Which effect of ultrasound is NOT considered a nonthermal effect?

The correct choice identifies the effect of increased extensibility of collagen structures as not considered a nonthermal effect of ultrasound.

When ultrasound is applied, two primary types of effects can occur: thermal and nonthermal. Nonthermal effects of ultrasound include mechanisms such as increased cellular metabolism, enhanced macrophage activity, and alterations in cell membrane permeability, which contribute to healing and tissue repair without causing a significant rise in temperature.

Increased extensibility of collagen structures, on the other hand, is regarded as a thermal effect because it typically involves a change in temperature that allows collagen fibers to relax and become more pliable. This thermal effect is important in improving flexibility and range of motion in tissues that have become stiff or contracted.

Considering these mechanisms helps clarify why the other choices are classified as nonthermal effects. Increased soft tissue repair and macrophage responsiveness can be driven by the mechanical effects of ultrasound, while increased cell membrane permeability can lead to enhanced nutrient exchange and waste removal at the cellular level, all without the influence of heat.