Therapeutic ultrasound in physical therapy is alternating compression and rarefaction of sound waves with a frequency in the area of injury or pain. This energy is absorbed in the soft tissue, connective tissue, ligaments, tendons, and fascia. Therapeutic ultrasound is sometimes recommended for muscle as well as joint pain. Therapeutic ultrasound has two types of benefits, thermal effects and non thermal effects. The thermal effects are due to the absorption of the sound waves. Non thermal effects are from cavitation, microstreaming and acoustic streaming. Cavitational effects result from the vibration of the tissue causing microscopic air bubbles to form, which transmit the vibrations in a way that directly stimulates cell membranes. This physical stimulation appears to enhance the cell-repair effects of the inflammatory response. Therapeutic ultrasound is sometimes recommended for muscles, joints, ligaments, tendons and soft tissues of the spine or the extremities.