting less force than is being placed on it, resulting in a lengthening of the muscle. “Less” and “lengthening” both have an “l,” which may help you to remember those terms are associated with eccentric movements. Eccentric actions move in the same direction as the resistance, decelerating or reducing force. An example would be lowering dumbbells during a bicep curl; the muscles are working to decelerate the force and thus exerting less force than is being placed on it.
When a muscle contracts isometrically, it exerts force equal to that placed on it, resulting in no change in muscle length. During exercises, isometric actions are used to stabilize the body, with no visible movement with or against resistance. For example, at the top of a bicep curl (after the concentric movement), there is an isometric contraction of the muscle while the body stabilizes itself before lowering the weight (during the eccentric movement).
When a muscle contracts concentrically, it exerts more force than is placed on it, resulting in a shortening of the muscle. This movement is the one most commonly associated with strength training because the muscle accelerates or produces force to lift the weight (as in the lifting portion of a bicep curl).
Source: Lauren McHugh