ThermaCare heat wraps work by using certain heat cell technology to release heat. The heat will be released against the body and into the body. With the heat wraps being attached to the skin, you can move around and do your normal activities. The chemical reaction that occurs will penetrate the muscle tissue and provide heat therapy while the heat wrap is being worn and then for a time after it's been taken off. You can find more information here: www.thermacare.com
The Thermacare Heat Wraps I have used, stick to my shirt as the heat side warms up and stays on my back. They have a chemical reaction in them that make them warm. They work pretty well for aches and pains.