Thermal heating of ultrasound increases the extensibility of soft tissues.

• A. Decreases
• B. Increases
• C. Increases viscosity
• D. No effect

Answer: (B)

Increasing tissue temperature through thermal ultrasound makes soft tissues more extensible. When connective tissues such as fascia, tendons, and joint capsules are warmed, their collagen fibers become more pliable and the tissue stiffness decreases. This enhanced pliability allows a greater amount of stretch for the same applied force, improving extensibility and making ROM gains easier during or after the heating. Ultrasound achieves this heating in the treated area based on frequency, intensity, duty cycle, and duration, so the amount of heat—and thus the degree of increased extensibility—depends on how the therapy is applied. Because heat reduces resistance to stretch, this statement is true. Heating does not increase viscosity and there is a physiological effect, so the other options are not correct.