There are two types of therapeutic ultrasound used in the Physical Therapy clinic: pulsed and continuous.
Continuous ultrasound can produce heat. The effects are increased circulation, increased tissue extensibility (ability to stretch), acceleration of cell metabolic rate and pain reduction. One might ask, why not just use a hot pack for heating the body part? Continuous ultrasound is actually much better at heating deeper, smaller areas, and tissue with high collagen content such as fascia, tendons and ligaments.
Pulsed ultrasound is delivered in waves, in an on/off cycle. It does not raise the temperature of body tissue, but it still has other useful treatment benefits during the inflammatory phase of tissue injury and repair. Pulsed ultrasound has been shown to increase cell membrane permeability, increase the rate of collagen synthesis by fibroblasts (collagen is the dominant protein in tendons, ligaments and fascia), and increase macrophage responsiveness (macrophages are white blood cells that promote healing and digest cellular debris). So in a nutshell, pulsed ultrasound enhances the cell activity necessary for tissue repair.
In summary, therapeutic ultrasound promotes healing with high-frequency sound waves that penetrate deeply into injured tissues. It can generate increased temperature and blood flow. Ultrasound disperses stagnant fluid that accumulates around an injury.