In the natural sciences, the term diathermy [di´ah-ther″me] means “electrically induced heat” the use of high-frequency electromagnetic currents as a form of physical or occupational therapy and in surgical procedures. The term diathermy is taken from the Greek words dia and therma, which literally means “heating through”. It is commonly used for muscle relaxation. Alternatively, it is a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine. Diathermy moat applied in the fields of physical therapy and occupational therapy; it delivers moderate heat directly to pathologic lesions in the deeper tissues of the body. Whether its achieved using short-wave radio frequency (range 1–100 MHz) or microwave energy (typically 915 MHz or 2.45 GHz), Diathermy tends to exert physical effects and elicits a spectrum of physiological responses. Both methods differ mainly in their penetration capability.

Additionally the extreme heat that can be produced by diathermy may be used to destroy neoplasms, warts, and infected tissues, and to cauterize blood vessels to prevent excessive bleeding. The technique is particularly valuable in neurosurgery and surgery of the eye. Diathermy was used for the first time in gynecology by the Spanish doctor Celedonio Calatayud in 1910. The three forms of diathermy employed by physical and occupational therapists are ultrasound, short wave and microwave. The application of moderate heat by diathermy increases blood flow and speeds up metabolism and the rate of ion diffusion across cellular membranes. The fibrous tissues in tendons, joint capsules, and scars are more easily stretched when subjected to heat, thus facilitating the relief of stiffness of joints and promoting relaxation of the muscles and decrease of muscle spasms.

Ultrasound diathermy employs high-frequency acoustic vibrations which, when propelled through the tissues, are converted into heat. This type of diathermy is especially useful in the delivery of heat to selected musculatures and structures because there is a difference in the sensitivity of various fibers to the acoustic vibrations; some are more absorptive and some are more reflective. For example, in subcutaneous fat, relatively little energy is converted into heat, but in muscle tissues there is a much higher rate of conversion to heat.

Short wave diathermy machines use two condenser plates that are placed on either side of the body part to be treated. Another mode of application is by induction coils that are pliable and can be molded to fit the part of the body under treatment. As the high-frequency waves travel through the body tissues between the condensers or the coils, they are converted into heat. The degree of heat and depth of penetration depend in part on the absorptive and resistance properties of the tissues that the waves encounter.

Microwave diathermy uses radar waves, which are of higher frequency and shorter wavelength than radio waves. Most, if not all, of the therapeutic effects of microwave therapy are related to the conversion of energy into heat and its distribution throughout the body tissues. This mode of diathermy is considered to be the easiest to use, but the microwaves have a relatively poor depth of penetration.