High Frequency Currents Explained: SWD and MWD Simply

Here's the puzzle that stops most students the first time they stand next to a diathermy machine. About 27 million cycles of current pass through the treated limb every second, yet the patient feels nothing except a gentle, spreading warmth. No tingle. No twitch. Where did the shock go? The answer sits at the centre of high frequency currents, and once it clicks, diathermy stops being a mystery box.

Table of Contents

Why Doesn't a 27 MHz Current Shock You?

High frequency currents are alternating currents with a frequency above about 100,000 Hz (100 kHz). Each phase is so brief that nerve membranes cannot respond, so the current causes no tingling and no muscle contraction. Its main effect is heat, which is why shortwave diathermy (27.12 MHz) and microwave diathermy (2450 MHz) are used to heat tissue.

Start with the word itself. Frequency (how many times a current completes its back-and-forth cycle in 1 second) is measured in hertz. Textbooks sort therapeutic currents into 3 bands, and the table shows the usual cut-offs. Books move the boundaries slightly, so match the numbers to your own syllabus.

Band	Frequency	Examples	What the nerve does
Low	Up to about 1,000 Hz	TENS, faradic-type currents	Fires easily: tingling or contraction
Medium	About 1 to 100 kHz	Interferential, Russian-type stimulation	Fires when many phases add up (summation)
High	Above about 100 kHz	Shortwave and microwave diathermy	Never fires; tissue heats

Figure 1: The 3 frequency bands, with shortwave and microwave diathermy sitting in the high frequency group.

Now the mechanism. A nerve fibre fires when its membrane (the thin skin around the fibre) is pushed past threshold by a current that lasts long enough. Think of that membrane as a heavy door. A steady push swings it open, but tapping it 27 million times a second never moves it. At 27.12 MHz, 1 full cycle lasts about 37 nanoseconds, so each phase gets roughly 18 nanoseconds. Light travels only about 5.5 metres in that time, about the width of a small classroom.

A typical TENS pulse lasts tens to hundreds of microseconds, thousands of times longer. Those pulses give the membrane time to charge, so the patient feels tingling. Diathermy gives it none.

Here's where most students slip. They assume a current that stimulates nothing must be a weak current. The reverse is true: a diathermy generator can deliver hundreds of watts. The energy is large, but it never gets a chance to excite a nerve.

The current is too fast to stimulate a nerve, yet it still shakes ions and water molecules around, and that shaking turns into heat.

That single property gives high frequency currents their clinical value. You can warm tissue deep below the skin without a twitch. Surgeons use the same trick in electrosurgery, with frequencies from hundreds of kHz to a few MHz, to cut and seal tissue without making the patient convulse.

Common Mistake! Ultrasound often sits in the same chapter as diathermy, and some syllabi even list it under high frequency modalities. It is sound, a mechanical wave at around 1 to 3 MHz in therapy, and it needs a gel to pass into the body. Diathermy uses electromagnetic energy, which needs no medium. Keep the 2 apart in exam answers.

Going deeper: why exactly 27.12 MHz, and what wavelength means

Diathermy machines work only in frequency bands set aside internationally for industrial, scientific and medical use, so they don't jam radio and television signals. For shortwave diathermy those bands are 13.56, 27.12 and 40.68 MHz. Wavelength equals the speed of light divided by frequency, which gives about 22 m, 11 m and 7.5 m (some books round the last one to 7.4 m). Most clinical units run at 27.12 MHz. Microwave diathermy sits far higher, at 2450 MHz (wavelength about 12.25 cm) or 915 MHz (about 32.8 cm).

So the current never talks to the nerve. Who does it talk to instead?

The Heating Chain: From Oscillating Field to Warm Tissue

Diathermy means "heating through". In plain terms, the machine makes tissue warm itself from the inside, at the depth where the field arrives, rather than warming the skin and waiting for heat to creep inward.

So what does tissue contain that a field can grab? Fluid, and plenty of it. Body fluid carries charged particles called ions (sodium and chloride are 2 familiar ones), and every water molecule is polar: slightly positive at 1 end, slightly negative at the other, like a tiny magnet. When the field reverses 27.12 million times per second, the ions shuttle back and forth over tiny distances and the water molecules flip to keep up. Textbooks call that flipping dipole rotation. All this frantic motion means constant bumping into neighbours. Friction follows, and friction is heat.

The flowchart below traces the whole chain, from field to clinical effect. It also works as a 1-minute revision card.

Figure 2: How an alternating field becomes heat inside tissue and then a clinical effect.

Notice the order. Heat comes first, and every benefit follows from it. Warmer tissue widens its blood vessels, so more blood arrives. Warm collagen (the protein that makes tendons, ligaments and joint capsules stiff) stretches more easily. Warm muscle relaxes.

Clinical Note Textbooks usually place the therapeutic range at about 40 to 45 °C in the target tissue. Below that, effects are small. Above it, damage begins. You can't push a thermometer into a patient's muscle during treatment, so the patient's own sensation of warmth becomes the working gauge.

Heat forms wherever the energy gets absorbed. And where that happens depends on how the energy enters the body.

Capacitor Field, Coil Field and Microwave: Where Does the Heat Go?

Shortwave diathermy (SWD) delivers its 27.12 MHz energy in 2 ways, and microwave diathermy (MWD) takes a different route altogether. The trick is to link each method with the field it creates.

Start with the capacitor field method. Picture a sandwich: 2 metal electrodes on either side, the patient's limb as the filling. The electrodes stay off the skin, separated by an air gap or a layer of dry towelling, and together with the tissue they behave like a capacitor, which is where the name comes from. Between the plates, an alternating electric field sweeps back and forth across the limb.

Many students picture current streaming from 1 plate through the limb into the other, as along a wire. The reality is an alternating electric field crossing a gap, with the limb sitting inside the field as part of the circuit.

Classic teaching says this field heats high-resistance tissue most, so skin and subcutaneous fat warm first. Real patients vary with electrode size, spacing and fat thickness, so treat that as a tendency. Two handy rules: a smaller electrode concentrates heat beneath itself, and wider spacing sends the field deeper while closer plates keep it near the surface.

The inductive (coil) field method works differently. A cable coiled around the limb, or a flat drum-shaped electrode called a monode, carries the alternating current. Its changing magnetic field sets up small whirlpools of current inside the tissue, called eddy currents. Think of the coil as a stirrer: the limb is the glass and the eddy currents are the swirling water. Tissue rich in water and ions conducts best, so muscle and other fluid-rich tissue heat most.

Figure 3: Capacitor and coil fields compared, with the tissues that classic teaching says heat most.

Exam Pointer "Compare capacitor and inductive methods" is a viva favourite. The anchor to carry in: capacitor field means an electric field, and skin and fat warm first. Coil field means a magnetic field, and muscle warms most.

Now microwave diathermy. No electrodes, no coil. A magnetron (the same kind of tube inside a kitchen microwave oven) generates the waves, and an emitter beams them at the skin from a short distance, a bit like a torch beam. The frequencies are 2450 MHz and 915 MHz. Water-rich tissue absorbs them, and some energy bounces off at tissue boundaries. MWD heats more superficially than SWD. Depth figures differ between books, roughly 1 to 4 cm, and 915 MHz reaches deeper than 2450 MHz.

Feature	Capacitor field SWD	Coil field SWD	Microwave diathermy
Frequency	27.12 MHz	27.12 MHz	2450 or 915 MHz
Delivery	2 plates, air gap or dry towel	Cable, monode or diplode	Emitter beamed at skin, no contact
Energy form	Electric field	Magnetic field with eddy currents	Microwave radiation
Heats most	Skin and fat (classic teaching)	Muscle, fluid-rich tissue	Water-rich tissue, near the surface
Depth	Deeper than hot packs	Deeper than hot packs	More superficial than SWD

That settles where heat forms. Next comes the dose: how much heat should form, and which dials control it.

Dosage: Continuous, Pulsed and the Schliephake Scale

Good news first: the frequency is fixed, so you never dial it. What you control is the mode, the output, the treatment time and the tuning. Textbooks usually describe sessions of roughly 15 to 20 minutes, though trial protocols vary widely.

In continuous shortwave diathermy the machine delivers a steady stream of energy, and tissue heats. In pulsed shortwave diathermy (PSWD) the same energy leaves in short bursts with silent gaps between them. 3 settings describe the bursts: pulse duration (how long each burst lasts), pulse repetition rate (bursts per second, written as pps or Hz) and peak power (the strength during a burst). Machines differ. The examples on the Electrotherapy.org page on pulsed shortwave therapy range from a 65 microsecond pulse at 26 pps up to a 400 microsecond pulse at 400 pps.

Here's the number that really matters: mean power, the average energy delivered per second. Mean power equals peak power × pulse duration × repetition rate. Take illustrative numbers: 150 W peak, 400 microsecond pulses, 100 pps. That works out to 150 × 0.0004 × 100 = 6 W on average. Think of a hose that gushes hard for a split second and then rests. Each gush is strong, yet the lawn barely gets wetter. The same Electrotherapy.org page makes the point that pulse duration and rate matter mainly through the mean power they produce together.

Laboratory work backs up that logic. Mills and colleagues (2002) found that PSWD at a mean power of 48 W for 10 minutes increased fibroblast proliferation in cell culture. With the mean power held at 6 W, changing pulse duration and repetition rate made no significant difference. Cells in a dish aren't patients, so read this as a clue about mechanism, not proof of clinical benefit.

Students often read "pulsed" as "non-thermal". Pulsed only means the energy comes in bursts. Push peak power, pulse duration or repetition rate up and the mean power climbs into thermal territory.

To turn mean power into something you can judge at the bedside, textbooks use the Schliephake scale, printed for example in the Chattanooga Intelect Shortwave 400 instructions.

Dose	What the patient feels	Usual pairing in student texts
I	No perceptible heat (athermal)	Acute conditions
II	Slight warmth, just perceptible	Acute and subacute
III	Pleasant, comfortable warmth	Subacute and chronic
IV	Strong warmth, still tolerable	Chronic conditions

The dose is judged by what the patient feels, not by the number on the dial. The reason: the same dial setting heats differently with fat thickness, electrode position, spacing and how well the machine is tuned. Pairings in the third column vary between books, but the logic holds: acute inflammation already has plenty of blood flow, so it gets the gentlest dose.

What if the patient can't feel heat properly, as can happen with nerve damage from diabetes? The sensation gauge fails, nobody notices overheating in time, and burns follow. That single chain of logic is why impaired thermal sensation counts as a contraindication.

Exam Pointer Expect "Define the Schliephake dose scale" and "Continuous vs pulsed SWD". Short answers: continuous is thermal; pulsed can be athermal at low mean power; dose is graded I to IV by sensation.

Quick pause. What do we have so far? A fixed frequency, a field type that decides where heat forms, and a mean power plus patient sensation that decide how much. Hold those 3 pieces; the rest of the article hangs on them.

Dose sorted. Now for what the tissue gains from all that warmth.

What Does the Heat Do, and Who Benefits?

Everything diathermy achieves comes from heat, so the effects list reads like a list of what warmth does to living tissue. The table links each effect to its mechanism and to the place you'll meet it in clinic.

Effect	How it happens	Where you'll meet it
More blood flow	Heat widens blood vessels (vasodilation)	Subacute soft tissue injury, slow-healing tissue
Faster cell activity	Warmer cells react faster, so metabolic rate rises	Tissue repair
Stretchier collagen	Warm collagen lengthens more easily under load	Stiff joints before mobilisation or stretching
Less spasm and pain	Muscle relaxes as warmth rises and pain eases	Muscle spasm, chronic back pain

Textbook indication lists centre on subacute and chronic musculoskeletal problems: osteoarthritis, chronic low back pain, muscle spasm, subacute sprains and stiff joints. Which of these you'll treat most often differs between clinics.

Now the part exams love and clinics argue about: the evidence. Classic teaching credits PSWD with non-thermal effects on oedema and healing, and your exam probably expects that answer. Current evidence refines it. A 2012 systematic review and meta-analysis by Laufer and Dar pooled 7 trials in knee osteoarthritis. It found small but significant effects on pain and muscle performance only when SWD produced a local thermal sensation, and studies using low mean power did not favour SWD for pain. The authors also warned that protocols differed so much that firm conclusions were hard to draw.

Here's a trap. Heat feels soothing, so students assume it suits any painful, swollen tissue. A fresh injury already has extra blood flow, and adding more can increase swelling. That's why acute conditions get the gentlest doses, if SWD is used at all.

Every one of those benefits comes from heat. And heat in the wrong place is exactly what burns.

Contraindications and Safe Set-Up: Why Each Rule Exists

The contraindication list looks long, but it collapses into a few causes. Each rule exists because of something the field or the heat does. Learn the reason and the list stops being a memory task. An Irish survey of physiotherapists in Physiotherapy (2004) found agreement above 90% on metal implants, pacemakers, malignancy, tuberculous joints and the eyes as contraindications to continuous SWD, and on malignancy and pacemakers for pulsed SWD.

Contraindication	Reason
Metal in or near the field (implants, jewellery, metal furniture)	Metal conducts and concentrates the field, so it and the tissue beside it can overheat
Cardiac pacemaker or other implanted electronics	The field can interfere with the device
Malignancy	Heat and extra blood flow could encourage growth or spread
Pregnancy (field over abdomen, pelvis or low back)	The developing foetus is sensitive to heat
Impaired thermal sensation or poor circulation	The patient can't report overheating, and blood can't carry heat away
Wet dressings or sweat in the field	Water heats strongly, producing hot spots and burns
Eyes and testes	Heat-sensitive tissue with little cooling blood flow
Bleeding tendency or recent haemorrhage	Vasodilation may restart or worsen bleeding
Growing bone ends in children	Texts advise caution near growth plates

What if a patient has a metal plate under the treated area? The field crowds into the metal, and metal has no nerve endings to complain. Tissue around it can heat deeper than the patient's skin can sense in time. Hold that image and the metal rule explains itself.

Here is how a textbook set-up runs, written as learning content for what you'll one day practise under supervision:

1. Screen against the contraindications above and test the skin's sensation to hot and cold.
2. Remove metal and electronics from the field, and avoid metal furniture near the treated part.
3. Check the skin is dry, then cover it with dry towelling that can absorb sweat.
4. Position the electrodes or coil with the spacing the chosen method needs, and keep cables apart so they don't cross or touch the patient.
5. Tune the machine so the patient circuit resonates with the generator for good energy transfer (many modern units tune automatically).
6. Raise the output until the patient reports the intended Schliephake level, then recheck after a few minutes because sensation can fade.
7. Stay with the patient, check the skin at the end, and record the settings.

Operator safety follows the same logic. Texts advise keeping some distance from applicators and cables (figures vary between books and device manuals), keeping other electronic equipment away, and keeping anyone with a pacemaker out of the room. Hold on to those reasons, and the rules read as common sense.

Quick Recap: High-Yield Points

• High frequency currents sit above about 100 kHz. They never stimulate nerve or muscle; the effect is heat.
• Each phase at 27.12 MHz lasts about 18 nanoseconds, far too short for a membrane to charge.
• Shortwave diathermy: 27.12 MHz (wavelength about 11 m). Microwave diathermy: 2450 or 915 MHz.
• Heating chain: field, then ions and water molecules move, then friction, then heat, then more blood flow, looser collagen and less spasm.
• Capacitor field is electric, with skin and fat heating first. Coil field is magnetic, with eddy currents heating muscle most.
• Microwave diathermy needs no electrodes and heats more superficially than SWD.
• Mean power = peak power × pulse duration × repetition rate. Pulsed does not automatically mean athermal.
• Schliephake dose I to IV is judged by the patient's sensation, from no heat to strong but tolerable.
• Evidence in knee osteoarthritis points to benefit mainly when the patient feels warmth.
• Contraindications trace to metal, device interference, heat-sensitive tissue, moisture, and an inability to report heat.

Questions Students Ask About High Frequency Currents

What are high frequency currents in electrotherapy?

High frequency currents are alternating currents above about 100 kHz. Shortwave diathermy (27.12 MHz) and microwave diathermy (2450 MHz) are the main examples. They change direction so fast that nerve membranes never respond, so the patient feels no tingling and no contraction. What they deliver is heat, produced when ions and water molecules shake inside the field. They warm tissue and never stimulate it.

Why can't high frequency currents stimulate nerves or muscles?

Each phase at 27.12 MHz lasts only about 18 nanoseconds. A nerve membrane needs far longer, tens to hundreds of microseconds in stimulating pulses, to charge up to threshold. The phases end before any charge builds, so no action potential fires. The energy still gets used: it moves ions and rotates water molecules, and that motion appears as heat in the tissue.

What is the difference between capacitor and inductive shortwave diathermy?

A capacitor field uses 2 electrodes held off the skin and creates an electric field across the limb. Classic teaching says skin and fat heat first. An inductive field uses a coil or drum whose changing magnetic field drives eddy currents inside the tissue, and fluid-rich muscle heats most. Electric field means skin and fat, magnetic field means muscle. Real patients vary with electrode size, spacing and fat thickness.

Is pulsed shortwave diathermy thermal or non-thermal?

It depends on mean power. Pulsed shortwave diathermy delivers 27.12 MHz in short bursts, and mean power equals peak power times pulse duration times repetition rate. Low mean power gives no perceptible heat (Schliephake dose I). Raise the settings and mean power climbs into the thermal range. A 2012 meta-analysis in knee osteoarthritis found benefit mainly when the patient felt local warmth.

What frequency and wavelength does shortwave diathermy use?

Most units run at 27.12 MHz, which has a wavelength of about 11 m. Two other permitted bands exist: 13.56 MHz (about 22 m) and 40.68 MHz (about 7.5 m). You get wavelength by dividing the speed of light by the frequency. Microwave diathermy sits much higher, at 2450 MHz (about 12.25 cm) or 915 MHz (about 32.8 cm).

Is ultrasound a high frequency current?

No. Therapeutic ultrasound is a mechanical sound wave, usually 1 or 3 MHz, that needs a gel to pass into tissue. Shortwave and microwave diathermy use electromagnetic energy, which needs no medium. Both can produce heat, which is why some syllabi teach them together, but the physics differs. Keep them in separate boxes in your exam answers.

What are the main contraindications of shortwave diathermy?

Traditional lists include metal implants in the field, cardiac pacemakers, malignancy, tuberculous joints, the eyes, pregnancy over the abdomen, impaired thermal sensation or circulation, wet dressings or sweat, and bleeding tendency. An Irish physiotherapist survey found over 90% agreement on several of these for continuous shortwave. Each one traces back to metal, device interference, heat-sensitive tissue, or an inability to report heat.

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

1. 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.
2. Mills PM, Hill J, Lewis M, Keilty C. "Pulsed short-wave diathermy effects on human fibroblast proliferation." Arch Phys Med Rehabil. 2002;83(6):832-836
3. Electrotherapy.org. "Pulsed shortwave therapy." (2026).
4. "Contra-indications to shortwave diathermy: survey of Irish physiotherapists." Physiotherapy. March 2004. Abstract hosted by the Chartered Society of Physiotherapy.
5. Chattanooga. "Intelect Shortwave 400: indications and Schliephake dosage levels" (instructions for use 28256, Rev C, 2012). (2026).