Transcutaneous electrical nerve stimulation (TENS). Mechanisms of analgesic action, principal stimulation modes and parameters, therapeutic parameters, dosage, indications, and contraindications
TENS sends small, short pulses of current through the skin to switch pain down in the spinal cord and brainstem. This page explains how it works, its modes and settings, how to dose it, where it helps, and where it must not be used.
- 12 min read
- 5 questions
- Checked 05 Nov 2026
Contents7 sections
At a glance
- TENS sends short electrical pulses through skin electrodes to reduce pain. It treats the feeling of pain, not its cause.
- It works mainly by the gate: pulses fire the thick touch fibres, and their signals close a gate in the spinal cord against pain signals. Strong, slow pulses also wake the brain's own pain-control system and its endorphins.
- Conventional TENS: about 50 to 150 Hz, short pulses of about 50 to 200 µs, a strong but comfortable tingling, no twitch. Fast, short relief. Acupuncture-like TENS: about 2 to 4 Hz, longer pulses of about 100 to 400 µs, strong enough to make muscles twitch. Slower, longer relief.
- The patient sets the intensity. Sessions last 20 to 60 minutes and can be repeated through the day. Pulses that vary help against tolerance.
- It is used for acute and chronic musculoskeletal pain, pain after surgery, labour pain and neuropathic pain. It helps while it runs; long-term benefit is unproven.
- No TENS near a pacemaker or defibrillator without cardiology advice, never over the front of the neck, the eyes or a pregnant uterus, and not on numb or broken skin.
What TENS is
Transcutaneous electrical nerve stimulation (TENS) means "stimulating nerves with electricity through the skin". A small battery box, about the size of a phone, sends pulses of current through two or four sticky pads placed on or around the painful area.
The idea is simple. Pain reaches the brain as signals in thin nerve fibres. If you flood the same part of the spinal cord with a harmless signal, the pain signal gets less of a hearing. It is the same thing you do without thinking when you bump your elbow and rub it: the rubbing helps.
TENS belongs to the low-frequency pulsed currents (pulses up to 1000 per second). Its own job is pain relief, which older texts call electroanalgesia. For muscle strengthening the right tool is NMES, not TENS.
Mechanisms of analgesic action
The gate in the spinal cord
In 1965 Melzack and Wall proposed the gate control theory. In the dorsal horn of the spinal cord, pain signals from thin fibres (Aδ and C) are passed on to the cells that carry them up to the brain. Small inhibitory cells in the dorsal horn work as a gate. Signals from thick, fast touch fibres (Aβ) excite these inhibitory cells, and they close the gate.
- 1
Pulses fire touch fibres
Short pulses at low intensity fire the thick Aβ fibres first, because thick myelinated fibres are the easiest to excite. - 2
The gate closes
Aβ signals switch on inhibitory interneurons in the dorsal horn, which release GABA and enkephalin onto the pain-relay cells. - 3
Less pain goes up
The pain-relay cells fire less, so less pain reaches the brain. The patient feels tingling instead of aching.
Point at a name or a part to light it.
Pain signals from thin fibres are handed on in the dorsal horn to the cells that carry them up to the brain, and small inhibitory interneurons work as a gate on that hand-over. Conventional TENS fires the thick touch fibres, which excite the interneurons, and the gate closes: less pain goes up, and the patient feels tingling instead of aching. Strong, slow pulses also make the brainstem close the gate from above.
Simplified: a diagram of the circuit, not the dorsal horn's true shape or layers; the touch fibre's own branch to the projection neuron, and the pain fibre's effect on the interneuron, are left out; the fibres' cell bodies, in the dorsal root ganglion, are left out; the projection neuron's axon crosses to the other side of the cord before it rises; one cell stands for many and one pad for the pair TENS uses; the brainstem's fibre is drawn ending on the interneuron, though it also acts on the projection neuron directly.
4 sources
- Melzack R, Wall PD. Pain mechanisms: a new theory. Science, 1965;150(3699):971–979.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Chapters on the physiology of electrical stimulation and on TENS.
- DeSantana JM, Walsh DM, Vance C, Rakel BA, Sluka KA. Effectiveness of transcutaneous electrical nerve stimulation for treatment of hyperalgesia and pain. Current Rheumatology Reports, 2008;10(6):492–499.
- Robinson AJ, Snyder-Mackler L. Clinical Electrophysiology: electrotherapy and electrophysiologic testing, 3rd edition. Lippincott Williams and Wilkins, 2008.
The brain's own pain control
Strong stimulation, especially slow pulses that make muscles twitch, also drives the descending inhibitory pathways. Signals go up to the periaqueductal grey and the rostral ventromedial medulla in the brainstem, and these send signals down that block pain at the dorsal horn. They release the body's own opioids, the endorphins and enkephalins.
Animal studies show a neat split: low-frequency TENS works mostly through μ-opioid receptors, high-frequency TENS mostly through δ-opioid receptors. This is why people who take a lot of opioids may get less from low-frequency TENS, and why steady daily use can build tolerance.
Other effects
- Distraction: a strong, pleasant sensation pulls attention away from pain.
- A local effect on the nerve: at high intensity, pulses can briefly slow conduction in the nerve, a mild "block".
- A placebo effect, as with any treatment that is felt and believed in. Good trials use sham TENS to separate it out.
Stimulation modes and parameters
The parameters
| Parameter | What it means | Usual range in TENS |
|---|---|---|
| Waveform | the shape of each pulse | rectangular, biphasic (one push, one pull) |
| Pulse duration (width) | how long each pulse lasts | about 50 to 400 µs |
| Frequency (rate) | pulses per second | 1 to 150 Hz, some devices up to 250 Hz |
| Amplitude (intensity) | how strong each pulse is | up to about 60 mA, set by the patient's feeling |
| Pattern | how pulses are delivered | continuous, burst, or modulated (changing) |
The pulses are biphasic: each positive phase is followed by an equal negative one. The net charge is zero, so there is no build-up of acid or alkali under the electrodes and no chemical burn, unlike galvanic current.
The main modes
| Mode | Frequency | Pulse width | Intensity, what the patient feels | How it works | Relief |
|---|---|---|---|---|---|
| Conventional (high-frequency, low-intensity) | about 50 to 150 Hz, usually 100 Hz | about 50 to 200 µs | strong, comfortable tingling, no twitch | the gate | starts in minutes, lasts minutes to an hour or two after |
| Acupuncture-like (low-frequency, high-intensity) | about 2 to 4 Hz (1 to 10 Hz in some sources) | about 100 to 400 µs | strong, with visible rhythmic twitches | descending control, endorphins | starts slower, can last hours |
| Burst | bursts of about 100 Hz, repeated 1 to 4 times a second | 100 to 200 µs | strong; twitches with each burst | both | a more comfortable way to get the acupuncture-like effect |
| Brief-intense | 100 to 150 Hz | 150 to 250 µs | as strong as can be borne, for minutes | a short local nerve block | for painful procedures such as dressing changes |
| Modulated | frequency, width or amplitude changes in cycles | varies | changing sensation | the gate, with less adaptation | as conventional, lasts longer in use |
Point at a name or a part to light it.
The modes of TENS differ in how fast, how long and how strong the pulses are, and so in what they fire. Conventional TENS sends fast, short, weak pulses that fire the touch fibres and close the spinal gate: a tingle and quick relief. Acupuncture-like TENS sends slow, longer, strong pulses that twitch the muscle and wake the brain's own pain control: slower, longer relief. Burst TENS packs fast pulses into slow trains, a gentler way to the same twitches. In every mode the pulses are biphasic, so the skin is not polarised.
Simplified: each mode is drawn at one value from its range, the pulse width it is quoted with drawn as the length of each of the pulse's two phases; sources and devices differ, and some count both phases together; in the trains a pulse is drawn as a line, far wider than it really is at this scale; its height shows how strong it is, not a measured current; the twitches are drawn by hand, a quick rise and a slower fall, not measured; brief-intense and modulated TENS are left out.
3 sources
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Chapter on transcutaneous electrical nerve stimulation.
- Johnson MI. Transcutaneous Electrical Nerve Stimulation (TENS): research to support clinical practice. Oxford University Press, 2014. Chapter 3, TENS equipment, techniques, and biophysical principles.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 1 handnotes. Medical University of Sofia, 2026. TENS: parameters and modes.
- conventional TENS, with short pulses (about 50–200 µs)
- 100Hz
- acupuncture-like TENS, with visible twitches
- 2Hz
- a session, repeated as needed
- 20–60minutes
Sources give slightly different ranges. The professor's older text gives conventional TENS as 80 to 200 Hz with 200 to 400 µs pulses; the current slides give 50 to 100 Hz and 50 to 100 µs for conventional, and 1 to 10 Hz with 200 to 400 µs for acupuncture-like TENS; widely used definitions give about 50 to 100 Hz (others up to 150 Hz, as in the table above) with 50 to 200 µs pulses for conventional and 2 to 4 Hz with 100 to 400 µs pulses for acupuncture-like TENS. All agree on the pattern: conventional is fast with short pulses, acupuncture-like slow with longer ones. In practice the patient's sensation matters more than the exact numbers: the right intensity for conventional TENS is a strong, comfortable tingling without muscle contraction.
Sequential modulation
The professor's course teaches a response-guided sequence, used on combined electroanalgesia devices: about one minute of plain 100 Hz pulses, then a few minutes of amplitude modulation, then up to about ten minutes of frequency modulation, roughly 16 minutes in all. Each change brings back an effect that was fading. The logic is the same as with any steady stimulus: the nervous system stops paying attention to something that never changes, so changing it keeps the effect going. The timings are a teaching scheme, not a validated protocol.
Therapeutic parameters and dosage
Electrode placement
- On the painful area or around it, so that the tingling covers the pain. The pads are usually 2 to 5 cm apart; further apart, the current reaches deeper but spreads more.
- Along the nerve that supplies the area, between the pain and the spinal cord, when the painful skin itself cannot be used (a wound, a cast, extreme sensitivity).
- Over the matching spinal segment (the dermatome) for pain coming from the spine or deep organs.
- Over trigger points or acupuncture points, often used with acupuncture-like TENS.
- Two channels with four pads can surround a large painful area, as in low back pain.
Dosage
- 1
Check
The skin is clean, intact and has normal feeling. Test sensation to sharp and blunt touch first. - 2
Place and set
Pads on, device at zero, choose the mode. Turn the intensity up slowly until the patient feels a strong but comfortable tingling (or, in acupuncture-like TENS, visible twitches that are still comfortable). - 3
Adjust
The feeling fades within minutes as nerves adapt, so the patient turns it up again a little. A modulated mode reduces this. - 4
Duration
A session of 20 to 60 minutes. Conventional TENS can be used for longer, on and off through the day, with breaks for the skin. Pain is rated before and after to see whether it helps.
Because TENS is portable, it is often used at home. This is its great strength: the patient controls it, when and as often as needed. The book notes it can run for hours or days, unlike other electrotherapy, because changing the stimulus overcomes adaptation. Even so, repeated use of the same settings builds tolerance, so the advice is to vary frequency, use mixed-frequency or modulated modes, and to take breaks.
Indications
TENS is a symptom treatment for pain that comes from the body's tissues or nerves. Because it only acts while it runs and a little after, it fits best where the patient needs relief to move, exercise and sleep, usually as part of a wider plan.
| Group | Examples |
|---|---|
| Musculoskeletal pain | osteoarthritis of the knee, neck pain, low back pain, sprains and strains, tendinopathy, myofascial pain |
| Pain after surgery or injury | after operations on the chest, abdomen or joints, rib fractures, as an add-on to drugs |
| Neuropathic pain | painful diabetic neuropathy, post-herpetic neuralgia, nerve injury, phantom limb pain |
| Visceral and other pain | primary dysmenorrhoea, labour pain (pads on the lower back) |
What the evidence says
The largest pooled analysis (381 trials, 2022) found that pain was clearly lower during and just after TENS than with a sham, across acute and chronic pain, though most trials were small. What is not shown is lasting benefit over weeks for common chronic conditions. NICE advises not to offer TENS for low back pain or for osteoarthritis, judging the evidence of benefit too weak. In labour, women who use TENS often say they would use it again, but trials show little difference in pain scores. So: a safe, cheap, patient-controlled tool, best used alongside exercise and other treatment, not instead of it.
Contraindications
The list makes sense once you ask what the current can reach and what it can disturb.
| Reason | Do not use, or only with special care | Why |
|---|---|---|
| The current can disturb an implanted electronic device | pacemaker, implanted defibrillator, other implanted stimulators | the device may read the pulses as heartbeats and stop pacing, or shock wrongly; avoided, or only with cardiology advice and ECG monitoring, never over the chest |
| The current can reach sensitive structures | front of the neck (carotid sinus, larynx); the eyes; across the chest in heart disease; the head and neck in epilepsy | a drop in blood pressure, laryngeal spasm, arrhythmia, a seizure |
| The fetus | over the abdomen, pelvis or lower back in pregnancy, and on acupuncture points with acupuncture-like TENS | the effect on the fetus is not known; TENS on the lower back in labour is the accepted exception |
| The patient cannot report the feeling | numb skin, confusion, dementia, small children who cannot explain | intensity cannot be judged safely |
| Damaged skin or tissue | open wounds, rashes, fresh scars, irritated skin | burns and pain; TENS can be placed around a wound instead |
| Pain that is not understood | undiagnosed pain | relief can hide a disease that needs treating |
| Disease in the area | active cancer, a deep vein thrombosis or infection in the treated region | the worry is theoretical: more blood flow or muscle twitching might loosen a clot or help a tumour or infection spread; there is little evidence either way, so it is avoided; in palliative care TENS over cancer pain is acceptable |
Safety
- The commonest problem is skin irritation under the pads, from the gel or adhesive or from long use. Pads are moved a little each time and the skin checked.
- The patient never sleeps with TENS running and never uses it while driving or operating machinery.
- Pads are not placed on the front or sides of the neck, on the eyes, or across the head.
- The device is switched off before pads are put on, moved or taken off, and the intensity always starts from zero, so there is no sudden jolt.
- The patient is taught to use it, given written settings, and asked to stop if the pain changes in character or gets worse.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1Conventional TENS is set to 100 Hz with 80 µs pulses at a strong but comfortable tingling, no muscle twitch. Which nerve fibres does it mainly excite, and why?
2A 70-year-old man with chronic knee pain from osteoarthritis has a demand pacemaker. What is the right answer about TENS?
3Which setting describes acupuncture-like TENS?
4A student says TENS is proven to cure chronic low back pain. What is the fairest reply?
5A woman uses high-frequency TENS at the same settings for many hours every day. After two weeks it no longer helps. What is the most likely reason and the best change?