High-frequency electromagnetic modalities. Biophysical principles and interactions with biological tissues

At a glance

1. High-frequency currents alternate more than 100,000 times a second (above 100 kHz). At these speeds the current cannot fire nerves or muscles, causes no polarisation or burns from electrolysis, and the skin no longer resists it.
2. The machine never touches the body as part of a circuit in the classic methods: the energy jumps across as an electric field (capacitive), a magnetic field (inductive) or a radiated wave (microwaves). TECAR is the modern exception, with electrodes on the skin.
3. In the tissues the energy becomes heat from inside: ions shake and collide, and water molecules twist back and forth. This is endogenous heat, deeper and longer-lasting than a hot pack.
4. Which tissue heats depends on the method: capacitive plates heat fat and other low-water tissue most; inductive coils and microwaves heat water-rich muscle and blood.
5. The family, from lowest to highest frequency: d'Arsonvalization (sparks on the skin, almost no heat), UHF or shortwave (27.12 MHz, or 40.68 MHz on Russian-school devices), decimetric waves and centimetric waves (microwaves). TECAR (about 0.5 MHz) sits outside this ladder.
6. Heat helps stiff, chronic, aching tissue and is dangerous where more blood or more metabolism would harm: acute inflammation, bleeding, tumours, poor circulation, lost sensation. Metal and implanted electronics in the field are the special danger of every heating method in this family.

01Physical characteristics

High-frequency electromagnetic therapy uses alternating current or fields that change direction more than 100,000 times a second. The course divides currents by frequency: low frequency up to 1 kHz, medium frequency from 1 to 100 kHz (see medium-frequency currents) and high frequency above 100 kHz.

Think of a skipping rope shaken by one end. Shake it slowly and the whole rope swings; shake it very fast and only short ripples run along it. Fast fields behave the same way: the faster they swing, the shorter their wavelength. Wavelength and frequency are tied by one rule: wavelength equals the speed of light divided by frequency. That is why the modalities are named after their wavelengths.

The therapeutic frequencies are not chosen freely: they are the bands set aside by international agreement for industrial, scientific and medical use, so the machines do not jam radio, aircraft or mobile phones.

How the current is made

Every high-frequency device has an oscillating circuit: a coil (an inductance, L) joined to a capacitor (C). Charge sloshes between them like water between two linked tanks, and it sloshes at one natural rate, f = 1 / 2π√(LC). Smaller coils and capacitors swing faster. Microwave devices use a magnetron instead, a tube with many tiny resonating cavities, the same part as in a kitchen microwave oven. The patient's circuit (plates, coil or antenna) is tuned to the machine, like a radio to a station, so that the energy passes into the body.

02Why the skin is no barrier and the nerves do not fire

High-frequency currents differ from low- and medium-frequency currents in four ways, and each one has a reason.

A nerve needs a push that lasts long enough to open its sodium channels (see electrotherapy essentials, chronaxie). One cycle at 27.12 MHz lasts about 37 billionths of a second, thousands of times too short. So the patient feels only warmth, and a strong current can flow without pain. This is why these currents could be used to heat the body at all.

03How the energy reaches the body

In galvanic and low-frequency therapy the body is a wire in the circuit. In classic high-frequency therapy it is not: the energy crosses the air gap or the insulation as a field. There are three ways.

1. 1

   Capacitive (electric field)

   The body part sits between two plates, like the filling between the two plates of a capacitor. The electric field swings back and forth through every layer between them. Used in UHF therapy and local d'Arsonvalization.

2. 2

   Inductive (magnetic field)

   A cable coil, a flat coil in a drum, or a solenoid that the limb sits inside carries the current. Its changing magnetic field induces small circular currents (eddy currents) in the tissue, as a transformer does. Used in UHF therapy; the Russian school calls inductive heating with a coil inductothermy.

3. 3

   Radiated (electromagnetic wave)

   An antenna in an applicator sends a beam of waves through the air, like a torch sends light. Used for decimetric and centimetric waves. The course calls this combined capacitive-inductive coupling, because a travelling wave carries both fields.

TECAR breaks the rule. It works at about half a megahertz with an active electrode moved over the skin and a large return plate, so the current runs through the body as in the old long-wave diathermy. Its insulated electrode works capacitively (CET), its bare metal electrode resistively (RET), with current passing straight through the tissue.

04What happens in the tissues

The absorbed energy becomes heat in two ways, which explain everything about which tissue heats.

• Ionic (conduction) losses. Ions in tissue fluid are shaken back and forth and bump into their neighbours. The more ions and water, the more this friction. This is how inductive fields heat: their eddy currents run best in good conductors such as muscle and blood.
• Dielectric (dipole) losses. A water molecule is a tiny magnet for electricity, a dipole, with a positive and a negative end. The field twists it one way, then the other, millions of times a second, and its rubbing against neighbours makes heat. This dominates at microwave frequencies, which is why a microwave oven heats wet food and leaves a dry plate cool.

Selectivity: which tissue heats most

The course's rule is short: capacitive plates heat low-water tissue, inductive coils and microwaves heat high-water tissue. The reason is in how each method passes through the layers.

So in a patient with thick subcutaneous fat, capacitive plates spend much of the dose warming fat, and an inductive coil or decimetric waves suit deep muscle better (centimetric microwaves, at 2450 MHz, can themselves overheat a thick fat layer). In a thin patient over a bony joint, capacitive plates reach the joint well.

Depth

The course gives these depths of useful effect:

d'Arsonvalization, the most superficial

5–8mmd'Arsonvalization, the most superficial

UHF (shortwave)

5–6cmUHF (shortwave)

decimetric waves, the deepest and broadest heat

6–9cmdecimetric waves, the deepest and broadest heat

centimetric waves, focused but shallower

5–6cmcentimetric waves, focused but shallower

Within the microwave range, the higher the frequency the shallower the heating, because more is absorbed near the surface and more is reflected, much as light is. So centimetric waves, despite their higher energy per photon, heat less deeply than decimetric ones. Western textbooks give shallower figures, especially for 2450 MHz microwaves, whose useful heating in muscle is only a few centimetres. The course's 2026 handnotes add a needed warning: these depths are rough. The real distribution depends on the power, the applicator and its distance, the thickness of fat, and blood flow, and a higher frequency never guarantees deeper or better treatment.

Is there an effect beyond heat?

Pulsed shortwave at a low mean power heats little, yet it was claimed to speed healing by non-thermal effects on cell membranes. Laboratory findings exist, but in patients the evidence is weak; a meta-analysis in knee osteoarthritis found small effects on pain only when the dose was warm enough to be felt.

05Endogenous and exogenous heat

Heat from outside (exogenous heat: hot packs, paraffin, mud, warm water, infrared lamps) is like warming a cold room through its wall: the wall gets hot first and the middle of the room last, if ever. Heat from inside (endogenous heat: high-frequency currents and ultrasound) is like a heater in the middle of the room.

The body cannot feel deep heat well, because most heat sensors are in the skin. That is why deep heating needs care in dose: the patient may feel only mild warmth while the deeper tissue is hot.

06Physiological effects of endogenous heat

Heat speeds up chemistry. Each degree of warming quickens most enzyme reactions by roughly a tenth, and the cells need more oxygen and fuel, so the vessels open to bring more blood. Everything else follows from that.

1. 1

   Metabolism rises

   Warmer enzymes work faster; the tissue uses more oxygen and makes more waste.

2. 2

   Vessels open

   Vasodilation and higher capillary pressure bring blood and carry the waste and heat away; fluid leaks a little more into the tissue.

3. 3

   Collagen softens

   Collagen in capsules, ligaments and scars stretches more easily at about 40 to 45 °C, so heat before stretching gains more range of motion.

4. 4

   Muscle relaxes, pain eases

   Muscle spindle firing falls, so muscle spasm eases, and warmth in the skin gates pain. Nerve conduction quickens.

Warming the tissue by about 1 °C is mild, 2 to 3 °C moderate and 3 to 4 °C vigorous. Above about 45 °C proteins start to break down and the tissue burns, which is also roughly where heat starts to hurt.

07The modalities compared

The four classic modalities form a ladder: as frequency rises, so do energy and the share of heat in the effect.

The details of shortwave and microwave in practice, with their electrodes, doses and full contraindications, are in shortwave and microwave diathermy. The two modalities without a topic of their own follow here.

Local d'Arsonvalization

A glass electrode filled with thin inert gas glows violet when a high-frequency, very high-voltage, very low current runs into it in bursts about 100 times a second. Tiny sparks jump to the skin. They do not heat; they prick the skin's touch receptors, open small vessels, and make a little ozone and nitrogen oxides, which kill surface bacteria.

• Electrodes: a ball or mushroom for skin areas, a comb for the scalp, a point for small spots and scars.
• Technique: contact (the electrode glides over the skin, labile, or rests on it, stable) or spark (held 1 to 2 mm away, stronger and more bactericidal). The electrode must not lift off during contact work: the many small sparks then merge into one big spark that can burn.
• Dose: set objectively in watts; 5 to 10 minutes, 10 to 15 sessions daily or on alternate days.
• Indications: itch, seborrhoea and dandruff, non-purulent chronic skin inflammation, slow-healing wounds and scars, neuralgia, muscle pain; and, because it adds no deep heat, varices, venous and arterial circulatory problems; the course also does not rule it out over metal implants.
• Contraindications: purulent processes (including acne), active tuberculosis, acute inflammation, bleeding tendency, tumours, fever, heart, lung or kidney failure, a pacemaker, the abdomen in pregnancy.

The evidence for d'Arsonvalization is old and thin; today it survives mainly in cosmetic practice.

TECAR

TECAR (capacitive and resistive energy transfer) is the newest member and not a higher form of shortwave. A machine at about 0.3 to 1.2 MHz drives current between a moving hand electrode and a large return plate under the body, with a conductive cream.

• Capacitive mode (CET), insulated electrode: the charge builds up under the electrode and heats the more superficial, water-rich soft tissue.
• Resistive mode (RET), bare electrode: current flows through the body and heats where resistance is high, so deeper fat, tendon and bone.

A study in healthy people found that it warms the skin and keeps it warm throughout the 45 minutes measured afterwards. It is used for musculoskeletal pain and stiffness and before exercise, but trials are small and claims of tissue regeneration are not established. Its cautions are those of any deep heating, plus poor contact: a lifted electrode or a small, dry return plate concentrates current and burns.

08Dosage in principle

Because deep tissue cannot report heat, the classic methods dose by what the patient feels, on a four-step scale named after Schliephake:

The scale is useful but crude: it measures the skin's feeling, not the deep temperature. Absent warmth does not prove absent heating, and a patient who cannot feel heat cannot be dosed this way at all. d'Arsonvalization alone is dosed objectively, by the watts on the device.

09Indications and contraindications: the logic

Every heating method in this family shares one set of rules, because they come from what heat does.

Two dangers are particular to electromagnetic heating and matter more than any other: metal concentrates the field and burns the tissue around it, and implanted electronics (pacemakers, defibrillators, neurostimulators) can be heated or upset. These, and the full lists, are set out in shortwave and microwave diathermy.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1A patient asks why the shortwave machine makes his knee warm but, unlike the TENS unit he used before, causes no tingling or twitching. What is the best explanation?

The current is too weak to reach the nervesEach cycle is far too short to depolarise a nerve membrane, so the ions only jiggle back and forthThe skin's high resistance blocks the current before it reaches the nervesThe heat blocks nerve conduction

2A thin knee and a knee under a thick layer of fat are both treated with capacitive plates at the same setting. Where is the heat greatest in the second patient?

In the muscle, because it holds most waterIn the joint cavityEvenly through all layersIn the subcutaneous fat

3According to the course, which method heats mainly water-rich tissues such as muscle and blood?

Inductive shortwave (UHF) and microwavesCapacitive shortwave (UHF) onlyLocal d'ArsonvalizationAny method, as long as the plates are close

4A patient with varicose veins and a slow-healing superficial wound on the shin wants a high-frequency treatment. Which one does the course allow here, and why?

Microwave diathermy, because it heats focallyCapacitive UHF, because it heats the skinLocal d'Arsonvalization, because it acts on the skin without deep heatNone of them, because all high-frequency currents heat

5Which of these is a property of endogenous (deep) heat compared with a hot pack?

It warms the skin far more than the deep tissuesIt reaches deeper, can favour some tissues over others and lasts longer after the sessionIt needs a conductive gel between the skin and the sourceIt loads thermoregulation more because the whole body warms