Ultrasound is a form of mechanical energy, not electrical. Mechanical vibration at increasing frequencies is known as sound energy but because of the high frequencies that are used in ultrasound, these vibrations are not heard by the human ear.
Bodily tissue, when exposed to sound waves, will move back and forth. Any increase in the molecular vibration (oscillation) in the tissue will result in heat generation, and thus can be used to produce thermal changes in the tissue.
These increases in temperature of a particular tissue can increase blood flow to the damaged tissue and help initiate the resolution of chronic inflammatory states which can be found in older more persistent injuries and conditions.
In addition to thermal changes, the vibration of the tissue appears to have effects which are generally considered non thermal. Amongst other things, this speeds up the rate of repair and enhances the quality of the repairing tissue so is more effective in the use of acute and sub acute injuries and conditions.
(Ref – Prof Tim Watson, Current concepts in Electrotherapy 2003).