Medium-frequency currents. Physical characteristics, physiological and therapeutic effects. Dosage, indications, contraindications, and safety considerations
Medium-frequency currents are alternating currents of about 1 to 10 kHz that pass the skin easily and are shaped by a slow rhythm to make nerves and muscles respond. This page covers their physics, why nerves react to them as they do, their kinds and effects, how they are dosed, and where they are and are not used.
- 10 min read
- 5 questions
- Checked 05 Nov 2026
Contents6 sections
At a glance
- Medium-frequency currents are alternating currents of about 1 to 10 kHz (the book's class spans 1 to 100 kHz).
- They pass the skin easily, because the skin behaves like a capacitor whose opposition falls as frequency rises. So they are more comfortable; the traditional claim that they also reach deeper is not reliably shown.
- A nerve cannot fire with every cycle. It responds to a slow rhythm in the current's strength, the modulation. So a medium-frequency current acts like the low-frequency current it carries: about 100 Hz relieves pain, 20 to 50 Hz makes muscles contract.
- The main kinds: sinusoidal modulated current (rhythm made in the device), interferential current (rhythm made by two currents meeting), Russian current and Aussie current (bursts, for muscle).
- A symmetrical alternating current has no direct component, so it does not polarise the skin, burns less, and is traditionally allowed over metal implants.
- Not over a pacemaker, a pregnant uterus, the front of the neck, a tumour, a thrombosis or an infection. Evidence for lasting benefit in pain is weak.
Physical characteristics
Where they sit among currents
Electrotherapy currents are sorted by frequency, the number of times a second the current changes.
| Class | Frequency | Examples | How it acts |
|---|---|---|---|
| Low frequency | up to 1 kHz | TENS, diadynamic currents, NMES | each pulse can fire a nerve |
| Medium frequency | 1 to 100 kHz, in practice 1 to 10 kHz | SMC, interferential current, Russian current | fires nerves only through a slow rhythm in its strength |
| High frequency | above 100 kHz | shortwave diathermy | does not fire nerves at all; heats |
Medium-frequency currents sit in the middle, and they have something of both neighbours: like high-frequency currents they pass the skin easily, and like low-frequency currents they can fire nerves, once they are given a rhythm.
The carrier and its modulation
The basic current is a smooth sine wave, the carrier, usually 2 to 5 kHz (often 4000 Hz). On its own it does little that a patient can feel for long. To make it useful, its strength is made to swell and fade at a low frequency, the modulation or beat frequency, from about 1 to 150 Hz. Picture a fast hum that is turned up and down in a slow rhythm; the ear, like the nerve, follows the slow rhythm.
There are two ways to build the rhythm:
- Made in the device, from one medium-frequency and one low-frequency current: sinusoidal modulated current, and premodulated interferential current. The book calls this exogenous derivation.
- Made in the body, where two medium-frequency currents of slightly different frequency cross and add up: classical interferential current. The book calls this endogenous derivation.
The skin as a capacitor
The skin's outer layer is a poor conductor sandwiched between conducting layers, so it acts like a capacitor. A capacitor blocks direct current but lets alternating current through, and the faster the current alternates, the more easily it passes. Its opposition, the capacitive reactance, is inversely proportional to frequency. Real skin is not a pure capacitor, so the actual fall in impedance is smaller than this simple sum suggests, but the direction is right.
Point at a name or a part to light it.
The skin's outer horny layer is a poor conductor between two good ones, the electrode's pad and the salty tissue, so it acts like a capacitor, which lets an alternating current through more easily the faster it alternates. So the skin opposes low-frequency currents strongly and medium-frequency currents much less: these need less voltage, stimulate the skin's nerves less, and a stronger current can be given comfortably.
Simplified: real skin is not a pure capacitor: a pure one's opposition would be 80 times lower at 4000 Hz than at 50 Hz, while the skin's falls less; the curve has no scale and is drawn from a simple model, not measured; real skin varies widely between people, places and how moist it is; the horny layer is drawn far thicker than it is, and the living skin and the tissues under it as one.
5 sources
- Ward AR. Electrical stimulation using kilohertz-frequency alternating current. Physical Therapy, 2009.
- Watson T. Interferential therapy. Electrotherapy on the web (electrotherapy.org), accessed 2026. The skin impedance at 50 Hz and at 4000 Hz.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 1 handnotes. Medical University of Sofia, 2026. Comparison with low-frequency currents: skin impedance and patient comfort.
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Electrotherapy, types of electric current; medium frequency currents.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Chapters on interferential therapy and electrical stimulation.
- the carrier in practice, often 4 kHz
- 1–10kHz
- the modulation the nerves respond to
- 1–150Hz
- skin capacitive opposition at 4000 Hz than at 50 Hz, in theory
- about 80×lower
Physiological and therapeutic effects
How nerves respond
A nerve fibre needs a certain charge to fire, and after firing it cannot fire again for about a millisecond, the refractory period. One cycle of a 4000 Hz current lasts a quarter of a millisecond, half of it in each direction. That is too short to fire the nerve by itself.
- 1
Summation
Many cycles in a row add up their small effects until the nerve fires. This is called the Gildemeister effect. - 2
Following the rhythm
If the current's strength swells and fades, the nerve fires on each swell. So it fires at the modulation frequency, not the carrier frequency. - 3
Adaptation and block
If the current is unmodulated and stays strong, the nerve fires irregularly at first, then adapts and stops responding. At high strength, a kilohertz current can even block conduction in the nerve.
This explains the book's claim that unmodulated medium-frequency current "does not irritate the receptors": the feeling fades within seconds as the nerves adapt. It also explains why the effects follow the modulation frequency, as they would with low-frequency pulses.
What the effects are
| Modulation | What the nerves do | Effect | Older term in the book |
|---|---|---|---|
| about 80 to 150 Hz, often 100 Hz | sensory fibres fire fast, closing the pain gate | pain relief | inhibitory, analgesic |
| about 20 to 50 Hz, in bursts with pauses | motor fibres fire, muscles contract and relax | muscle training, pumping of oedema, improved blood flow | irritative, excitomotor |
| about 1 to 10 Hz | muscles twitch, small vessels open | circulation, relaxation of spasm | trophic |
| changing across a wide range | mixed sensory and motor effects | combined effects, less adaptation | trophic and revulsive (counter-irritant hyperaemia) |
Compared with low-frequency currents
The professor's book lists six advantages over low-frequency currents. Each follows from the physics above:
| Claimed advantage | Why | How well it holds |
|---|---|---|
| Easier passage through the skin | the skin's capacitive opposition falls with frequency | true |
| Higher intensity with less risk of chemical burn | symmetrical alternating current has no net direct component | true for unrectified currents |
| No polarisation | the same | true for unrectified currents; rectified SMC does polarise |
| No irritation of receptors when unmodulated | nerves adapt to a steady kilohertz current | true at moderate strength |
| A wider gap between a motor contraction and pain | less sensory stimulation at the skin | partly; it depends on burst length and frequency |
| A deeper effect | less skin opposition, so more current reaches deep tissue | not reliably shown: depth depends on electrodes, tissues and intensity, and studies disagree |
The kinds
| Kind | What it is | Main use |
|---|---|---|
| Sinusoidal modulated current (SMC, amplipulse) | a 2 to 10 kHz carrier, often 5 kHz, modulated 10 to 150 Hz in the device, four working types | pain, muscle stimulation, drug delivery in its rectified form |
| Interferential current (IFC) | two carriers near 4 kHz differing by 1 to 200 Hz, crossing in the tissue, or premodulated in the device | pain, oedema, muscle stimulation |
| Russian current | a 2500 Hz carrier cut into 50 bursts a second (10 ms on, 10 ms off) | strong muscle contraction for strengthening |
| Aussie current | a 1 or 4 kHz carrier in short bursts of 2 to 4 ms | 1 kHz for muscle strength, 4 kHz for comfortable sensory stimulation |
Russian current was introduced by Kots in the 1970s, with claims of strength gains of 30 to 40 per cent in trained athletes. These were never reproduced. It strengthens muscle about as well as other neuromuscular stimulation. A common scheme is 10 seconds of contraction, 50 seconds of rest, for 10 minutes (10/50/10).
Dosage
The dose of any medium-frequency current is a set of choices, each with a reason.
| Parameter | Usual choice | Why |
|---|---|---|
| Carrier | 2 to 5 kHz; 4 kHz for comfort, lower (1 to 2.5 kHz) for strong muscle contraction | lower carriers fire motor nerves more efficiently but feel less pleasant |
| Modulation frequency | 80 to 150 Hz for pain, 20 to 50 Hz for muscle, 1 to 10 Hz for circulation | the nerve follows the modulation |
| Modulation depth | 50 to 100 per cent | deeper modulation gives a stronger, more stimulating rhythm |
| Intensity | to a strong but comfortable sensation for pain; to a visible, tolerated contraction for muscle | sensation and contraction are the measure; mA differs between people |
| Time | 10 to 20 minutes, sometimes 2 or 3 programmes of a few minutes each | longer sessions add little |
| Course | daily or every other day, traditionally about 8 to 15 sessions | as for other electrotherapy courses; no trial has fixed the number |
Intensity is usually raised again after a few minutes, as nerves adapt and the sensation fades. If a rhythm is changing (a sweep), the patient feels it come and go, which is normal.
Indications
Grouped by what the current does:
| Because it… | It is used for |
|---|---|
| relieves pain | musculoskeletal pain: osteoarthritis, back and neck pain, radicular pain, neuralgia, periarthritis of the shoulder, epicondylitis |
| makes muscles contract | weakness after immobilisation or surgery, disuse atrophy, stress urinary incontinence (pelvic floor), partially denervated muscle |
| helps circulation and fluid move | post-traumatic oedema, sprains and contusions after the first days, slow-healing soft-tissue injury |
| relaxes spasm | muscle spasm with pain |
| passes the skin with little irritation | patients who cannot tolerate low-frequency currents; areas with metal implants, when the current is unrectified |
The evidence is modest. Medium-frequency currents relieve pain during and soon after treatment, but trials rarely show lasting benefit, and NICE advises against interferential therapy for low back pain. They are best used to make active exercise possible, not instead of it.
Contraindications
Again, the logic is where the current goes and what it can disturb.
| Reason | Contraindication or care |
|---|---|
| The current can disturb an electronic implant | pacemaker or implanted defibrillator, other implanted stimulators |
| The current can reach sensitive organs | over the pregnant uterus or the front of the neck (carotid sinus); current paths across the abdomen and pelvis in pregnancy; across the chest in heart disease; across the head |
| More blood flow and movement could spread or worsen disease | active cancer in the area, deep vein thrombosis or thrombophlebitis, active infection, acute inflammation, bleeding tendency or recent haemorrhage |
| The patient cannot judge the sensation | numb skin, confusion, inability to communicate |
| The skin cannot take the current | open wounds, skin disease, fresh scars under the electrodes |
| General state | fever, severe heart failure, uncontrolled epilepsy |
The book lists the contraindications of medium-frequency currents as those of galvanization, but allows metal implants for the unrectified forms. This is the key difference: rectified (monopolar) forms have a direct component, so they are not used over metal and can burn like galvanic current.
Safety
- Check the skin and sensation before each session, with a test on the healthy side if feeling is reduced.
- Start at zero, raise slowly, and stop on any pain or burning.
- Keep the stimulator well away from a working shortwave or microwave diathermy machine (device manuals often say at least 3 metres), because its field can disturb the stimulator's output; never use both on a patient at the same time.
- Use clean, moist sponges or gel pads of the right size; a small, dry contact concentrates the current and can burn.
- Look at the skin after treatment; slight redness is normal, a blister is not.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1Why does a 4000 Hz current pass the skin more comfortably than a 50 Hz current of the same strength?
2Why is a medium-frequency current modulated at a low frequency for most treatments?
3A patient has a plate in the forearm after a fracture. Which of these electrical treatments over the plate is the most acceptable?
4What is Russian current?
5A woman 20 weeks pregnant has pain over the sacroiliac joint. Why is medium-frequency current with electrodes across the lower trunk avoided?