Thermal energy is used in ultrasonic therapy. When sound is propagated through tissues, the degree that any medium is exposed to heat depends on the tissue thickness. Muscle and bone have been found to absorb more energy at interfaces with other heterogeneous tissues, because at these surfaces, the longitudinal waves of ultrasound are reflected and transformed into transverse waves, creating a heating effect. This happens commonly in the areas in between the muscle and bone or between the muscle and tendon. By applying ultrasonic waves to these areas it can reduce inflammation and increase mobility in the joints.
Ultrasound is a deep heating modality that is most effective in heating tissues of deep joints. It has been found to be helpful in improving the distensibility of connective tissue, which facilitates stretching. It is not indicated in acute inflammatory conditions where it may serve to exacerbate the inflammatory response and typically provides only short-term benefit when used in isolation. It is perhaps best used to improve limitations in segmental spinal range of motion following recurrent or chronic low back pain as an adjunct in facilitating soft tissue mobilization and prolonged stretching.