Now consider extending your elbow joint from 90 degrees whilst holding a heavy object. The muscle which had shortened to produce the flexion to raise the arm to 90 degrees undergoes a controlled lengthening as the forearm is returned to its original position. What happens on muscle shortening is that the muscle generates enough force for its fibres to be able to shorten to lift your arm and the heavy object against the force of gravity.

During isometric action the muscle generates enough force to keep its fibres short and to prevent gravity from pulling the limb down. When you wish to lower part of your body slowly the already shortened muscle fibres generate force which is not quite enough to produce further shortening or to maintain a static position but which is enough to produce a slowing down effect. This is called eccentric muscle work.