Magnetotherapy. Low- and high-frequency magnetic fields. Biological and therapeutic effects, application methods, dosage, indications, and contraindications
Treatment with magnetic fields. What a magnetic field is and how it is measured, the static, alternating and pulsed low-frequency fields and the high-frequency and high-intensity ones, what they do to cells and what they have been shown to do for patients, and how they are applied, dosed and ruled out.
- 9 min read
- 5 questions
- Checked 05 Nov 2026
Contents7 sections
At a glance
- A magnetic field is measured as flux density in tesla. Therapeutic low-frequency fields are microtesla to millitesla, far weaker than an MRI scanner (1.5 to 3 T).
- Low-frequency magnetotherapy uses static fields (permanent magnets), alternating (sinusoidal) fields and, most often, pulsed fields (PEMF), usually at 1 to 100 Hz. It does not heat.
- A changing magnetic field induces small electric fields and currents in tissue (Faraday's law). Effects on ion channels, signalling and growth factors are shown in the laboratory; in patients the evidence is modest.
- The best evidence is for bone healing in delayed union and non-union, and for pain in osteoarthritis, and even there it is weak to moderate. The field passes through plaster, clothes and dressings.
- High-frequency magnetic fields act differently: inductothermy heats like shortwave diathermy, and high-intensity magnetic stimulation (about 1 T) fires nerves through the skin with little pain.
- Ruled out: pacemakers and other implanted electronics, pregnancy, a tumour in the field, active bleeding. A metal implant rules out diathermy, but not automatically low-frequency magnetotherapy.
Physical characteristics
Magnetotherapy treats with magnetic fields. A magnetic field is the invisible pull around a magnet or around any wire carrying current; iron filings show its lines. Its strength is the magnetic flux density (B), measured in tesla (T). One tesla is a lot, so therapy speaks in millitesla (mT) and microtesla (µT).
- the Earth's field
- about 0.05mT
- low-frequency magnetotherapy
- µT–mTrange
- a hospital MRI scanner
- 1.5–3T
- peak of a magnetic stimulator, for a fraction of a millisecond
- about 1–2.5T
The fields are sorted by how they change in time.
| Field | How it changes | Typical B | Typical frequency | Source |
|---|---|---|---|---|
| Static (constant) | not at all | mT | 0 Hz | permanent magnets |
| Alternating | swings smoothly back and forth (sinusoidal) | µT to mT | 1 to 100 Hz, up to kHz | coil with alternating current |
| Pulsating | sinusoidal half-waves in one direction | µT to mT | up to 100 Hz | coil with rectified current |
| Pulsed (PEMF) | short square or trapezoid pulses with pauses | µT to mT at peak | 1 to 100 Hz; pulses of microseconds to milliseconds | PEMF devices, bone growth stimulators |
| Travelling (running) | the field moves along a row of coils | mT | low | Russian multi-coil devices |
Low-frequency magnetotherapy covers fields from 0 to about 100 Hz (some devices to a few kHz). High-frequency magnetic fields are a different family, in two forms:
- Inductothermy: the magnetic field of a coil carrying shortwave current (13.56 or 27.12 MHz). Its eddy currents heat water-rich muscle and blood, so it is really a form of shortwave diathermy and follows its rules (see shortwave and microwave diathermy).
- High-intensity magnetic stimulation: a capacitor discharges into a coil, and a field of about 1 T or more rises and falls within a fraction of a millisecond. It induces a current strong enough to fire nerves with little pain, because the current is made inside the tissue and does not have to be pushed through the skin and its pain receptors. Over the brain it is transcranial magnetic stimulation (see brain stimulation in psychiatry); over nerves and muscles it is peripheral magnetic stimulation.
Penetration
A magnetic field passes almost unhindered through skin, fat, bone, clothing, plaster and dressings, because none of them is magnetic. It weakens quickly with distance from the coil, and its shape depends on the coil: a small ring gives a shallow field, a large one a deeper and wider field, and a figure-of-eight coil a focused one.
Point at a name or a part to light it.
A coil carrying pulsed current makes a magnetic field that passes through skin, muscle, bone and even plaster as if they were not there, because none of them is magnetic, so the coil can sit over a cast. The field itself does nothing to still tissue; while it changes, it induces a weak electric field and small currents in the tissue (Faraday's law), which the cells may respond to. Usually 1 to 100 pulses a second, it gives no meaningful heat. Its best-studied use is a fracture slow to unite, and even there the evidence is weak.
Simplified: not to scale: the leg, its layers, the cast and the coil are drawn to fit, and the coil is shown with only a few turns; only a few field lines are drawn, as a solenoid's field looks in empty space; the tissue barely changes it; the induced currents are drawn as two loops ringing the limb; they flow in rings round the axis all along the inside of the coil, and the dashes stand for weak, not for broken; the pulses are a sketch: devices differ in shape, width and rate, and their pulses come in bursts; the blocks of induced current are drawn flat because the drawn ramps are straight.
5 sources
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Ultra-high frequency therapy, inductive electrodes (cables and solenoids).
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 2 handnotes (magnetotherapy). Medical University of Sofia, 2026.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2020. Chapter on pulsed electromagnetic fields.
- Halliday D, Resnick R, Walker J. Fundamentals of Physics, 10th edition. Wiley, 2014. Induction and inductance: Faraday's law; the magnetic field of a solenoid.
- Griffin XL, Costa ML, Parsons N, Smith N. Electromagnetic field stimulation for treating delayed union or non-union of long bone fractures in adults. Cochrane Database of Systematic Reviews, 2011.
Biological effects
The course's 2026 handnotes separate three levels of certainty, and the distinction is worth keeping.
- 1
Established physics
A changing magnetic field induces an electric field and small eddy currents in tissue (Faraday's law). The faster B changes, the bigger the induced field. At therapeutic low frequencies this is non-ionising and non-thermal: no bonds are broken and the tissue is not heated. A static field induces nothing in still tissue; it only exerts tiny forces on moving ions, such as those in flowing blood.
- 2
Proposed cellular mechanisms (laboratory and animal studies)
Changes in calcium entry through voltage-gated channels and in membrane potential; activation of second messengers and growth signalling; more growth factors (BMPs, TGF-β, VEGF) and markers of bone formation; less of the inflammatory cytokines TNF-α and IL-1β; effects on mitochondria and on new vessels.
- 3
Clinical effects (in patients)
Strongest for bone healing, promising but modest for pain in osteoarthritis, and variable for everything else.
Therapeutic effects
The course lists these effects, each with its proposed reason:
| Effect | Proposed reason |
|---|---|
| Osteogenic (bone healing) | induced currents mimic the small electrical signals of loaded bone and stimulate osteoblasts and callus |
| Analgesic | less sensitisation of pain pathways and less inflammation |
| Anti-inflammatory and anti-oedema | lower inflammatory cytokines, better lymph and microcirculation |
| Trophic and regenerative | growth factors, new vessels, collagen synthesis in tendons and skin |
| Chondroprotective (proposed) | chondrocyte activity and matrix synthesis in cartilage, seen in laboratory work |
What the evidence shows
| Use | Evidence in patients |
|---|---|
| Delayed union and non-union of long bones | small placebo-controlled trials suggest more fractures unite, but the pooled result is not statistically certain; used in specialist practice |
| Acute fractures, after osteotomy and spinal fusion | adjunct use; evidence limited |
| Osteoarthritis | a Cochrane review of nine small trials found it may give moderate pain relief; no clear gain in function or quality of life |
| Low back pain, tendinopathies, soft tissue injuries | low to moderate; small effects at best |
| Chronic wounds and diabetic foot | adjunct; limited evidence |
| Osteoporosis, complex regional pain syndrome | inconsistent or little evidence |
Application methods
- Large circular applicator (tunnel): the patient lies inside a ring for the trunk, pelvis or whole limb.
- Local applicators: a small ring for a joint or soft-tissue spot, a figure-of-eight coil for a focused field, a cylinder (solenoid) that the limb passes through, and flat rectangular or flexible pads that follow the body's shape.
- Through plaster and dressings: the coil is placed over the cast; the field passes, though it is weaker at the target if the coil is further away.
- Bone growth stimulators: small coils built into a strap or brace, worn at home.
The coil is placed so that the target lies in the strongest part of the field, close to the coil, and the distance prescribed by the device is kept. The patient feels nothing during low-frequency treatment and can lie clothed.
Dosage
There is no single dose: each device and indication has its own validated protocol, and the course asks that every prescription record what was given so that it can be repeated.
| Parameter | What it means | Usual range |
|---|---|---|
| Flux density | field strength at the target | µT to tens of mT |
| Frequency | pulses or cycles per second | 1 to 100 Hz |
| Waveform | sinusoidal, rectangular, trapezoidal, static | by device |
| Pulse width and duty cycle | how long each pulse lasts; share of time on | µs to ms; per cent |
| Session time | minutes per session | often 15 to 30 min with clinic devices; by protocol |
| Course | number and frequency of sessions | often 10 to 20, daily; by protocol |
Bone growth stimulators are different: they are worn for several hours every day for months, until the fracture unites.
Indications
The indications follow the effects: bone that will not heal, joints and soft tissue that hurt, tissue that is swollen or slow to repair.
- Bone: fractures with delayed union or non-union, after osteotomy and spinal fusion, slow-healing fractures in a cast.
- Joints: osteoarthritis, especially of the knee, as an adjunct to exercise.
- Soft tissue and spine: tendon, ligament and muscle injuries, tendinopathies, low back pain, muscle spasm.
- Circulation and skin: post-traumatic oedema, chronic wounds and diabetic foot ulcers, as an adjunct.
- High-intensity stimulation: pelvic floor training in urinary incontinence, spasticity and pain; rTMS after stroke, where European guidelines rate low-frequency rTMS over the unaffected hemisphere as definitely effective (level A) for hand motor recovery in the post-acute stage, and high-frequency rTMS over the affected side as probably effective (level B).
Contraindications
Two things matter here: what a field does to devices, and what extra blood flow and cell activity would do.
| Because... | Contraindication or precaution |
|---|---|
| the field switches and disturbs electronics | pacemakers, implanted defibrillators, neurostimulators, cochlear implants, insulin pumps: avoided unless the manufacturers clear it. A magnet over a pacemaker switches it to fixed-rate pacing |
| the fetus should not be exposed without need | pregnancy, in the field |
| growth and regeneration could feed a tumour | malignancy in the treatment area |
| more microcirculation may bleed | active bleeding, bleeding tendency (caution with anticoagulants) |
| treatment must not delay the right one | acute infection and fever, unexplained pain or a complicated fracture, treated first |
| a strong field moves loose iron | ferromagnetic fragments, such as metal in the eye, with strong fields |
| the course adds | severe heart disease and low blood pressure (decided case by case), epilepsy (low risk with low-frequency fields; follow the device), children and very old people (care and supervision), lost sensation (applicator pressure or heat unnoticed) |
What is not automatically a contraindication with low-frequency fields: a passive metal implant (plate, joint replacement), because the field hardly heats; and an open wound, if the applicator is clean and the skin protected. This is a key contrast with diathermy. For high-intensity magnetic stimulation and inductothermy, metal in the field is ruled out, as for diathermy.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1How does a pulsed low-frequency magnetic field act on tissue, according to established physics?
2A man with a cardiac pacemaker asks for magnetotherapy for knee osteoarthritis. What is the main reason it is ruled out?
3A woman has a tibial fracture that has not united after six months and is in a cast. Why is a pulsed electromagnetic field device a practical choice for her, and what does the evidence show?
4A patient has a plate in the forearm. Which statement fits the course's 2026 teaching?
5Which of these is a high-frequency magnetic field treatment?