Humans love stimulants! We have always enjoyed zapping ourselves and each other, just a little bit, or even quite a lot.123 For many decades, by far the most popular (and tame) form of electrotherapy or neuromodulation4 has been transcutaneous electrical nerve stimulation (TENS). Other kinds of electrical stimulation are also interesting, and more promising, like deeper stimulation — zapping the brain and spinal cord. This is an overview of all forms of neuromodulation with a strong focus on TENS for pain.
TENS therapy has been a staple in physical therapy and chronic pain clinics for decades. There are many types of consumer TENS machine on the market. It has a huge following of believers, both providers and customers.
I do think we are heading into an era of using … both electrical & magnetic stimulation as a therapeutic tool.
Dr. Steven Novella, Nerve Stimulation for Relaxation
But people will believe anything! (You may have noticed this.) Much like ultrasound, this old-school techy therapy isn’t all it’s cracked up to be: the effectiveness of TENS “is based on individual experience rather than scientific evidence,” which seems odd for such a popular therapy. But it’s probably not entirely useless — interesting sensations, presented in the right way to patients, can always get something done (sensation-enhanced placebo).
What does “TENS” stand for? Transcutaneous electrical nerve stimulation. “Trans” means across or through, and “cutaneous” means affecting skin, so TENS refers to the stimulation of electricity passing through skin between electrodes.
What does a TENS unit do? Why would anyone want to use one?
If you stick a couple of electrodes on your skin and pump an electric current through your tissues, you will tickle your nerves and stimulate interesting sensations and/or some mild muscle contractions, which may temporarily drown out pain or “distract” us from it. More about the biology below.
You can fiddle with various dials for different flavours of TENS. For instance, you can boost the intensity and frequency to get more muscle contraction.5 With a powerful enough TENS machine (“These go to eleven.”), it is possible to “taze yourself.”
And so it’s possible that just the right setting makes all the difference, just like with phasers. This makes the topic endlessly debatable, even though it’s obvious that typical TENS doesn’t work many miracles. In fact, it probably mostly doesn’t work much better for most pain than an aspirin, and it’s not much good for anything else except maybe some relaxation or gently exercising muscles in rehab.
And central stimulation? Tickling the brain and spinal cord with electricity
And then there’s central stimulation — stimulation of the spinal cord or brain — the other major kind of neuromodulation. It’s possible to do this non-invasively, but probably not possible to do it well (more below). To get it right, you’ve probably got to get right in there with an actual implant: more invasive, profound, and dangerous, but also more impressive results for more serious problems like Parkinson’s diseases, major depression, and severe chronic pain. This approach has roots in electroconvulsive therapy — “electroshock” therapy back in the day — but the modern era of electrical therapy began when deep brain stimulation was introduced in 1987, and it’s been steadily improving since then, mostly thanks to better surgical techniques for implants.
Now, if only we knew how it worked! No one actually does, and there are several possibilities, like vagus nerve stimulation to control inflammation for rheumatoid arthritis — extremely interesting, but far from an established effect.6 That’s an advanced topic, beyond the scope of this article, but it's a topic worth watching over the next decade.
As brain-machine interfaces continue to improve, we are likely to see some real progress. Consider a 2021 study that demonstrated substantial pain relief using a smart implant in rat brains.7 Cool stuff! But it’s about as close to TENS as rocket science is to paper airplanes.
Maybe if you zap this, it will be happier? Kind of a strange hypothesis when you think about it. Why would brains respond well to electromagnetic forces? I mean, hey, anything’s possible, but it’s not like there’s any obvious reason to think so.
Non-invasive brain stimulation techniques for chronic pain
Why invade when you can just surround? Spoiler: because it doesn’t work as well. Non-invasive brain-stim aims to stimulate brain tissue from the outside by various means,8 but all of them are “transcranial” stimulation with something, mostly magnetism and direct current — just like TENS, but with the electrodes attached to your head.
To date, there isn’t much reason to think that any of them are actually effective for pain. A 2015 British Medical Journal editorial ruled “not recommended; early promise is fading fast as trial methods improve” (commenting on a rare good quality trial9). In 2018,the Cochrane Database of Systematic Reviews just declared the evidence to be pathetic — not enough good studies like Luedtke et al. But the evidence so far is already following a classic pattern: slightly promising results from sloppy studies, discouraging results from the better ones that follow. The null hypothesis will probably rule the day, as it usually does.10
TENS costs and safety
TENS machines can be had for around $100, or even less for a few bargain models, but it is possible to spend several hundred on more power, options, or trendiness.
Although there is a tiny risk of mild shock from faulty devices, consumer TENS units are just too weak to be dangerous, and the risk of too much juice in a medical setting is extremely low.11 Overzealous self-treatment might cause some skin irritation around the electrodes.
Could TENS cause pain or other neurological symptoms?
Other than direct injury, it’s conceivable that TENS could backfire and actually cause pain instead of relieving it. I’ve wondered about this, and went looking for evidence of it — and found none. If it occurs, it’s rare and relatively minor.
But I still think it’s plausible. Any unusual stimulus can potentially be a trigger for the symptoms of certain diseases (headache triggers like chocolate and wine are a classic example). A TENS backfire would not involve any harm to the nerves themselves, just a sensory over-reaction.12 Many cases of backfiring might not quite reach the level of pain. For instance, mild numbness, tingling, and other paresthesias are also possible. These are extremely common symptoms in people even when we aren’t pumping electric current through our flesh, and they usually are completely harmless and temporary, triggered by a variety of minor physiological and psychological circumstances. The strange sensations of TENS, for some patients, might trigger an “attack” of tingling or numbness.
The best defense against such reactions to TENS — or any weird stimulus — is mental confidence and optimism.
Do TENS machines work? Is there any scientific evidence that TENS can treat pain?
People tend to ask “how does TENS unit work?” before asking the much more important question: “do tens machines work?” There is some evidence that TENS helps pain, but it’s a bit sketchy. Superficial TENS is well-studied as these things go: there have been enough studies that there are several reviews of the studies. However, a lot of the reviews were inconclusive, because they’re reviewing a lot of weak data. Par for the course in musculoskeletal medicine, unfortunately: garbage in, garbage out.
|Milne 200113||negative review of 5 trials of TENS for chronic low back pain|
|Johnson 200714||positive review of 38 trials of TENS for chronic musculoskeletal pain, “effective”|
|Nnoaham 200815||inconclusive review of 25 studies of TENS for chronic pain|
|Khadilkar 200816||inconclusive review of 4 trials of TENS for chronic low back pain|
|Walsh 200917||inconclusive review of 12 trials of TENS for acute pain|
|Hurlow 201218||inconclusive review of 3 trials of TENS for cancer pain|
|Vance 201419||mixed review: “it’s complicated” but promising|
|Chen 201520||negative review of 18 trials for knee osteoarthritis|
|Desmeules 201621||inconclusive, discouraging review of 6 trials for rotator cuff tendinopathy|
|Li 201722||barely positive meta-analysis of 5 weak trials for pain after knee replacement|
|Salazar 201723||barely positive meta-analysis of 9 weak trials of stim for fibromyalgia|
That doesn’t look great: five disappointingly non-positive,24 two spankings for back pain and osteoarthritis, two that damn TENS with faint praise, and just the one clearly positive review out of eleven (which is also one of the oldest and perhaps the least picky).25
But that 2014 one … “it’s complicated” is an understatement! Vance et al is a more formal and thorough version of what I’m doing here.26 Although it’s dense reading, they do a good job of putting it all in perspective, and point out several reasons why the evidence reviewed here is probably not the whole story, and why they remain optimistic that the right TENS (intensity, duration) for the right kind of pain could still be good medicine.
That kind of talk is often a red flag,27 especially when the same researchers have also argued that there is probably a “strong placebo component” with TENS.28 But sometimes it’s fair, and this might be one of those times. Maybe science hasn’t asked the right questions yet. Despite the messy evidence, Vance et al. believe that “both high-frequency and low-frequency TENS have been shown to provide analgesia specifically when applied at a strong, non-painful intensity.”
My official position for now is that it probably works well enough for some patients to be worth trying if you’re desperate … but keep your expectations low. There are solid reasons to doubt that TENS can work well.
If TENS works, how does it work? Digging deeper into the biology
What does a tens unit actually do to treat pain? The mechanism cited most often is the counterstimulation effect: pre-empting pain with other sensations. This effect is real but minor, and particularly unlikely to be helpful with severe or chronic pain.
The obligatory, canonical example is that animals instinctively rub on or near acute minor injuries. We self-treat with counterstimulation all the time, and sometimes we even counterirritate ourselves (like pounding on your thigh after a toe stub).
This is often informally explained as “distracting” us from pain — we’re basically flooding sensory pathways with competing sensations, and (weirdly) the nervous system gives priority to tactile sensation over noxious stimuli. This is the phenomenon of counterstimulation, which is based on some “simple” neurology, sensory bottlenecks in the spinal cord (“gate control theory”). It’s not a high-level, top-down phenomenon. It exploits a limitation and quirk of our wiring. The effect is real, but generally minor and transient. Citing it as a mechanism for TENS is pretty weak sauce (but often done, because it sounds science-y).
But what if it wasn’t always so minor and transient? But what if there was a fancier, better version of the same thing?
Fancy counterstimulation: novel sensory input descending modulation
In this section, I will give TENS the benefit of the doubt and spitball optimistically about how a bit of tingling might deliver something a little more robust more than a trivial counterstimulation effect. This hypothesis can be applied to any unusual sensation, and may in fact explain why many kinds of therapy are mildly effective. But even if it explains why it works better than simple counterstimulation, it probably also explains its modest limits.
This is basically about amusing and calming the brain with interesting, non-threatening sensations, resulting in some “descending modulation”: the brain turning the alarm volume down.
All pain is ultimately generated by an overprotective brain that likes to sound the alarm too loudly, too often, regardless of what’s actually going on in tissues. This means that many kinds of chronic pain are partially and briefly treatable with tricks and hacks and virtually any novel stimulation.29 Unfortunately, your brain is stubborn and it’s hard to convince it to shut up about pain completely — short of knocking it out, which is why anaesthesia is the only truly effective analgesia.
The brain decides what hurts based on many sources of information. TENS messes with that system. It blasts the nervous system with static, sensory white noise. By stimulating nerves in this strange way, TENS doses the brain with unusual sensory information, which either “drowns out” information about tissue damage, and/or simply distracts the brain and forces re-evaluation. If it happened without warning or explanation, it would probably be scary and make the pain worse. But pay good money for it and call it “therapy,” and it can be reassuring instead. Context is everything!
But TENS cannot actually stop the brain from doing its job! Unless you turned it up enough to actually disable your brain.30 If your brain thinks you need to be in pain, the alarm is going to start going off again sooner or later — maybe right after the TENS is turned off.
Basically what all of this means is that TENS might deliver what I call sensation-enhanced placebo. Placebo is “relief from belief,” and the belief can be (greatly) enhanced by novel sensations. Every placebo involves some degree of fundamentally dishonest exaggeration to make the patient believe that it’s better than it actually is. Treatments that produce unusual sensations have an advantage, because the unusual sensation makes the treatment seem much more legitimate and special. If you take this far enough, it seems less like a placebo and more like we’re doing something therapeutic to the body. But if it boils down to convincing the patient’s brain to dial down the pain, it’s still a placebo at heart — just a fancy one.
TENS for muscle knots
The most common kind of musculoskeletal pain that doesn’t have an obvious cause is probably the “muscle knot” or “trigger point” — a patch of sensitive soft tissue, which is still mysterious after decades of sparring with it scientifically. All we really know for sure is that they feel like aching muscle, and they’ve inspired a lot of theories and half-baked cures.31 Trigger points are as likely a target for TENS as anything else, and it’s safe and cheap enough to be worth trying in difficult cases.
A 1997 study of TENS specifically for treating these little monsters found positive results.32 Yes, you read that right: positive! Some good news for once. Of course, the “just one small study” disclaimer applies in a big way: one study never proves anything, and I think this is the only one of its kind. But it was a reasonably good experiment, and the results were promising.
They compared nerve stimulation, muscle stimulation, and a placebo for 60 patients with knots in their shoulders (trapezius). They measured pain, pressure sensitivity, and range of motion (neck sidebend) before and after treatment. Muscle stimulation improved range of motion (but not pain/sensitivity), and nerve stimulation improved pain/sensitivity (but not range).33 The authors concluded:
ENS is more effective for immediate relief of myofascial trigger point pain than EMS, and EMS has a better effect on immediate release of muscle tightness than ENS.
The effects were statistically significant, but not clinically dramatic enough for them to boast about. But there’s enough here that I really wish someone would try the experiment again with more people. Maybe someday!
Electrical muscle stimulation: EMS vs. ENS
Just as electrical stimulation can be shallow or deep, there’s also electrical stimulation of muscle rather than nerves: electrical muscle stimulation (EMS), or neuromuscular electrical stimulation (NMES). (They are all “transcutaneous,” but for some reason T-for-transcutaneous is usually only use when referring to nerve stim. There is no “TEMS.”)
The right electrode placement and settings will stimulate muscles more than nerves, forcing them to contract. And so EMS may be a useful trick for exercising muscles, slightly useful in rehabilitation and, perhaps, performance enhancement. EMS feels like doing light isometric muscle contraction exercises — that is, clenching muscles without moving joints.
Or you could just do isometric muscle contractions, of course.
Forcing the issue with electrodes may be more about novelty than utility. But why not? I like gadgets as much as the next guy (or more, my office is pretty thick with them). It might just be fun to use EMS.
What EMS might be good for
Whereas traditional TENS is primarily used to try to treat pain, EMS is mostly meant for athletic training and performance enhancement — which is speculative and somewhat optimistic but not necessarily totally out to lunch — and rehabilitative scenarios like preventing muscles from atrophying from disuse after trauma, which makes sense and seems to work at least a little.34
Do beware of using it on actual muscle injuries though (muscle strains). Electrically stimulating a damaged muscle to contract is very “What could possibly go wrong?” EMS isn’t brutal. It’s unlikely to be a problem. But still! Be careful, for pity’s sake.
Many readers are curious to know specifically if EMS can help them maintain fitness when otherwise resting or incapacitated from an injury, especially stubborn overuse injuries like runner’s knee or plantar fasciitis that may require a period of substantial resting before a tedious phase of “baby steps” rehabilitative exercise. That’s exactly what EMS is good for, in theory, but in practice the benefits are probably trivial. For instance, in one study it didn’t help people with moderate knee osteoarthritis.35
Arthrogenic muscle inhibition (AMI)
AMI is the weird semi-paralysis of muscle in response to injury, most often seen in the quads after knee injuries.36 It’s a protective mechanism that gets out of hand (which is, interestingly, exactly analogous to a lot of chronic pain).37
AMI seems like the kind of thing that EMS was made for. If you can’t zap dysfunctionally inhibited muscles back into action, what can you zap? One blogger claims that EMS is “an integral part of the rehabilitation pre and post ACLR.” O rly? Sadly, the reference she gave doesn’t actually support that38 — and that’s how these things end up sounding better than they are. Other studies do lean that way,39 but not all.40 If my quads were still emaciated months after an injury, I would certainly try EMS — but not until the AMI was clearly dysfunctional, rather than just routine wisdom-of-the-body stuff after an injury.
Even if EMS works a little, in most cases the effect is nothing you couldn’t get just as well or more so with isometrics over time, and much more conveniently and cheaply to boot. Half-arsed optimization of muscle conditioning in the early stages of rehab should not be a high priority for most people. The exceptions might be people who are more profoundly injured, have unusually persistent AMI, or have specialized training needs due to a progressive disease.
And there is that entertainment value! EMS is kinda neat to make your muscles vibrate.
More TENS claims: relaxation
Are you tense? Try some TENS! It’s probably at least a little bit relaxing, and anything truly relaxing is at least a little bit good for pain — but only a little bit. Relaxation is weak medicine, as anyone with chronic pain can tell you. TENS has always been seen as a trivially relaxing therapy, just like a vibrating massager is: it literally vibrates muscle, as well as producing buzzing and tingling sensations that are quite similar to the feeling of vibration. Actually, I suspect that vibration is a major reason why TENS is a thing.
These days, the relaxation claims are fancier. (Almost everything about TENS is fancier these days.41)
What if TENS could stimulate specific nerves in a specific way, to create a more profound effect? That’s what the Thync (pictured at the top of the article) is supposedly doing, and either relaxing or invigorating depending on what mode it’s in: “Thync uses neurosignaling to activate42 specific cranial and peripheral nerves … signalling brain regions to change the way you feel.” Unsurprisingly, skeptical neurologist Dr. Steven Novella’s late 2015 review of the Thync is discouraging.43 For me, $200 is not an outrageous price for a little interesting experimenting on myself, and I’ll probably try it for fun. But I’m keeping my expectations low. If I’m really serious about getting into a deeply relaxed state, I suspect practicing meditation would offer better bang for buck. Or even just a hot bath.
And how about the more traditional approach? For instance, how about using TENS to reduce jaw clenching in a patient with temporomandibular joint syndrome? It’s a good example to consider, because it’s a classic example of the combination of muscle clenching and pain. If TENS is relaxing, it should be able to help this problem. The evidence is very limited and mixed. A little 2001 study is the only reasonably well-designed test I could find. It compared TENS to biofeedback in 20 patients. Unfortunately, neither method had any effect: those jaw muscles kept right on clenching, as measured by EMG.44 That is a bummer.
And I’d still try it if I had temporomandibular joint syndrome. The right settings could make the difference!
“Just the right settings”
With any therapy that depends on physics and technology, there’s always a handy justification for optimism: you just need the right combination of frequency, amplitude, intensity, [insert exotic parameter here]. There’s always someone who claims that their patented device is the only one that gets it right. No matter how poor the evidence for conventional approaches is, the gadget marketers can always say, “Yes, obviously ordinary TENS will fail — you need our special sauce.”
It’s a great strategy because it’s impossible to definitively contradict.45 But the longer people try to find “just the right settings” without ever proving that such a combination does exist, the more it turns into special pleading — claiming to be a “special” case, an exception to the trend.46
This is a pitch perfect example of TENS special pleading based on this trope, from the marketing for “Action Potential Simulation Therapy”:
The APS therapy wave form is an [sic] direct copy of an action potential, the body’s own electrical signalling system, which is what makes it unique. We are often asked if this is the same as TENS. The answer is no. To compare, a TENS machine uses a current of a thousandth of an amp, or 0.001, to creat [sic] a tingle that distracts from pain. APS Therapy uses a current of 0.00 0001, or a millionth of an amp., to get the cells firing more effectively, to address the underlying cause of pain, amongst other things.
The body does not have any one clear type of “action potentials” to replicate, even if doing so is what actually matters. Electrical engineering contains a bewildering diversity that takes serious expertise to even begin to master, but biology is dramatically more complex! But this company claims that they make a “direct copy of an action potential,” as it could ever be one replicable thing, and as if we know that would be an effective strategy even if it was possible. Classic therapy babble.
MOAR! Whole body TENS, anyone? Old timey galvanic baths
As if there aren’t enough TENS options with a few electrodes and several dials, you can — if you’re eccentric — run current through your whole body in an electric bath. This current would hopefully only be direct (galvanic) current for safety.47 But for the same reason it’s safer, direct current is also less stimulating: it causes a smooth, sustained muscle contraction, whereas the alternating current of TENS makes your muscles vibrate. Sounds relaxing, eh? Spasm baths!
Do not try at home.
Galvanic baths are mostly an old-timey thing you would have a hard time trying even if you wanted to, but they are an entertaining example of electric therapy.
“What is so strange about a galvanic bath?” one reader asked me. I suppose it’s not quite as outrageous as it looks, but it does reek of what could possibly go wrong? And, medically speaking, it’s quite a bit farther out in left field than TENS, quite unlikely to be good for anything. There’s no known or plausible therapeutic mechanism other than “it feels kinda funny.”
If nothing else, galvanic bathing was doomed to extinction by economics: the high cost of the infrastructure and equipment to deliver such treatments is way out of proportion to the slim hope of any significant benefit. Plus, can you imagine the lawsuits it would generate today?
TENS has a fascinating cousin: pulsed electromagnetic field therapy
Pulsed electromagnetic field therapy (PEMF) is clearly kin to TENS, a member of the electrotherapy family. It’s also related to simple (“static”) magnet therapy, which is mostly far more snake oily48 and less plausible than electrical stimulation.49
But this is a different beast, it’s very own thing. It’s more exotic, with a more mysterious mechanism of action. PEMF is hypothesized to directly stimulate cellular repair, and not for nothing: it actually seems to do that, and the effect is almost magical, speeding up bone fracture healing, and even restoring it in cases where healing has failed completely.50
Amazingly (and completely unexpectedly for me), the scientific reviews of PEMF used for this purpose are unstintingly positive.5152 When does this happen in musculoskeletal medicine? Never, that’s when. Which is suspicious in itself.
If PEMF can help fractures heal, it’s certainly reasonable to wonder what else it can do. But only recently have PEMF devices gotten small and cheap enough for consumers and less critical medical applications. Can they work on more ordinary problems? Like arthritis? Something TENS can only treat effectively with just the right settings and variables, that no one can seem to confirm?
Some trials of PEMF for these purposes are unambiguously positive.53 Even though there’s some hope for TENS with just the right settings, the best evidence is only lukewarm, and confirmation awaits better trials of just the right sort of TENS for the right sort of patients. Meanwhile, PEMF is in that fragile “promising” stage that few therapies actually emerge from unscathed. A cynic would wisely predict that higher quality studies are going to be disappointing.
TENS compared to nerve conduction testing
These two things seem similar: in both cases, electrodes are applied to the skin, and current is passed through the tissues between them. But while TENS is tame and (hopefully) therapeutic, nerve conduction testing is diagnostic and rather unpleasant — higher voltage and amperage makes it feel much more like an series of unpleasant electrical shocks.
The purpose of a nerve conduction test is to determine the health of peripheral nerves by measuring how quickly they conduct electricity. Several pathologies interfere with conduction in distinctive ways. To get the speed, mild electric shocks are applied at very specific locations so that the electricity will mostly zap its way down a specific peripheral nerve axon, to an electrode several inches away.
This causes some discomfort at the location of the shock, and also stimulates the nerve quite strongly. Shocking of motor and sensory nerves causes muscles to twitch and sensations to blare. It feels pretty much like what you’d expect: zappy!
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About Paul Ingraham
I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter, or subscribe:
- Does Ultrasound Therapy Work? — Many concerns about the widespread usage of therapeutic ultrasound, especially extracorporeal shockwave therapy (ESWT). It’s amazing how analagous TENS and ultrasound are: two almost identical methods of stimulating tissue with a gadget.
- Pseudo-Quackery in the Treatment of Pain — The large, dangerous grey zone between evidence-based care and overt quackery in musculoskeletal and pain medicine. TENS is one of the best examples of a popular therapy in that zone.
- Electromagnetic Sensitivity Absurdity — Electrosensitivity is an imaginary, debunked energy allergy. Ironically, while most people want electricity for therapy, some believe it’s toxic!
- Cold Laser Therapy Reviewed — A critical analysis of treating pain and injury with frickin’ laser beams
- Infrared saunas are yet another good example of the idea that the right kind of stimulation will make biology work better. Biological effects beyond warming tissues are conceivable, but have never actually been documented. I explore this in more detail here: Heat for Pain and Rehab: A detailed guide to using heat as therapy for acute and chronic pain and recovery from injury.
- The Road to Wellville is a 1994 comedy/drama that depicted fashionable old-timey zapping at an unusual early 20th century health care facility, run by the eccentric Dr. Kellogg. Although it’s not a fantastic film, it’s not bad, and probably worthwhile for many health care professionals.
What’s new in this article?
Eleven updates have been logged for this article since publication (2016). All PainScience.com updates are logged to show a long term commitment to quality, accuracy, and currency. more
When’s the last time you read a blog post and found a list of many changes made to that page since publication? Like good footnotes, this sets PainScience.com apart from other health websites and blogs. Although footnotes are more useful, the update logs are important. They are “fine print,” but more meaningful than most of the comments that most Internet pages waste pixels on.
I log any change to articles that might be of interest to a keen reader. Complete update logging of all noteworthy improvements to all articles started in 2016. Prior to that, I only logged major updates for the most popular and controversial articles.
See the What’s New? page for updates to all recent site updates.
Mar 28, 2022 — Added some context about simple, “static” magnet therapy to the pulsed electromagnetic field therapy section.
2021 — Science update: cited Zhang on pain relief from brain implants.
2021 — Elaborated significantly on the mechanism of action of TENS, especially distinguishing between simple counterstimulation and the "fancier" version of counterstimulation: descending modulation triggered by novel sensory input.
2021 — Added summary and citations for non-invasive brain stim. Added two new scientific review citations to the “Overview of TENS reviews” table. Added a new section, “Just the right settings.” Added a dense couple paragraphs about the unclear role of muscle stim in treating arthrogenic muscle inhibition.
2018 — Science update, cited Luedtke et al on transcranial direct current stimulation (tDCS).
2018 — Minor new section: “TENS compared to nerve conduction testing.” Also added a citation to Vance.
2016 — Added a note about the potential of vagus nerve stimulation to control inflammation.
2016 — Added a mobile-only article summary. Added a paragraph about sensation-enhanced placebo.
2016 — Minor science update, added a new review (Desmeules) of TENS for rotator cuff tendinopathy.
2016 — Added more information about possible harms, a really bad pun, a link to The Road to Wellville, and a reader comment about the “fancyness” of TENS devices over the years.
2016 — Added section about pulsed electromagnetic field therapy.
2016 — Publication.
- Most famously, in 1963, the Milgram experiment supposedly showed that people are surprisingly willing to inflict extremely painful and even dangerous shocks on other people. To this day, it is cited as an demonstration of the human capacity for evil done by “just following orders,” but this is a misinterpretation that has become a myth. Happily, there are many sound criticisms of the classic interpretation, and lots of follow-up science has shown that people are actually quite reluctant to zap each other.
- Running current through our bodies is just the tip of the iceberg when it comes to potentially self-destructive treatments. You’ll think I’m bullshitting you, but voluntary lobotomy was actually faddish once. What could possibly go wrong?! See Popular but Weird & Dangerous Cures.
- There have been many abuses of electrotherapies, but the winner for paving the road to hell with good intentions must surely be the chapter in the history of autism treatment when cattle prods were promoted to parents as “tingle sticks” to modify the behaviour of their autistic children. Seriously. See Neurotribes: The Legacy of Autism and the Future of Neurodiversity.
- Neuromodulation is the umbrella concept for all kinds of nerve stimulation, mostly with electricity. It’s defined by the International Neuromodulation Society as “the alteration of nerve activity through the delivery of electrical stimulation or chemical agents to targeted sites of the body.” The point of neuromodulation of any kind is to normalize nerve function, to get nerves to behave differently, and hopefully better. Many different kinds of electromagnetic stimuli are used for this.
- Push that far enough and things get much more “interesting.” Fortunately, consumer units don’t go to 11 on that scale — to simulate rigor mortis, you’d need a proper generator.
- Koopman FA, Chavan SS, Miljko S, et al. Vagus nerve stimulation inhibits cytokine production and attenuates disease severity in rheumatoid arthritis. Proc Natl Acad Sci U S A. 2016 Jul;113(29):8284–9. PubMed #27382171 ❐ PainSci #53670 ❐
Vagus nerve stimulation sure sounds great (maybe a little too great): stimulate your vagus nerve with an implant, et voila, less systemic inflammation. There’s broad biological plausibility here, but almost no evidence — this idea hinges only on the results of this one study so far. Koopman et al. tested it on humans and reported that “these results establish that vagus nerve stimulation targeting the inflammatory reflex modulates TNF production and reduces inflammation in humans.”
Established, eh? Not without replication! That’s overconfident for sure — this needs replication before celebration.
All kinds of data hijinks could be hiding in a study this technical. My main concern is the use of the word “significantly” in the abstract, without any details (effect size in particular). All too often that wording, without clarification, means there was a statistically significant but clinically trivial result. With many treatment trials I can go digging for the effect size to confirm, but not here, the reading is too difficult for me to form any meaningful impression without spending an hour, and even then it might not be clear. And even if the paper does indicate a clinically meaningful result it’s still got “too good to be true” written all over it and may well prove to be difficult to reproduce.
But it’s a genuinely interesting topic, I think.
- Zhang Q, Hu S, Talay R, et al. A prototype closed-loop brain-machine interface for the study and treatment of pain. Nat Biomed Eng. 2021 Jun. PubMed #34155354 ❐
This study advances the science of brain-machine interfaces with a test of an implanted computer chip in rat brains, designed to treat chronic pain. The chip reads the rat’s minds: it detects patterns of brain activity in the cingulate gyrus that are consistent with pain, and then stimulates part of the frontal lobe to mute pain: specifically, to “exert top-down nociceptive regulation.” Obviously this is very invasive, and even if it works there’s a risk of adaptation and dependence, and human applications are many years off.
Doing this for humans is probably still many years away. But if it works? It’s extremely precise, responding in real-time, only working when there’s pain to treat — completely unlike the continuous, always-zapping approach that has dominated the field so far.
And it really did work amazingly well. The treated rats withdrew from painful stimuli about 40% slower, and greatly preferred spending time in a chamber where the implant was functional to one where it wasn’t. These were strong results, and a very promising demonstration of the principle.
This kind of approach is likely to improve as we continue to improve brain-machine interface technology, and knowledge of brain circuity gets more precise.
See neurologist Dr. Steve Novella’s more detailed explanation of this experiment.
- repetitive transcranial magnetic stimulation (rTMS)
- cranial electrotherapy stimulation (CES)
- transcranial direct current stimulation (tDCS)
- transcranial random noise stimulation (tRNS)
- reduced impedance non-invasive cortical electrostimulation (RINCE)
Acronyms, acronyms, acronyms!
- Luedtke K, Rushton A, Wright C, et al. Effectiveness of transcranial direct current stimulation preceding cognitive behavioural management for chronic low back pain: sham controlled double blinded randomised controlled trial. BMJ. 2015;350:h1640. PubMed #25883244 ❐ PainSci #54171 ❐
- Null hypothesis primer: in plain English, the null hypothesis is that “Most ideas turn out to be wrong.” And therefore most weakly positive study results will turn out to be the product of bias and wishful thinking, doomed to be overturned by more rigorous studies.
- As of 2016, there seem to be hardly any complaints on the Internet about trouble with these devices — which is remarkable, considering that we’re talking about very large numbers of humans playing with devices that are intended to deliver electric current.
- It would be a case of the brain interpreting the unusual stimulation as threat rather than therapy. Mental context is everything, and so I would expect it to occur mainly in vulnerable and sensitive tissues, and people with higher anxiety and lower pain tolerance for any reason.
- Milne S, Welch V, Brosseau L, et al. Transcutaneous electrical nerve stimulation (TENS) for chronic low back pain. Cochrane Database Syst Rev. 2001;(2):CD003008. PubMed #11406059 ❐
- Johnson M, Martinson M. Efficacy of electrical nerve stimulation for chronic musculoskeletal pain: a meta-analysis of randomized controlled trials. Pain. 2007;130(1-2):157–165. PubMed #17383095 ❐
- Nnoaham KE, Kumbang J. Transcutaneous electrical nerve stimulation (TENS) for chronic pain. Cochrane Database Syst Rev. 2008;(3):CD003222. PubMed #18646088 ❐
- Khadilkar A, Odebiyi DO, Brosseau L, Wells GA. Transcutaneous electrical nerve stimulation (TENS) versus placebo for chronic low-back pain. Cochrane Database Syst Rev. 2008;(4):CD003008. PubMed #18843638 ❐
- Walsh DM, Howe TE, Johnson MI, Sluka KA. Transcutaneous electrical nerve stimulation for acute pain. Cochrane Database Syst Rev. 2009;(2):CD006142. PubMed #19370629 ❐
- Hurlow A, Bennett MI, Robb KA, et al. Transcutaneous electric nerve stimulation (TENS) for cancer pain in adults. Cochrane Database Syst Rev. 2012;3:CD006276. PubMed #22419313 ❐
- Vance CG, Dailey DL, Rakel BA, Sluka KA. Using TENS for pain control: the state of the evidence. Pain Manag. 2014 May;4(3):197–209. PubMed #24953072 ❐ PainSci #54034 ❐
- Chen LX, Zhou ZR, Li YL, et al. Transcutaneous Electrical Nerve Stimulation in Patients with Knee Osteoarthritis: Evidence from Randomized Controlled Trials. Clin J Pain. 2015 Mar. PubMed #25803757 ❐
- Desmeules F, Boudreault J, Roy JS, et al. Efficacy of transcutaneous electrical nerve stimulation for rotator cuff tendinopathy: a systematic review. Physiotherapy. 2016 Mar;102(1):41–9. PubMed #26619821 ❐
- Li J, Song Y. Transcutaneous electrical nerve stimulation for postoperative pain control after total knee arthroplasty: A meta-analysis of randomized controlled trials. Medicine (Baltimore). 2017 Sep;96(37):e8036. PubMed #28906393 ❐ PainSci #51742 ❐
- Salazar AP, Stein C, Marchese RR, Plentz RD, Pagnussat AD. Electric Stimulation for Pain Relief in Patients with Fibromyalgia: A Systematic Review and Meta-analysis of Randomized Controlled Trials. Pain Physician. 2017 02;20(2):15–25. PubMed #28158150 ❐
- “Inconclusive” is definitely closer to a negative conclusion than a positive one: if TENS worked well, even small and poor quality studies would probably show it, all the more so because many of them were probably done by researchers who were hoping to get a positive result. A high risk of bias combined with scanty data usually produces tentatively positive conclusions!
- The number of studies included generally reflects how picky the authors were about excluding poor quality studies, and it might be important that the one positive review included the largest number of studies — obviously including plenty of data that other review authors considered to be poor quality.
- That is, it’s not a meta-analysis, a statistical summary of the results of many studies, but “review” in the more familiar sense: exploring and critiquing the data we have so far, and speculating about its significance.
- You see it all the time in defence of snake oils and quackery. It’s called “special pleading”: the scientific equivalent of a bratty kid coming up with reasons why the rules shouldn’t apply to them.
- Vance CG, Rakel BA, Blodgett NP, et al. Effects of transcutaneous electrical nerve stimulation on pain, pain sensitivity, and function in people with knee osteoarthritis: a randomized controlled trial. Phys Ther. 2012 Jul;92(7):898–910. PubMed #22466027 ❐ PainSci #54295 ❐
Researchers tested transcutaneous electrical nerve stimulation (TENS) on 75 arthritic knee patients. They were given a high or low frequency stimulation or a placebo. Several different measurements of pain were taken before and after, such as resting pain and pressure tolerance. Pressure tolerance and an activity test improved a bit, but the effects were nil or trivial by all other measures. The researchers concluded that there is “a strong placebo component” to the effect of this type of treatment. (Ya think?)
- Almost any reassuring and/or distracting input has some potential to persuade the brain to dial pain down a bit, by fooling a brain into thinking there’s no cause for alarm, at least for a little while. Brief, modest treatment results for chronic pain are mostly about how pain works… not how the treatment works.
- And that was exactly the literal point of olde timey electroshock therapy originally: to completely overwhelm the brain and “short circuit” whatever undesirable thing it was up to, which was usually mental illness. A brain-disabling treatment. Interestingly, pain was rarely the rationale for electroshock therapy, because no one yet understood that the brain was the source of all pain — just like all other sensations and perceptions — and not the body as everyone assumed.
- They are probably like tiny cramps — contracted, stagnant, swampy sections of muscle tissue — but that’s just a hypothesis, denounced by some experts. However they work, no one doubts that these sensitive spots in muscle are common. The pain often spreads in confusing patterns, and they grow like weeds around other painful problems, which makes them clinically interesting and tricky. Since we don’t really understand why trigger points occur in the first place, reliable and effective treatment is impossible. The best anyone can do is to experiment with therapeutic approaches, starting with the safest and cheapest things that make some sense. TENS is not at the top of that list of options, but it’s probably somewhere in the top third of it.
- Hsueh TC, Cheng PT, Kuan TS, Hong CZ. The immediate effectiveness of electrical nerve stimulation and electrical muscle stimulation on myofascial trigger points. Am J Phys Med Rehabil. 1997;76(6):471–476.
- That’s an interesting difference — it suggests that the effect of a trigger point on muscle extensibility exists independently of how sensitive it is, which is a little surprising. Those properties “should” come and go together, according to the main hypothesis of how trigger points work: the point hurts and resists elongation because it’s contracted. It’s not clear why ENS/EMS would each relieve one without affecting the other. But such puzzles are typical of trigger point biology. No one really knows what the heck is actually going on.
- Hasegawa S, Kobayashi M, Arai R, et al. Effect of early implementation of electrical muscle stimulation to prevent muscle atrophy and weakness in patients after anterior cruciate ligament reconstruction. J Electromyogr Kinesiol. 2011 Aug;21(4):622–30. PubMed #21334221 ❐
This was a test of electrical muscle stimulation (EMS) for rehabilitation after knee surgery. Twenty patients had acute anterior cruciate ligament tears repaired. Half of them got EMS after surgery, and half did not. “Muscle thickness of vastus lateralis and calf increased significantly 4 weeks after surgery in the EMS group, while it decreased significantly in the control group. The decline of knee extension strength was significantly less in the EMS group than in the CON group at 4 weeks after the surgery, and the EMS group showed greater recovery of knee extension strength at 3 months after surgery.” The anti-atrophy effect of EMS was not dramatic, but it was clear.
- Palmieri-Smith RM, Thomas AC, Karvonen-Gutierrez C, Sowers M. A Clinical Trial of Neuromuscular Electrical Stimulation in Improving Quadriceps Muscle Strength and Activation Among Women With Mild and Moderate Osteoarthritis. Phys Ther. 2010 Jul. PubMed #20671100 ❐
After knee replacements, “electric exercise” has already been shown to be useful in rehabilitation: using NMES, (neuromuscular electrical stimulation) can improve the strength and activatation of quadriceps. Can the same approach be used to exercise knees with mild to moderate osteoarthritis? In this randomized controlled trial, 30 women with slightly weak knees were assigned to receive either nothing, or NMES 3 times per week for a month. The results showed no difference between the two groups.
- Hart JM, Pietrosimone B, Hertel J, Ingersoll CD. Quadriceps activation following knee injuries: a systematic review. J Athl Train. 2010;45(1):87–97. PubMed #20064053 ❐ PainSci #51715 ❐ “Quadriceps activation failure is common in patients with anterior cruciate ligament deficiency (ACLd), anterior cruciate ligament reconstruction (ACLr), and anterior knee pain (AKP).”
- Is it really “out of hand,” though? In a few cases, almost certainly, yes — but since most people with AMI do recover within a few weeks, a bit of extra paralysis in the short term could well just be functional protection, nature’s way of saying, “Don’t rush this.”
- The reference given by the author, Snyder-Mackler et al, establishes the existence of AMI in a group of patients only. In fact, the paper has literally nothing to do with EMS at all! This was a classic “clean miss” — the most popular of the 13 Kinds of Bogus Citations. Not everything referenced is well referenced.
- Toth MJ, Tourville TW, Voigt TB, et al. Utility of Neuromuscular Electrical Stimulation to Preserve Quadriceps Muscle Fiber Size and Contractility After Anterior Cruciate Ligament Injuries and Reconstruction: A Randomized, Sham-Controlled, Blinded Trial. Am J Sports Med. 2020 08;48(10):2429–2437. PubMed #32631074 ❐ PainSci #51741 ❐ This trial looks impressive, and identifies clear cellular and strength improvements in the NMES patients in the early stages — but there was also no long-term difference in results. There is a NMES difference, but it may not be an important difference.
- Kim KM, Croy T, Hertel J, Saliba S. Effects of neuromuscular electrical stimulation after anterior cruciate ligament reconstruction on quadriceps strength, function, and patient-oriented outcomes: a systematic review. J Orthop Sports Phys Ther. 2010 Jul;40(7):383–91. PubMed #20592480 ❐ This is a review of 8 low-quality trials done prior to 2010 (and not much was added to that literature in the next decade). It’s basically a garbage in, garbage out study: some trials found an effect and some didn’t, but none of them found a big one, and it’s clear (as usual) that the question just hasn’t actually been studied enough yet to get a good answer.
- This reader comment from a long-time TENS user was interesting: “I’ve used TENS for 17 years on and off. One thing I have noticed over the years is that they sure have got a lot fancier in settings but the basic premise is still the same. But the new features don’t seem to make one jot of difference mentally or physically. The intensity can be increased or decreased, and that’s all.”
- A mostly meaningless oversimplification, since the nerves aren’t “dormant” in any sense, and making them to produce signals is not in itself meaningful to the brain. Nerve stimulation cannot have any specific or inevitable effect on brain state, because it’s completely up to the brain to interpret the significance of nerve impulses — that’s the brain’s job, and there’s no known way to control that precisely with an unnatural external stimulus.
- “The Thync device, in my opinion, is jumping the gun and making claims that go beyond the evidence. All they currently have is one small in-house study looking at one of their two claims (relaxation). Also, using one device stimulating at the same location for both relaxation and stimulation stretches plausibility.”
- Wieselmann-Penkner K, Janda M, Lorenzoni M, Polansky R. A comparison of the muscular relaxation effect of TENS and EMG-biofeedback in patients with bruxism. J Oral Rehabil. 2001 Sep;28(9):849–53. PubMed #11580823 ❐
- We can never prove that there isn’t some special combination of complex, exotic variables that works like a hot damn. The history of science is full of surprising examples of things that work only when you do them juuuuust right — and not at all if you don’t. Unlike a perpetual motion machine, which blatantly defies the laws of physics, we truly have no idea what kooky combo of properties might be a medical marvel, because biology is so messy that it’s extremely hard to be sure of anything.
Special pleading is an informal fallacy: claiming an exception to a general trend or principle without actually establishing that it is, either using a thin rationalization or even just using the exception as evidence for itself (“the rules don’t apply to my claim because my claim is an exception to the rule”).
In the case of TENS settings, the special pleading is that the right settings are an exception to the trend of disappointing research. “Yes, the wrong settings will fail, of course — but my special settings will succeed because they are different and special.”
- Both kinds of current can be lethal if it’s enough current for long enough, but AC is much less comfortable at the same magnitude, and it’s better at making hearts freak out (fibrillation). It takes quite a bit more DC current to cause a heart attack. For instance, the “let-go current” is the highest current at which you can let go of a conductor: above the limit, you can’t let go! The limit for AC is just 22 mA, but 88 mA in DC.
- Cepeda MS, Carr DB, Sarquis T, et al. Static magnetic therapy does not decrease pain or opioid requirements: a randomized double-blind trial. Anesth Analg. 2007 Feb;104(2):290–4. PubMed #17242082 ❐
A growing multibillion dollar industry markets magnetic necklaces, bracelets, bands, insoles, back braces, mattresses, etc., for pain relief, although there is little evidence for their efficacy. We sought to evaluate the effect of magnetic therapy on pain intensity and opioid requirements in patients with postoperative pain. We designed a randomized, double-blind, controlled trial. One-hundred-sixty-five patients older than 12 yr of age were randomized to magnetic (n = 81) or sham therapy (n = 84) upon reporting moderate-to-severe pain in the postanesthesia care unit. Devices were placed over the surgical incision and left in place for 2 h. Patients rated their pain intensity on a 0-10 scale every 10 min and received incremental doses of morphine until pain intensity was < or =4 of 10. Pain intensity levels were similar in both groups. The magnet group had on average 0.04 U more pain intensity (95% confidence interval, -0.4 to 0.5) than the sham group. Opioid requirements also were similar in both groups. The active magnet group required 1.5 mg more morphine (95% confidence interval, -1.8 to 4.0) than the sham magnet group. Magnetic therapy lacks efficacy in controlling acute postoperative pain intensity levels or opioid requirements and should not be recommended for pain relief in this setting.
- Magnet therapy is broadly implausible. For contrast, electrical stimulation therapies make more superficial sense, because cells do at least have many well-known “electrical” functions — and so perhaps they do respond to some non-biological sources of electricity (not that the evidence actually shows that). Magnetism is closely related to electricity in physics terms, but human physiology makes no obvious use of magnetism in general. And so it is generally unlikely that simple magnetism is “stimulating” to tissue in any sense (never mind actually stimulating in a way that is actually beneficial). That doesn’t mean it’s impossible, just that there’s an obvious reason to me more skeptical from the get-go.
- This is known as “fracture nonunion,” a rare but extremely serious complication. “Normally a broken bone will begin to grow together in a few weeks if the ends are held close together to each other without movement. Occasionally, however, a bone will refuse to knit despite a year or more of casts and surgery. This is a disaster for the patient and a bitter defeat for the doctor, who must amputate the arm or leg and fit a prosthetic substitute.” Becker
- Shi Hf, Xiong J, Chen Yx, et al. Early application of pulsed electromagnetic field in the treatment of postoperative delayed union of long-bone fractures: a prospective randomized controlled study. BMC Musculoskelet Disord. 2013;14:35. PubMed #23331333 ❐ PainSci #53405 ❐ “Fracture patients treated with an early application of PEMF achieved a significantly increased rate of union and an overall reduced suffering time compared with patients that receive PEMF after the 6 months or more of delayed union, as described by others.”
- Assiotis A, Sachinis NP, Chalidis BE. Pulsed electromagnetic fields for the treatment of tibial delayed unions and nonunions. A prospective clinical study and review of the literature. J Orthop Surg Res. 2012;7:24. PubMed #22681718 ❐ PainSci #53378 ❐ “PEMF stimulation is an effective non-invasive method for addressing non-infected tibial union abnormalities. Its success is not associated with specific fracture or patient related variables and it couldn't be clearly considered a time-dependent phenomenon.”
- Bagnato GL, Miceli G, Marino N, Sciortino D, Bagnato GF. Pulsed electromagnetic fields in knee osteoarthritis: a double blind, placebo-controlled, randomized clinical trial. Rheumatology (Oxford). 2016 Apr;55(4):755–62. PubMed #26705327 ❐ PainSci #53404 ❐
This was a rigorous test of wearable pulsed electromagnetic fields (PEMF) for older patients with osteoarthritis of the knee: moderate to severe cases with X-ray evidence and pain of at least 4/10 for more than six months, despite maximum tolerated medication. Sixty patients wore either a real PEMF device for 12 hours per day, or a fake; neither they nor the researchers knew who got real PEMF (double-blind). PEMF is particularly easy to test properly, because it causes no sensation, making it much easier to compare to an active placebo.
The placebo devices do not emit a radiofrequency electromagnetic field but are identical to the active devices, including a light-emitting diode light showing operation. The energy from the active device is not felt by the user, and the active device cannot be distinguished in any way from the placebo device.
Their pain and knee function were compared. PEMF won decisively: the real-PEMF patients enjoyed a 25.5% reduction in pain, compared to a 3.6% reduction for the fake-PEMF patients. Knee function improved as well, though not as much. I hope everyone got a real PEMF device at the end!
That’s compelling evidence. Not that there aren’t caveats. There are always caveats.
Although the results seem straightforwardly positive, the authors explain that “some of the effects of this therapeutic approach might be derived from neuromodulation of the pain mechanism”: that is, it might be “just” a pain-killer, as opposed to actually helping to heal arthritic cartilage. (But killing pain effectively would be a pretty good second place.)
Also, the device used is extremely low-power (a tiny battery) — so low that it's quite implausible that it could possibly have a therapeutic effect, and these results need replication to be believed, no matter how good the study seems.
The Bioelectronics Corporation manufactures PEMF devices, and provided the pulsed electromagnetic fields and placebo devices, but they did not fund the study and the authors declared no conflict of interest. These devices are widely available to consumers: see ActiPatch®.