Ultrasound is an inaudible high-frequency mechanical vibration generated by a transducer. The sound waves are transmitted through the tissue and progressively lose its intensity due to absorption and dispersion to the surrounding tissues.
During physiotherapy treatment, your therapist will determine the appropriate frequency and intensity based on the depth and density of the injured tissue (fat/tendon/muscle).
Additionally, therapeutic ultrasound may be used for its thermal or non-thermal effects. Therapeutic ultrasound can generate localized heat to relieve pain and muscle spasm to increase tissue extensibility. And at the same time reduce inflammation.
On the other hand, some proposed effects of non-thermal therapeutic ultrasound are stimulation of tissue regeneration and bone healing, increase circulation and increase cell permeability which also optimize tissue healing.
What is the purpose of the gel?
The cold gels are used to prevent reflection of the sound waves away from the soft tissue by excluding air between the transducer and the patient. The gel also allows easy movement of the transducer over the skin surface.
Not all soft tissue injuries will benefit from therapeutic ultrasound. Frankly, research studies have demonstrated a mixed result of therapeutic ultrasound, possibly due to inappropriate dosage, inadequate delivery of ultrasound to the injured site, or a true lack of effect of the treatment. To say the least, more promising research are needed on therapeutic ultrasound in the future.
Reference
Speed, C. A. (2001). Therapeutic ultrasound in soft tissue lesions. Rheumatology, 40(12), 1331-1336.