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, and don’t miss the good news at the end of the article about TENS’s complicated, awesome cousin: pulsed electromagnetic field therapy.
TENS has been a therapeutic staple in physical therapy and chronic pain clinics for decades. There are many consumer TENS units 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.
But people will believe anything! (You may have noticed this.) Much like ultrasound, this old-school techy therapy definitely isn’t all it’s cracked up to be: the effectiveness of TENS [NHS] “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.
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 or disrupt pain. 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 unit (“These go to eleven.”), it is possible to “taze yourself.”
And so it’s possible that the right setting makes all the difference, just like with phasers. This makes the topic endlessly debatable, even though it’s obvious that 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 then there’s central stimulation — stimulation of the spinal cord or brain — the other major kind of neuromodulation. It’s kind of a big deal: more invasive, profound, and dangerous, and also used with more impressive results for more serious problems like Parkinson’s diseases, major depression, and serious chronic pain. It 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. That’s an advanced topic, beyond the scope of this article for now.
TENS units 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.6 Overzealous self-treatment might cause some skin irritation around the electrodes.
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 (migraine 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.7 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.
There’s 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 20018||negative review of 5 trials of TENS for chronic low back pain|
|Johnson 20079||positive review of 38 trials of TENS for chronic musculoskeletal pain, “effective”|
|Nnoaham 200810||inconclusive review of 25 studies of TENS for chronic pain|
|Khadilkar 200811||inconclusive review of 4 trials of TENS for chronic low back pain|
|Walsh 200912||inconclusive review of 12 trials of TENS for acute pain|
|Hurlow 201213||inconclusive review of 3 trials of TENS for cancer pain|
|Vance 201414||mixed review: “it’s complicated” but promising|
|Chen 201515||negative review of 18 trials of TENS for knee osteoarthritis|
That doesn’t look great: five disappointingly non-positive,16 two spankings for back pain and osteoarthritis, and just the one clearly positive review out of seven (which is also one of the oldest and perhaps the least picky).17
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.18 Maybe science hasn’t asked the right questions yet. Despite the messy evidence, Vance et al. believe that “TENS has been shown to provide analgesia specifically when applied at a strong, non-painful intensity.” 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,19 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.
The reason it might work is probably the same reason it can only work so well.
Pain is completely controlled by an overprotective brain that likes to sound the alarm too loudly, too often, regardless of what’s actually going on in tissues. This makes most kinds of pain partially and briefly treatable with tricks and hacks.20 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.21 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.
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.22 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.23 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).24 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!
Just as electrical stimulation can be shallow or deep, there’s also electrical nerve stimulation (ENS) or electrical muscle stimulation (EMS).25 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, useful either in rehabilitation or, perhaps, performance enhancement. EMS feels like doing light isometric muscle contraction exercises — that is, clenching muscles without moving joints.
Whereas ENS 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.26
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, and nothing you couldn’t get just as well or more so with isometrics, and more conveniently 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 would be people who are more profoundly injured, or have specialized training needs due to a progressive disease.
And then there’s the entertainment value. EMS is kinda neat. If you just like the idea of messing around with it for the kicks and novelty, knock yourself out — figuratively speaking.
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.
These days, the relaxation claims are fancier. (Almost everything about TENS device is fancier these days.27)
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 activate28 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.29 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.30 That is a bummer.
And I’d still try it if I had temporomandibular joint syndrome. The right settings could make the difference!
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.31 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!
“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?
Pulsed electromagnetic field therapy (PEMF) is clearly kin to TENS, part of the electrotherapy family. And yet it’s a different beast, much more exotic, with a more mysterious mechanism of action. PEMF is hypothesized to directly stimulate cellular repair, and not for nothing: it seems to really do that, and the effect is almost magical, speeding up bone fracture healing, and even restoring it in cases where healing has failed completely.32
So PEMF has been used on fractures for a long time now, 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?
Apparently so. Although the evidence for this isn’t unanimous, some of the best and most recent PEMF trials are unambiguously positive.35 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 just looking rather good, and I think it’s probably worth a try.
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
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.
— 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.
— Added section about pulsed electromagnetic field therapy.
This was 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 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.BACK TO TEXT
This was a scientifically 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!
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®.BACK TO TEXT