Ultrasound machines utilize high or low frequency sound waves. As the ultrasound waves pass from the treatment head into the skin they cause the vibration of the surrounding tissues. This increased vibration leads to the production of heat within the tissue. In most cases this cannot be felt by the patient themselves. This increase in temperature may cause an increase in the extensibility of structures such as ligaments, tendons, scar tissue and fibrous joint capsules. In addition, heating may also help to reduce pain and muscle spasm and promote the healing process.