What distinguishes isotonic contractions from isometric contractions?

Isotonic contractions are characterized by the active shortening of muscle fibers during contraction while generating sufficient tension to overcome resistance. This type of contraction allows the muscle to change length as it actively moves a load, which is essential for many types of physical movement, such as lifting or walking.

In contrast, isometric contractions occur when the muscle generates tension without changing its length, meaning that the muscle is engaged but not actually shortening or lengthening. This differentiation is key in understanding how muscles work during various activities.

The other options reflect misunderstandings about isotonic contractions: they do involve changes in muscle length and can occur during various phases of physical activity, not just relaxation. The tension generated during isotonic contractions is indeed significant and vital for performing work against an opposing force.