A shortwave diathermy (SWD) machine is basically a small radio transmitter. It broadcasts at 27.12 MHz, right in the neighbourhood of CB radio, yet the patient feels warmth deep inside the thigh and no shock or tingle. So how does a radio signal end up heating a hamstring from the inside? The answer explains most of what exams ask about SWD: electrode placement, dosage and safety.
Table of Contents
What Is Shortwave Diathermy, and Why Does It Use 27.12 MHz?
Shortwave diathermy (SWD) is an electrotherapy modality that uses high-frequency electromagnetic energy, most commonly at 27.12 MHz with a wavelength of about 11 metres, to produce deep heat in body tissues. It can be delivered continuously for thermal effects or in pulses for lower mean heating, using either capacitor plates or an induction coil.
The name gives away the goal. Diathermy means "through heating" (from the Greek dia, through, and therme, heat). A hot pack warms the surface, and that warmth fades quickly with depth. SWD makes heat inside the tissue itself, so it can reach a large muscle or a joint capsule that a hot pack never touches.
Quick recap before the numbers. Frequency is how many wave cycles happen each second, and 1 MHz means 1 million cycles per second. Wavelength is the distance 1 cycle covers in space, and the faster the wave, the shorter that distance. Divide the speed of light (about 300 million metres per second) by 27.12 million cycles per second and you get 11.06 metres, roughly the length of a city bus.
| Frequency | Wavelength | Where you meet it |
|---|---|---|
| 13.56 MHz | About 22 m | Allocated, rarely used |
| 27.12 MHz | About 11 m | Standard in clinical units |
| 40.68 MHz | About 7.4 m | Allocated, rarely used |
Why only 3 fixed values? Radio frequencies are shared with communication services, so regulators reserve a few narrow bands for medical and industrial equipment. An SWD unit stays inside one of those bands, which keeps it from jamming anyone's radio. Robertson and colleagues, in Electrotherapy Explained (2006), list these 3 allocated frequencies, with 27.12 MHz as the clinical standard.
That covers the wave itself. What the wave does once it meets tissue is where the physiology starts.
How Radio Waves Turn Into Heat Inside Tissue
Tissue is basically salty water. It holds ions (charged particles such as sodium and chloride) and water molecules, and each water molecule is polar, meaning 1 end carries a slight positive charge and the other end a slight negative one. SWD acts on exactly these 2 things.
The wave brings an electric field that reverses direction 27.12 million times every second. Here's what the tissue does with it, step by step:
- The field pushes ions 1 way, then the other, millions of times a second, and the ions keep bumping into neighbouring molecules (conduction heating).
- Water molecules flip back and forth to stay lined up with the reversing field (dipole rotation), and every flip rubs against nearby molecules.
- All that jostling converts movement energy into heat.
- Warmth builds inside the tissue itself, while blood flow carries part of it away.
Think of it like rubbing your palms together, except the "palms" are countless molecules and the rubbing repeats 27 million times a second. The flowchart below puts the whole chain on 1 screen.
Here's where most students slip. They picture the metal plates getting hot and passing warmth into the skin, like a hot plate on a table. The plates stay much cooler than the tissue. The heat is born inside the tissue, and the applicator only delivers the field.
You might also wonder why a patient feels no tingle or twitch, since this is still an electric field. Each cycle lasts about 37 billionths of a second, far too short for nerve and muscle membranes to react. They never get the chance to depolarise, so the only thing the patient notices is warmth.
Quick pause. Field reverses, ions shuttle, water rotates, friction becomes heat. Hold those 4 links, because everything that follows is about controlling where that heat lands and how much of it you make.
The applicator decides the first part, so that's the next decision on the machine.
Capacitor Field vs Coil Field: Which Tissue Heats More?
Same wave, 2 very different applicators. The capacitor field method (also called the condenser method) places the body part between 2 insulated plates. Picture a sandwich: plate, gap, limb, gap, plate. The electric field runs through the whole sandwich, and the tissue plays the part of the insulating layer in a capacitor.
The inductive (coil field) method works differently. A coil, either a flat drum over the area or a cable wrapped around the limb, carries the high-frequency current. That current creates a rapidly changing magnetic field, and a changing magnetic field drives swirling currents inside the tissue. Those swirls are called eddy currents, and they make the heat.
Here's the single difference to carry into the exam: capacitor plates work through the electric field, and coil applicators work through the magnetic field. Each field heats the tissue it suits best. Robertson and colleagues (2006) and most electrotherapy texts agree that the capacitor method heats high-resistance tissue such as fat and skin more strongly, while the coil method heats water-rich, low-resistance tissue such as muscle more strongly.
Same machine, same wave: change the applicator and you change which tissue gets the heat.
The panel below sets the 2 methods side by side.
| Feature | Capacitor (condenser) field | Coil (inductive) field |
|---|---|---|
| Field doing the work | Electric field | Magnetic field, which induces eddy currents |
| Applicator | 2 plates or pads, with a gap or towelling to the skin | A drum placed over the area, or a cable wrapped around the limb |
| Tissue heated most | High-resistance tissue such as fat and skin | Water-rich, low-resistance tissue such as muscle |
| Common arrangements | Contraplanar (plates on opposite sides), coplanar (plates on the same side) | Single drum, or cable coil around the part |
Plate spacing also shapes the field. When the 2 plates sit on the same side of the body, placing them close together keeps the field shallow, while placing them wide apart pushes it deeper. Texts also keep a layer of towelling or an air gap between plate and skin, which helps keep the field even.
Either applicator can run the field continuously or in bursts, and that's the next switch on the panel.
Continuous or Pulsed SWD: What Does the Pause Change?
In continuous mode the field never switches off, so heat keeps building. In pulsed SWD (PSWD) the same 27.12 MHz wave arrives in short bursts with long silent pauses between them. During each pause, blood flow carries heat away, so the tissue usually warms far less. The trace below shows the difference.
To describe a pulsed setting you need 4 numbers. Exact ranges vary between units and textbooks.
| Parameter | Plain meaning | What raising it does |
|---|---|---|
| Pulse duration | How long each burst lasts, usually tens to hundreds of microseconds | More energy in every burst |
| Pulse repetition rate (PRR) | How many bursts arrive each second | More energy every second |
| Peak power | Power during a burst, up to about 1,000 W on some units | More energy in every burst |
| Mean power | Average power over time | Directly sets how much the tissue warms |
The 4th number is the one that matters, and you can calculate it: mean power = peak power × pulse duration × pulse repetition rate. Take a peak of 1,000 W, bursts of 400 microseconds (0.0004 s) and 100 bursts per second. That gives 1,000 × 0.0004 × 100 = 40 W. The unit is switched on for only 4% of every second, so a machine that peaks at 1,000 W behaves like a 40 W machine.
Now a what-if. Double the pulse rate and leave everything else alone, and mean power doubles to 80 W, so the tissue warms to match. That's the point of the formula, and it plants a clinical lesson: read the mean power before assuming a pulsed setting is gentle.
So why bother with the pause at all? Pulsed SWD is mainly promoted for effects that don't depend on heating, called athermal (non-thermal) effects. Proposed mechanisms involve changes at the cell membrane and in calcium binding, but the evidence for them is still developing.
Whatever mode you pick, you still have to decide how much energy to give, and the scale for that is surprisingly low-tech.
How Do You Dose Heat You Can't See?
You ask the patient. Continuous SWD dosage follows the patient's own sensation of warmth, graded on a 4-level scale. A US FDA device record (K083433, 2009) lists these levels, and electrotherapy texts teach the same scale.
| Dose | What the patient feels | Heating level |
|---|---|---|
| I | No sensation of heat | Athermal |
| II | Mild heating sensation | Low thermal |
| III | Moderate, pleasant heating | Medium thermal |
| IV | Vigorous heating, tolerable below the pain threshold | Heavy thermal |
Many texts pair the lower doses with acute and subacute conditions and the higher doses with chronic ones. Session times quoted in many texts run roughly 10 to 30 minutes, with longer times for higher doses.
Here's the logic, and its weak point. Warmth receptors in the skin report how hot things feel, so the dose is only as reliable as the patient's ability to feel and report warmth. Remember that, because it explains several contraindications in the next section.
Does the Heat Actually Help? Classic Teaching vs Current Evidence
Textbooks such as Cameron's Physical Agents in Rehabilitation (5th ed., 2018) give the classic list of thermal effects: wider blood vessels and more blood flow, faster metabolism, less muscle spasm, pain relief and easier stretching of collagen-rich tissue. That's the list your exam expects.
Trials add a twist. Laufer and Dar (2012) pooled 7 trials of SWD in knee osteoarthritis. They found small but significant improvements in pain and muscle performance only when SWD produced a local feeling of warmth, while studies using low mean power did not favour SWD for pain.
In those knee osteoarthritis trials, the benefit appeared when the patient felt warmth. That links straight back to dose. The authors also called for more research with comparable protocols, and the case for pulsed SWD is still developing.
Heat that helps can also harm, and that's where the dose scale's blind spot starts to matter.
When Must SWD Stay Switched Off?
Every contraindication traces back to 1 of 5 reasons. Learn the reasons and you can rebuild the list yourself in a viva, instead of memorising a long list of separate items.
| Reason | Examples | Why it's a problem |
|---|---|---|
| Metal and electronics | Pacemakers and other implanted devices, metal implants in the field, jewellery, an IUD, metal treatment tables | Metal can overheat and burn the tissue around it, and the field can disrupt electronic devices |
| Heat can't escape | Poorly perfused (ischaemic) tissue, eyes, testes | Blood flow is the body's cooling system, and these tissues clear or tolerate heat poorly |
| Heat can't be reported | Impaired thermal sensation | Sensation-based dosing fails when the patient can't feel warmth |
| Heat or field may cause harm | Malignancy, pregnant or possibly pregnant uterus, growth areas (epiphyses) of children, tuberculous joints | Texts cite concern about effects on tissue growth and on a developing foetus |
| Setup errors | Moist dressings, sweat, wet clothing | Fluid concentrates the field and can create hot spots and burns |
In a survey of Irish hospital physiotherapy departments (Shields and colleagues), more than 90% of respondents agreed that metal implants, pacemakers, malignancy, tuberculous joints and treatment over the eyes contraindicate continuous SWD. For pulsed SWD the strongest agreement was on malignancy and pacemakers. The authors added that most contraindications come from common sense, with little research behind them.
What if a patient can't feel warmth? The dose scale collapses, because the warning system is gone. Tissue can heat past safe limits without a single complaint.
Put the 6 pieces together and SWD becomes 1 chain: wave, heat, applicator, mode, dose and safety. Break any link and either the treatment fails or the tissue pays for it.
- Shortwave diathermy (SWD) uses high-frequency electromagnetic energy for deep heating. Clinical standard: 27.12 MHz, wavelength 11.06 m. Also allocated: 13.56 MHz (22.12 m) and 40.68 MHz (7.37 m).
- Heat comes from ion movement and water dipole rotation inside the tissue. The frequency is too high to stimulate nerve or muscle.
- Capacitor method: electric field, heats high-resistance tissue such as fat. Coil method: magnetic field and eddy currents, heats water-rich muscle.
- Pulsed SWD: same wave in bursts. Mean power = peak power × pulse duration × pulse repetition rate. Pulsed output can still heat if mean power is high.
- Dose I to IV follows sensation: no heat, mild, moderate pleasant, vigorous below the pain threshold.
- Knee osteoarthritis evidence: small benefits, mainly when the patient felt warmth (Laufer and Dar, 2012).
- Contraindications fall into 5 groups: metal and electronics, heat can't escape, heat can't be reported, possible harm, setup errors.
SWD Quick Answers: Common Student Questions
What is the difference between shortwave diathermy and ultrasound therapy?
Both warm deep tissue, but they use different energy. SWD sends a high-frequency electromagnetic field, and the tissue turns that field into heat through moving ions and rotating water molecules. Therapeutic ultrasound sends sound waves, which are mechanical vibrations that tissue absorbs as heat. SWD warms a large area, which suits big muscle groups, while ultrasound is aimed at a smaller spot.
Why is 27.12 MHz used in SWD machines?
Radio frequencies are shared with communication services, so regulators reserve a few narrow bands for medical and industrial equipment. 27.12 MHz, with a wavelength of about 11.06 metres, is the band almost every clinical SWD unit uses. 13.56 MHz and 40.68 MHz are also allocated but rarely used. Staying inside a reserved band keeps the machine from interfering with radio communication.
Is pulsed SWD thermal or non-thermal?
It can be either. Pulsed SWD sends the same 27.12 MHz wave in short bursts with pauses between them. If the mean power (peak power × pulse duration × pulse repetition rate) is low, heat escapes during the pauses and the effects are described as athermal. If the mean power is high enough, the tissue warms. Check the mean power before judging a setting by the word pulsed.
Which heats muscle better in SWD, the capacitor method or the coil method?
Most electrotherapy texts say the coil (inductive) method favours muscle. Its magnetic field drives eddy currents through water-rich, low-resistance tissue, and that tissue heats. The capacitor method uses an electric field and tends to heat high-resistance tissue such as fat more strongly. Heating also shifts with electrode size and spacing, so treat this as the standard exam answer; real set-ups vary.
Why can't a patient with a pacemaker or metal implant have SWD?
Two separate dangers apply. Metal in the field can overheat and burn the tissue around it. Electronic implants such as pacemakers can be disrupted by the electromagnetic field. In a survey of Irish physiotherapy departments, more than 90% of respondents agreed that pacemakers contraindicate both continuous and pulsed SWD, and they agreed on metal implants for continuous SWD.
How is SWD dosage decided?
Continuous SWD dosage follows what the patient feels. Dose I is no sensation of heat, Dose II is mild heat, Dose III is moderate and pleasant, and Dose IV is vigorous heat that stays below the pain threshold. Because the scale relies on sensation, the patient must be able to feel and report warmth. Real prescribing always depends on patient factors and clinical judgment.
Does shortwave diathermy actually work for knee osteoarthritis?
Evidence is modest. A 2012 meta-analysis by Laufer and Dar pooled 7 trials in knee osteoarthritis and found small but significant improvements in pain and muscle performance only when SWD produced a local feeling of warmth. Studies with low mean power did not favour SWD for pain. The authors asked for more trials using comparable protocols.
This article is written for educational purposes for health-science students. It is not medical advice and never replaces clinical training, professional judgment, or your institution's curriculum. Real clinical decisions always require proper patient assessment under qualified supervision.
References
- Ward AR, Robertson VJ, Low J, Reed A. Electrotherapy Explained: Principles and Practice. 4th ed. Butterworth-Heinemann; 2006.
- Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. 5th ed. Elsevier; 2017.
- Laufer Y, Dar G. "Effectiveness of thermal and athermal short-wave diathermy for the management of knee osteoarthritis: a systematic review and meta-analysis." Osteoarthritis Cartilage. 2012;20(9):957-966. doi:10.1016/j.joca.2012.05.005. PMID 22659070.
- Shields N, et al. "Contra-indications to shortwave diathermy: survey of Irish physiotherapists." Chartered Society of Physiotherapy.
- U.S. Food and Drug Administration. "510(k) K083433: Intelect SWD 100 / Senior Solutions Shortwave Diathermy." Record mirrored by Innolitics. (2009).
Some reference links above are affiliate links to Amazon. As an Amazon Associate, MedLyre earns from qualifying purchases made through these links, at no additional cost to you. Every book listed is a genuine source consulted for this article, and the link always points to the exact edition cited. No author, publisher, or equipment company paid for inclusion in this reference list.
