Heat relieves pain as a counterirritant and by direct effect on the tissues treated.
In the first situation, the sensation of heat acts to reduce the transmission of pain signals in the dorsal horn of the spinal cord and may also induce inhibitory stimuli from the brain stem. The local direct effects of heat include muscle relaxation, increased blood flow, and tissue compliance.

Superficial heating of tissue is achieved with hot packs, hot water bottles, electric heating pads, radiant heat lamps (white and infrared), and hydrotherapy. Heating of deeper tissues may be achieved with ultrasound, shortwave diathermy, and microwave treatment, in which electromagnetic energy is converted into heat in the tissues. They should not be used near metal or plastic prostheses or areas where bone cement has been used.

Heat therapy is of particular benefit in the treatment of muscle spasm, myofascial pain, and the general musculoskeletal discomforts associated with immobility and debility.
Heat treatment can cause tissue damage and should not be used (or should used with caution) in areas where there is diminished sensation or paralysis and where tissues are ischemic. It should not be used where there is infection or directly over tumor tissue.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 165.