Final answer:
The variation in the amount of shortening during an isotonic contraction with changes in afterload is due to the length-tension relationship, as increased afterload requires more tension for the muscle to shorten.
Step-by-step explanation:
The amount of shortening during an isotonic contraction varies with afterload because of the length-tension relationship. During an isotonic contraction, muscle length changes to move a load; however, if the afterload (the resistive force) is increased, the muscle may not be able to shorten as much because it can only produce a certain maximum amount of tension. The cross-bridges can only form at the overlap points between actin and myosin, which means that the length of the muscle fiber at the onset of contraction influences how many cross-bridges can form and therefore how forcefully the muscle can contract to shorten.
When afterload increases, the muscle must generate more tension to overcome the additional resistance. If the tension reaches the point where it cannot any longer produce enough force to continue shortening, the contraction will result in less muscle shortening. This response is inline with the length-tension relationship, which describes how the forcefulness of a muscle contraction is dependent on the length of the sarcomeres within the muscle at the onset of the contraction.