Superficial heating modalities can be very beneficial in some chronic pain situations. Superficial heat, however, will not heat deep tissues such as muscles, because of the subcutaneous layer of fat beneath the surface of the skin which acts as a thermal insulator and inhibits heat transfer into tissues. There are deep heating modalities such as ultrasound and shortwave diathermy, but we will not be talking about those since they generally need to be provided by a physical therapist or occupational therapist in a clinical setting. We are focusing on self-help treatment for chronic pain problems at this time.
Superficial heat does have effects on underlying tissues, however. It does cause increased blood flow, but in addition it causes relaxation of underlying muscles. Its effect on reducing pain is largely a "covering up effect," in that while the heat is being applied, the pain signals are less perceptible because of the sensory overload caused by the heat. In other words, the heat causes a "gating effect" in that it fills up sensory nerve channels with heat stimulation and does not allow the pain signals to get through as readily. That means that your awareness is of the heat instead of the pain. This, of course, is good and heat can be soothing because of this, but it does not have the same effects as cold therapy, in general, when treating muscular pain problems. The cold therapy is much more effective once one has become acclimated to the initial cold sensation.
It should be noted that superficial heat and other heating modalities can exacerbate inflammation and swelling, so as a general rule, as a practitioner, I do not prescribe superficial or deep heat where there is significant swelling or inflammation. This would of course include any acute injury where clearly swelling and inflammation would be the result of the injury. One would use cold therapy, at least, for the first 72 hours after an acute tissue injury.
There are a number of modalities or methods that one can use to apply superficial heat. Hot packs or hydrocolator packs are generally heated in a hot water tank and then applied to the skin over several layers of towels for 20 to 30 minutes at a time. Electric heating pads can also be used, but they can be dangerous. If they are used excessively, they can cause burns and should be used with great caution. The other danger with electric heating pads, of course, is the danger of electrical shock and one should never use an electric heating pad when trying to go to sleep. The healing pad may produce enough analgesia, or covering up of the pain, that burns can be produced without the person's knowledge until it is too late.
In general, I do not prescribe heat therapy nearly as often as cold therapy. I find cold therapy to be much more effective even in situations where most others seem to prefer heat therapy. Cold therapy tends to penetrate tissue better and has had clearly a better effect on treating muscular pain syndromes, such as myofascial pain syndrome or fibromyalgia.
Other methods of providing superficial heat therapy include hydrotherapy, that is, a tub of hot water which, of course, can be very relaxing, paraffin baths and fluidotherapy.
Now there are chemical heat packs available that one can activate simply and utilize, some of which can be used repeatedly.
One simple method of providing moist heat is to fold a damp towel and heat it up in a microwave oven and utilize this. Of course, one needs to be very careful about the possibility of burns.
In any case, be very careful in your use of superficial heat.