I went to a physical therapist for a shoulder problem. Ultrasound was used to heat the tissue before stretching and exercise. I was told it's a form of heat, but I didn't feel any heat. Can you explain this?

Ultrasound (US) is a form of energy that can be applied to the body through the skin. High frequency sound waves absorbed by tissues causes a vibration that's turned into heat energy. US raises the tissue temperature to around 100 to 110 degrees Fahrenheit.

Since the effect is deep in the tissues, no sense of heat is felt over the skin. You may feel some burning pain at the bone. This signifies the US is too high or being applied improperly.

Pain relief occurs with US as increased blood flow to the area sweeps away toxins and damaged cells. It also has a beneficial effect on irritated nerves.