The goal of therapeutic ultrasound is often to create heat in the underlying tissue. This only requires rubbing the transducer over a small area. The size of the area depends on the size of the transducer. Most transducers in the U.S. come in two sizes. The 3/4-inch transducers should only be used in an area the size of a golf ball. The bigger 2.5-inch transducers should be used on an area about the size of a softball.

Ultrasound is really not appropriate for treating larger areas. You may want to ask your therapist about other treatments, including short-wave diathermy, moist heat, electrical stimulation, and ice.