Voted Best Answer
Feb 14, 2013 - 06:46 AM
Concentric muscle actions occur when tension is generated in the muscle that is greater than gravity or external resistance. The muscle in turn shortens and moves associated joints through a range of motion. For example, the biceps brachii shortens concentrically and flexes the elbow as the bar is lifted upwards during a biceps curl or the latissimus dorsi shortens concentrically as the bar is pulled down during a lat pull-down.
Eccentric muscle actions occur when a muscle lengthens under tension and allows associated joints through a range of motion. Like lowering the weight down during a bicep curl or letting the bar move upwards during a lat pull-down. There is tension in the muscle, but the tension is less than gravity or external resistance so the associated joints are allowed to move.
Isometric muscle actions occur when a muscle activates to match gravity or external resistance. There is no change in muscle length and no change in associated joint position. This could be the pause at the top, middle or bottom of a biceps curl.
So you can ask yourself "is the muscle shortening? (concentric action) Lengthening? (eccentric action) Or staying the same length? (isometric action)
Source: Eric Beard