Diathermy is a medical technique that uses electromagnetic energy or sound waves to generate heat deep inside body tissues, well below what a hot pack or heating pad can reach. The word itself comes from the Greek for “heating through,” and the principle is straightforward: energy passes into the body, and as the tissue absorbs it, molecules vibrate or rotate faster, producing warmth from the inside out. Depending on the form of energy used and the clinical goal, diathermy shows up in rehabilitation clinics for pain relief, in operating rooms for cutting and sealing tissue, and in oncology suites for destroying tumors.
How Energy Becomes Heat Inside the Body
The core mechanism behind all forms of therapeutic diathermy is the same. An external device delivers energy into the body at a frequency that causes molecules in the target tissue to move. Water molecules, proteins, and ions in muscle, tendon, and joint capsules absorb that energy and convert it into heat. The result is a rise in tissue temperature at depths of several centimeters, something surface-applied heat cannot achieve. Shortwave diathermy, the most widely studied form, uses high-frequency electromagnetic fields and produces deep tissue warming along with increased blood flow and vasodilation.
The vascular response to that internal warming is well documented. When tissue heats up, the nervous system dials back the signals that normally keep blood vessels slightly constricted. The vessels relax, and blood flow increases. Research in human subjects shows that this vasodilation is driven largely by the local release of nitric oxide, a molecule that signals blood vessel walls to widen. Two processes contribute: a fast response mediated by nerve reflexes and a slower, sustained response that depends on nitric oxide production in the tissue itself.1Brazilian Journal of Physical Therapy. Application of shortwave diathermy to lower limb increases arterial blood flow velocity and skin temperature in women: a randomized controlled trial That increased circulation is thought to accelerate healing by delivering more oxygen and nutrients to damaged or inflamed tissue while carrying away waste products.
Shortwave Diathermy and Its Two Modes
Shortwave diathermy (SWD) is the form you are most likely to encounter in a physical therapy clinic. The device generates electromagnetic waves, typically at 27.12 MHz, and delivers them to the body through electrodes or an inductive coil placed near the skin. It comes in two flavors: continuous and pulsed.
Continuous shortwave diathermy delivers a steady stream of energy, producing a noticeable rise in tissue temperature. This is the more aggressive heating mode and can warm skin substantially. A systematic review of tissue-heating studies found that continuous SWD applied through a capacitive technique raised skin temperature by as much as about 8°C, while muscle temperature increases were somewhat lower.2PubMed. Tissue heating in different short wave diathermy methods: A systematic review and narrative synthesis The warming effect depends heavily on the electrode arrangement, power level, and how long the treatment lasts.
Pulsed shortwave diathermy (PSWD), by contrast, delivers bursts of energy with rests in between. Because the energy is intermittent, less total heat accumulates in the tissue. At low doses, some pulsed applications raise tissue temperature by less than half a degree Celsius.2PubMed. Tissue heating in different short wave diathermy methods: A systematic review and narrative synthesis This makes pulsed SWD attractive for treating conditions where you want the electromagnetic energy’s biological effects without aggressively cooking the tissue, such as in acute inflammation. Some researchers have argued that pulsed fields stimulate healing through non-thermal pathways, though that area of the science remains less settled than the thermal effects.
Inductive applicators, which use a coil rather than flat plates, tend to heat muscle more effectively than capacitive (plate) applicators, which heat superficial fat and skin. The same systematic review found that pulsed SWD delivered through an inductive technique produced the highest muscle temperature increase, roughly 4.6°C, and that the warmth in the muscle decayed more slowly after the device was turned off.2PubMed. Tissue heating in different short wave diathermy methods: A systematic review and narrative synthesis In practical terms, clinicians choose the applicator type and mode based on the depth and location of the tissue they want to treat.
Ultrasound Diathermy
Not all diathermy relies on electromagnetic waves. Therapeutic ultrasound uses mechanical sound waves, typically at frequencies of 1 to 3 MHz, to generate heat through tissue vibration. This is the diathermy you might receive from a sports medicine clinician who glides a small handheld transducer over your skin with a layer of coupling gel.
A newer variation called low-intensity continuous ultrasound (LICUS) is designed to be worn as a wearable patch for hours rather than applied for the typical 10-to-15-minute clinic session. LICUS devices work at lower power levels but over a longer duration, aiming to sustain gentle warming and promote tissue healing through both heat and mechanical stimulation of cells. Research on these devices has shown that the therapeutic ultrasound intensity can be sustained to depths of several centimeters, and that the addition of a coupling gel containing diclofenac (an anti-inflammatory drug) extends the effective signal propagation while preserving the heating profile at depth.3PubMed Central. Clinical Diathermy Performance Evaluation of Multi-hour Sustained Acoustic Medicine Treatment with 2.5% Diclofenac Ultrasound Coupling Patch
Knee Osteoarthritis and Joint Pain
The clinical condition with the strongest body of diathermy research is knee osteoarthritis. Multiple trials have tested whether shortwave diathermy can reduce pain and improve function in people with worn-down cartilage, and the results suggest it can, with an important caveat about dosing.
A meta-analysis pooling data from knee osteoarthritis studies found that treatments using a dose strong enough to produce some thermal effect showed a statistically meaningful benefit for pain reduction, while studies that used very low power with minimal heating did not outperform placebo.4Osteoarthritis and Cartilage. The effectiveness of short-wave diathermy in the management of knee osteoarthritis: a systematic review and meta-analysis The implication is that for joint pain, the treatment actually needs to warm the tissue to be effective. Setting the machine too low may mean the patient gets electromagnetic exposure without the heating that drives the clinical benefit.
A multicenter randomized trial in women with knee osteoarthritis confirmed the pattern: both low-dose and high-dose pulsed shortwave treatments produced meaningful reductions in pain and improvements in function compared with sham treatment and a no-treatment control group.5Physical Therapy. Pulsed Shortwave Treatment in Women With Knee Osteoarthritis: A Multicenter, Randomized, Placebo-Controlled Clinical Trial Separately, a pilot study comparing continuous and pulsed SWD head-to-head in knee osteoarthritis patients found that both modes reduced pain scores by roughly a third and improved overall joint function, with no significant difference between the two.6PubMed Central. Effectiveness of continuous versus pulsed short-wave diathermy in the management of knee osteoarthritis: A randomized pilot study So the evidence suggests the type of shortwave matters less than whether enough energy reaches the joint.
Does Diathermy Improve Flexibility?
One popular idea in sports rehabilitation is that heating a muscle or tendon before stretching should make the tissue more pliable and help improve range of motion. Diathermy seems like a natural tool for this: warm the hamstrings from the inside, then stretch them. The theory makes intuitive sense because collagen, the protein that gives connective tissue its stiffness, becomes more extensible at higher temperatures.
However, the evidence on whether diathermy alone improves flexibility is disappointing. A controlled study specifically testing whether a single pulsed shortwave treatment increased hamstring extensibility found no significant improvement when no stretching was performed alongside the heat.7International Journal of Athletic Therapy and Training. Effect of a Single Pulsed Shortwave Diathermy Treatment on Extensibility of the Hamstrings The takeaway is that deep heating may help prepare tissue for stretching, but it does not appear to be a substitute for actually stretching. If your physical therapist applies diathermy before range-of-motion exercises, the heat is meant to make the subsequent stretching more effective and comfortable, not to replace it.
Pelvic Inflammatory Disease and Other Deep-Tissue Uses
Because shortwave diathermy can deliver heat to tissues that are difficult to reach with surface methods, it has been explored for conditions deep inside the body. Chronic pelvic inflammatory disease (PID), a persistent infection and inflammation of the reproductive organs, is one area where the technique has shown promise.
A randomized controlled trial found that shortwave diathermy had a significant effect on pain and resolution of inflammation in chronic PID compared with both analgesic-only treatment and a control group.8PubMed. Short wave diathermy in the symptomatic management of chronic pelvic inflammatory disease pain: A randomized controlled trial An earlier case report described a woman with an eight-year history of PID who became completely pain-free after nine shortwave diathermy sessions using a “cross-fire” technique, where the applicator is repositioned to heat the pelvic region from two different angles in a single session.9PubMed. Management of chronic pelvic inflammatory disease with shortwave diathermy. A case report These results are encouraging, but this remains a niche application with limited large-scale evidence.
Surgical Diathermy Is a Different Animal
The word “diathermy” also appears in operating rooms, but surgical diathermy (more properly called electrosurgery) works on an entirely different principle from the therapeutic version. Instead of gently warming a wide region of tissue to promote healing, surgical diathermy concentrates high-frequency electrical current at the tip of an instrument to cut tissue or seal bleeding blood vessels. The temperatures generated at the contact point are high enough to cause immediate protein coagulation, effectively cauterizing as the surgeon operates.
Surgical diathermy comes in two main forms. Monopolar instruments pass current through the patient’s body to a large grounding pad (return electrode) placed elsewhere on the skin. Because all the current flows through the small instrument tip, the energy density there is high enough to cut or coagulate, while the broad grounding pad spreads the current out so no damage occurs at the exit site. Bipolar instruments, by contrast, confine the current between two closely spaced tips (like the tines of forceps), so the energy stays localized and does not travel through the body. Bipolar mode is generally considered safer for work near delicate structures.
Complications from surgical diathermy are primarily burns, and they usually trace back to predictable causes. Incorrect power settings are the most common culprit: too high a setting causes deep burns, while too low a setting forces the surgeon to make repeated passes, extending operating time. Improper placement of the return electrode pad can also produce burns, especially if the pad does not make full contact with the skin due to hair, bony prominences, or dry skin. In laparoscopic (keyhole) surgery, insulation failure in long instruments is a particular concern, since tiny cracks in the insulation can leak current to adjacent tissues without the surgeon noticing. Patients with more subcutaneous fat face a higher risk of return-pad burns, and metallic surgical instruments near the active electrode can pick up stray current through a phenomenon called capacitive coupling.10Medical Research Archives. Complications in Surgical Diathermy: Causes and Prevention
Radiofrequency Ablation as a Diathermy Offshoot
Radiofrequency ablation (RFA) takes the heating principle of diathermy and turns it up to a destructive level on purpose. A needle-like electrode is inserted into a tumor, and radiofrequency energy heats the surrounding tissue until proteins denature and cells die. RFA has become a standard option for patients with liver tumors who are not candidates for surgical removal, and its use has expanded to other cancers and even to cardiac arrhythmias, where targeted heat destroys the small patches of heart tissue that cause abnormal electrical signals.11PubMed Central. Radiofrequency ablation: mechanisms and clinical applications It is worth knowing that RFA shares the same physical foundation as the shortwave device in a rehab clinic; the difference is entirely in dose and intent.
Safety Rules and Who Should Avoid Diathermy
For therapeutic diathermy in a rehabilitation setting, the biggest safety concerns involve metal inside the body and certain medical devices. Any metallic implant, whether a joint replacement, a surgical plate, or even an intrauterine device (IUD), can act as an antenna for electromagnetic energy and heat up far more than the surrounding tissue, creating a localized burn risk. Cardiac pacemakers are an absolute contraindication for continuous shortwave diathermy, because the electromagnetic field can interfere with the device’s electronics or heat its leads.
The safety picture gets muddier with pulsed shortwave diathermy, which delivers less total energy. A survey of physiotherapists found that while most agreed continuous SWD should never be used on patients with metal implants or pacemakers, confidence dropped when it came to pulsed SWD. Roughly half of the therapists surveyed believed pulsed shortwave was always off-limits for patients with metal implants, and only slightly fewer said the same for IUDs. Worryingly, over a third of respondents thought pulsed SWD could be used in patients with pacemakers or applied to other body parts away from the device.12PubMed Central. Physiotherapists’ Understanding of Shortwave Diathermy Contraindications: A Questionnaire Survey That level of uncertainty among clinicians is a red flag. If you have any metal implant or electronic medical device, make sure your therapist knows about it before diathermy is applied, regardless of the mode.
Other standard contraindications include pregnancy (the electromagnetic field should not cross the abdomen), active malignancy in the treatment area (heat can promote tumor growth), active bleeding or hemorrhage, and impaired sensation in the treatment region (because the patient cannot report if the tissue is overheating). Wet dressings and synthetic materials should also be kept out of the field, since they can concentrate heat unpredictably.
What a Treatment Session Feels Like
If you are receiving therapeutic shortwave diathermy for the first time, the experience is less dramatic than you might expect. The clinician positions electrodes or a drum-shaped applicator near (but usually not touching) the skin over the target area. You lie still for 15 to 30 minutes. With continuous SWD, you typically feel a gentle, pleasant warmth that builds gradually. Your therapist will ask you to speak up if the warmth becomes uncomfortable, since the goal is a comfortable therapeutic temperature, not a burn.
With pulsed SWD, the sensation depends on the dose. At low pulse repetition rates and low power, you may feel nothing at all, which can be disconcerting when you are told the machine is on. Research on patient perception confirms that there is a significant relationship between pulse repetition rate and whether people report feeling warmth. At lower pulse rates, subjects are less likely to feel a “definite” thermal sensation, even though the electromagnetic field is active.13PubMed. Effect of pulse repetition rate on the perception of thermal sensation with pulsed shortwave diathermy This matters practically: just because you do not feel the heat does not mean the device is not working. Conversely, any sharp or burning sensation during treatment is abnormal and should prompt you to alert the therapist immediately, as it may indicate current concentration near metal (jewelry, clothing snaps) or a device malfunction.
Ultrasound diathermy feels different. The therapist presses a smooth transducer against your skin with gel and moves it in slow circles over the painful area. You may feel mild warmth or nothing at all, depending on the intensity. The moving technique is important: if the transducer sits still, the ultrasound energy can concentrate in one spot and cause a periosteal burn at the bone surface underneath, which is painful. A competent therapist will keep the transducer gliding at all times.
Where the Evidence Is Strong and Where It Thins Out
The best-supported application of therapeutic diathermy is pain reduction in knee osteoarthritis, where multiple randomized trials and a meta-analysis agree that doses producing genuine tissue warming outperform placebo.4Osteoarthritis and Cartilage. The effectiveness of short-wave diathermy in the management of knee osteoarthritis: a systematic review and meta-analysis The evidence for chronic pelvic inflammatory disease is smaller but positive, and the vascular effects of diathermy on blood flow are well documented in controlled experiments.1Brazilian Journal of Physical Therapy. Application of shortwave diathermy to lower limb increases arterial blood flow velocity and skin temperature in women: a randomized controlled trial
Where the evidence gets thin is in claims about non-thermal effects. The idea that pulsed electromagnetic fields at sub-heating doses can stimulate cell repair, reduce swelling, or accelerate wound healing independently of temperature has circulated in the rehabilitation literature for decades. Some in-vitro studies support the concept, but clinical evidence in human patients is less convincing. The distinction matters because non-thermal claims are sometimes used to justify applying pulsed SWD at doses so low they produce almost no measurable heating, and the knee osteoarthritis data suggest that very low power settings may not be clinically useful.4Osteoarthritis and Cartilage. The effectiveness of short-wave diathermy in the management of knee osteoarthritis: a systematic review and meta-analysis The honest summary is that most of the proven benefit from therapeutic diathermy traces back to good old-fashioned deep heating.
Diathermy also competes with simpler and cheaper alternatives. Hot packs, warm baths, and infrared lamps can all raise tissue temperature, just not at the same depth. For superficial muscles, these options may be perfectly adequate and far less expensive than a clinic visit with shortwave equipment. Diathermy’s real niche is in treating structures that lie deep enough that surface heating cannot reach them effectively: the hip joint capsule, deep knee structures, pelvic organs, and the spine. If your condition involves deep tissue and your therapist has access to diathermy equipment, it can be a useful addition to a broader rehabilitation plan. On its own, though, it is rarely the whole answer. The flexibility data make this clear: heat without exercise accomplishes less than heat combined with movement.