Answer
In an concentric isotonic contraction, the muscle shortens as it generates force. In an isometric contraction, the muscle generates force, but it does not change length.
We do very many isotonic contractions each day involving walking, stair climbing, lifting things etc. When one tries to lift a heavy object with a fore limb, if the object is raised to a certain level, strain (discomfort) of varying degrees may be felt in the contracting muscles. There are may factors acting here, but the immediate feeling of discomfort is due to some degree of mechanical trauma done to myofibers.
Work Step by Step
Repeated contractions will bring other factors into play more significantly.
