Eccentric Action (Can Someone Help Explain This?)

[Keith S.]

Ok, so I finally got my NSCA-CPT materials and I've started reading and taking notes but eccentric action has got me pretty damn confused. The book states:

An eccentric action occurs when a muscle cannot develop sufficient tension and is overcome by an external load, and thus progessively lengthens. Eccentric actions are commonly involved in the deceleration of joint motions. For example, walking down stairs involves eccentric action of the quadriceps muscle group as it decelerates the flexion action of the knee.

I know this is probably easy to understand, but I'm just not getting it. Is it the action of your muscle to let yourself down slowly from stair to stair? (sticking to the example given)

[Simon.]

The eccentric action is from the point you place your straight foot on the lower step to the point at which your knee is as flexed as it gets when going down the stairs and the other leg is straight on the next step below...
When you place your leg straight on a lower step and allow it to bend your quad is 'giving in' to the load and lengthens thus by the definition you quoted preforming an eccentric action.