What is the main action that occurs during an isotonic contraction?

During an isotonic contraction, the key action that occurs is that the muscle contracts and either shortens or lengthens while generating tension. This means that as the muscle fibers contract, they actively change length to produce movement. For instance, when lifting weights, the muscle shortens during the lifting phase, and during eccentric movements (like lowering the weights), the muscle lengthens while still being under tension. Isotonic contractions are essential for various physical activities and are responsible for the movements in many sports and exercises.

The other options do not accurately describe the characteristics of isotonic contractions. For instance, if a muscle remains relaxed, it signifies no contraction is taking place. Generating tension without changing length refers to isometric contractions, not isotonic ones. Lastly, if the muscle fibers are not activated, there would be no contraction occurring at all. Therefore, the choice indicating that the muscle contracts and either shortens or lengthens defines isotonic contractions correctly.