When heat is applied to the skin, it causes more blood to flow into the area. Heat affects the skin as well as the underlying tissues. How deeply these effects travel depends on what type of heat is used. For instance, a heating pad may only target the “shallow” tissues, while therapeutic ultrasound can penetrate into the deeper muscles.
How Heat Helps Pain:
When blood flow increases to an area, it brings along oxygen and nutrients that can help to speed healing. Heat helps to relax muscles, which can decrease some types of pain sensations. The sensation of heat on the skin also provides something called an analgesic effect: it alters the perception of pain so you don’t hurt as much.
When NOT to Use Heat for Pain:
Heat is best for injuries or conditions that are not in the acute phase. In other words, don’t use heat on a fresh injury: you could increase swelling, which in some cases could increase your discomfort. In these cases, ice is a better choice. Also, you shouldn’t apply heat over irritated skin or open wounds (including incisions that are still healing). Finally, people with cancer should not use heat to treat pain, as there is a chance of increased tumor growth.