Sinusoidally modulated currents (SMC). Characteristics, physiological effects, application methods, dosage, indications, and contraindications
Sinusoidal modulated currents (SMC, amplipulse therapy) are a medium-frequency carrier whose strength the device swells and fades at a low frequency, in four working types. This page covers how they are built, what each type and setting does, how they are applied and dosed, and where they are and are not used.
- 9 min read
- 5 questions
- Checked 05 Nov 2026
Contents7 sections
At a glance
- SMC is a medium-frequency carrier, usually 5 kHz (2 to 10 kHz), whose strength the device swells and fades at 10 to 150 Hz. The carrier gets the current through the skin easily; the slow rhythm tells the nerves what to do.
- Four types: 1 continuous modulation (pain, the strongest but shortest relief), 2 modulation with pauses (muscle stimulation), 3 modulation alternating with plain carrier (pain relief that lasts longer), 4 two modulation frequencies alternating (the longest-lasting, gentlest relief).
- Modulation depth (0 to 100 per cent, and above): the deeper, the more stimulating. Frequency: about 100 Hz for pain, about 50 Hz for innervated muscle, 10 to 20 Hz for denervated muscle.
- Unrectified (bipolar) SMC has no direct component: gentler, allowed over metal. Rectified (monopolar) SMC flows one way: more stimulating, can carry drugs into the skin, but must not be used over metal.
- In the traditional scheme, 2 or 3 types per session, 3 to 5 minutes each, 10 to 20 minutes in all, a course of about 8 to 15 sessions.
- Indications and contraindications follow those of diadynamic current. Evidence is mostly from older eastern European practice.
Characteristics
What it is
Sinusoidal modulated currents (SMC) were developed in the Soviet Union in the 1960s, and the therapy is often called amplipulse therapy after the first device. They are one of the two classical medium-frequency currents; the other is interferential current.
The idea: take a fast alternating current that passes the skin comfortably, and make its strength swell and fade at a slow rhythm. The nerves cannot follow the fast current, but they follow the slow swelling. Think of a fast drum roll played louder and softer in a slow rhythm: you hear the slow rhythm. In SMC the device makes this rhythm, unlike classical interferential current, where it is made inside the tissue.
- the usual carrier (2 to 10 kHz)
- 5kHz
- the modulation frequency
- 10β150Hz
- modulation depth, and above 100
- 0β100per cent
The parameters
| Parameter | What it is | Usual range | What it changes |
|---|---|---|---|
| Carrier frequency | the fast current | 2 to 10 kHz, usually 5 kHz | comfort and passage through the skin |
| Modulation frequency | how many times a second the strength swells | 10 to 150 Hz | what the nerves do: pain relief or contraction |
| Modulation depth | how far the strength dips between swells | 0 to 100 per cent, and above 100 | how stimulating the rhythm feels |
| Type (kind of work) | the time pattern | types 1 to 4 | analgesic or stimulating, short or long effect |
| Burst and pause times | in types 2 to 4 | 1 : 1.5 s, 2 : 3 s, 4 : 6 s | the rhythm of contraction and rest, or of change |
| Mode | unrectified or rectified | bipolar or monopolar | gentle or stimulating; drug delivery |
Modulation depth
At 0 per cent the current is a plain carrier: nerves soon adapt and little is felt. At 50 per cent the strength dips to half between swells. At 100 per cent it dips to zero, so the current comes as separate bursts. Above 100 per cent (over-modulation), the bursts get shorter and the gaps longer, giving sharp, distinct bursts.
The rule from the book: the deeper the modulation, the stronger the stimulating (irritative) effect, and the shallower, the gentler. For pain relief, 50 to 75 per cent; for muscle stimulation, 100 per cent or above.
The four types
| Type | Pattern | Traditional effect | Main use |
|---|---|---|---|
| 1 Continuous modulation | the carrier modulated all the time at one frequency | the strongest but shortest pain relief, at about 100 Hz | acute pain |
| 2 Modulation with pauses | bursts of modulated current, then no current | the most stimulating: rhythmic contraction and rest | muscle stimulation, about 50 Hz for innervated, 10 to 20 Hz for denervated muscle |
| 3 Modulation alternating with plain carrier | bursts of modulated current, then the unmodulated carrier | weaker but longer-lasting pain relief, at 80 to 100 Hz | subacute and chronic pain, circulation |
| 4 Alternating frequencies | a chosen frequency (10 to 140 Hz) alternating with 150 Hz | the gentlest but longest-lasting pain relief | chronic pain, with less adaptation |
In types 2 to 4 the two parts of the cycle last 1 and 1.5 seconds, 2 and 3 seconds, or 4 and 6 seconds.
Point at a name or a part to light it.
Sinusoidal modulated current is one medium-frequency carrier, usually 5 kHz (2 to 10 kHz), whose strength the device swells and fades at 10 to 150 Hz: the carrier passes the skin easily, and the slow rhythm tells the nerves what to do. The four types are the same current in four rhythms. A steady rhythm works strongly, but nerves soon adapt (type 1); a changing one works more gently but for longer (types 3 and 4); real pauses let muscle contract and rest (type 2). The deeper the modulation, the more stimulating the current.
Simplified: time is squeezed: each drawn swell stands for about 25 real ones, and the carrier swings a few times in each swell instead of 50; the types are drawn at 75 per cent depth and type 2 at 100; each can be set to any depth; in type 3 the plain carrier is drawn at the swells' full strength; the rectified (one-way) form of each type is not shown.
2 sources
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Medium frequency currents, sinusoidal modulated current: types and deepness of the modulations.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 1 handnotes. Medical University of Sofia, 2026. 10, Sinusoidal modulated currents: four classical types, modulation depth, main parameters.
Unrectified and rectified mode
| Unrectified (alternating, bipolar in the book) | Rectified (monopolar in the book) | |
|---|---|---|
| How the current flows | both ways, symmetrically | one way only, as half-waves |
| Direct component | none | yes |
| Polar effects under electrodes | none | yes: acid under the anode, alkali under the cathode |
| Effect | less stimulating, more inhibitory | more stimulating, less inhibitory |
| Metal implants | allowed | not allowed |
| Drug delivery | no | yes, as with iontophoresis |
Because rectified SMC has a direct component, it can drive charged drugs into the skin, like iontophoresis; this is called amplipulsophoresis. It also carries the risks of galvanic current: chemical burns under small or dry electrodes, and harm around metal.
Physiological effects
SMC acts through the same physiology as low-frequency currents, because nerves follow the modulation, not the carrier. One cycle of a 5 kHz carrier is too short to fire a nerve; many cycles add up, and the nerve fires on each swell of strength.
| Effect | How it comes about | Settings |
|---|---|---|
| Pain relief | fast firing of thick touch fibres closes the pain gate in the spinal cord; strong stimulation also drives the brain's own pain control | 80 to 150 Hz, types 1, 3 and 4, moderate depth |
| Muscle contraction | motor nerves fire in bursts; muscle contracts and relaxes | type 2, about 50 Hz for innervated muscle, deep modulation |
| Better circulation, less swelling | rhythmic contraction pumps blood and lymph; local vessels open | types 2 and 3, lower frequencies, moderate depth |
| Relaxation of spasm | pain relief breaks the cycle of pain and spasm | types 3 and 4 |
| Trophic effect | more blood flow to tissues | types 3 and 4 |
Because the carrier passes the skin easily, SMC feels smoother than low-frequency pulses, and the skin's own sensory nerves are stimulated less. This lets a higher intensity reach the tissues below.
Application methods
Electrodes
- Two plate electrodes with moist pads, or self-adhesive pads, placed across or along the area, as for other currents. The lecture describes the bipolar arrangement with two similar electrodes, and the monopolar one with a small active electrode and a larger dispersive one.
- Transverse placement (on opposite sides of a joint) for deep targets, longitudinal (along a limb or muscle) for nerves and muscles.
- Over a muscle's motor point for stimulation.
- Over the spinal segment or the painful area for pain.
- For amplipulsophoresis, the drug-soaked pad goes under the electrode with the same charge as the drug, in rectified mode.
Procedure
- 1
Check
Contraindications, skin, sensation tested with sharp and blunt touch, metal in the area. - 2
Choose
Mode (unrectified unless drug delivery or a strong stimulating effect is wanted and there is no metal), types, frequency, depth and burst times for the aim. - 3
Raise intensity
From zero, slowly, to a clear, comfortable vibration or tingling for pain; to a visible, tolerated contraction for muscle. - 4
Change type
Usually two or three types in one session, switched without changing the intensity. - 5
Finish
Intensity back to zero, electrodes off, skin checked.
Dosage
| Parameter | Pain relief | Muscle stimulation |
|---|---|---|
| Mode | unrectified | unrectified, or rectified if no metal |
| Types | 1, then 3 or 4 | 2 |
| Modulation frequency | 80 to 150 Hz, often 100 Hz | about 50 Hz innervated, 10 to 20 Hz partially denervated |
| Modulation depth | 50 to 75 per cent | 100 per cent or more |
| Burst and pause | 2 : 3 s or 4 : 6 s | 1 : 1.5 s or 2 : 3 s |
| Intensity | a strong, comfortable vibration | a visible contraction, not painful |
| Time | 3 to 5 minutes per type, 10 to 20 minutes in all | 10 to 20 minutes |
| Course | daily or every other day, about 8 to 15 sessions | daily, about 10 to 15 sessions |
These timings come from the eastern European amplipulse tradition; schools vary (courses of 6 to 10 sessions are common too) and no trial has fixed them. As a rule: in acute pain use higher frequencies, shallower modulation, shorter sessions; as pain settles, move to lower frequencies, deeper modulation and types that stimulate more.
Indications
The book gives SMC the indications of diadynamic current, including stimulation of muscles that still have their nerve. Grouped by what the current does:
| Because it⦠| It is used for |
|---|---|
| relieves pain | neuralgia, radicular pain from the spine, osteoarthritis, periarthritis of the shoulder, epicondylitis, back and neck pain |
| makes muscles contract | weakness and wasting after immobilisation, injury or surgery; partially denervated muscle; atony of bowel or bladder in the old tradition |
| improves circulation and moves fluid | post-traumatic swelling, sprains, contusions, peripheral circulatory disorders without thrombosis |
| relaxes muscle | painful muscle spasm |
| can carry drugs, in rectified mode | local delivery of analgesic or anti-inflammatory drugs, as with iontophoresis |
| passes the skin easily and has no direct component, in unrectified mode | areas with metal implants; patients who cannot tolerate low-frequency currents |
Contraindications
The contraindications are those of other electrical currents (the book refers back to galvanization and diadynamic current), with the logic of where the current goes and what it can worsen.
| Reason | Contraindication or care |
|---|---|
| Electronic implants | pacemaker, implanted defibrillator, other stimulators |
| Sensitive regions | the pregnant uterus and the front of the neck; across the chest in heart disease; across the head |
| More blood flow or movement could spread disease or bleeding | active cancer, deep vein thrombosis or thrombophlebitis, active infection, tuberculosis, suppuration, bleeding tendency, acute inflammation or trauma in its first days |
| Unreliable feedback | numb skin, confusion, inability to communicate |
| Skin | wounds, skin disease, fresh scars |
| General | fever, severe heart, lung or kidney failure, uncontrolled epilepsy |
| Rectified mode only | metal in the area, and allergy to the drug in amplipulsophoresis |
Safety
- Use unrectified mode by default; choose rectified only for a reason and never over metal.
- In rectified mode, treat it like galvanic current: wet pads of good thickness, no burning sensation allowed, skin checked afterwards.
- Raise and lower intensity slowly; switch types without changing the intensity setting, then adjust if needed.
- Keep the stimulator well away from a working diathermy machine (device manuals often say at least 3 m), and never use both on a patient at the same time.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1You want to make a weak but innervated quadriceps contract and relax rhythmically with SMC. Which type and setting fit best?
2A patient has a metal plate in the ankle. Which form of SMC must not be used over it?
3What happens when the modulation depth of SMC is raised from 50 to 100 per cent?
4Why does type 4 SMC alternate between two modulation frequencies?
5What makes it possible to deliver a drug through the skin with SMC (amplipulsophoresis)?