Therapeutic heating — “thermotherapy” for therapy geeks, the opposite of its trendier cousin, cryotherapy — is more useful than most people realize, mainly because muscle is a source of more and worse pain than most people realize, and muscle pain seems to respond well to heat. Muscle pain caused by over-exertion, muscle cramps and spams, and especially trigger points (muscle “knots”) are all common and sometimes severe, but often mistaken for other kinds of problems.
This therapeutic staple has mostly been ignored by science: its benefits are far from proven, and obviously it’s no miracle cure.1 Nevertheless, it’s probably doing more than “just” relieving symptoms. (Not that we should knock symptom relief.2) Everyone should understand heating the same way everyone knows how to put on a bandaid: it is a cheap, drugless way of taking the edge off an amazing array of common painful problems, especially neck and back pain, and maybe more.
What heat is for: mostly non-inflammatory body pain
Heat is primarily for relaxation, comfort, and reassurance, and taking the edge off of several kinds of body pain, mostly duller and persistent pains associated with stiffness, cramping, and/or sensitivity, which can be loosely categorized:
- Acute soreness from over-exertion: the pain you get after the first ski trip of the season. (Interestingly, not only is heat likely helpful for this kind of pain, it’s almost the only thing that is! More about this below.)
- Stiffness and pain in specific areas related to osteoarthritis, muscle “knots” or trigger points, and most kinds of cramping/spasm3 (menstrual, neuropathic, restless leg syndrome, for example, or even just stiffness from postural stress). But not, of course, cramps from heat exhaustion!
- “Hurts all over” pain and sensitivity. There are many kinds, but primarily: fibromyalgia, the rheumatic diseases, drug side effects,4 vitamin D deficiency, and sleep deprivation.
There are many other kinds of pain, of course, but these are the ones most likely to benefit from heat. No one with appendicitis or a 2nd degree burn wants a nice hot water bottle.
What heat is not for
Heat will make some conditions much worse. Never apply heat to an infection or fresh injury! Or any other acute inflammation, like a flare-up of arthritis. Just don’t do it! That’s what ice is for: soothing inflamed tissue.
And what’s a “fresh” injury? Any time tissue has been physically damaged, it will be inflamed for a few days, give or take. If superficial tissue is sensitive to touch, if the skin is hot and red, if there is swelling, these are all signs that your injury is still fresh, and should not be heated.5
If there’s no obvious/severe injury or infection, it’s okay to try a heat treatment. But it can be surprisingly unclear!
Sometimes it’s really tough to tell if pain is due to an injury. This is the puzzle at the centre of many chronic pain problems: the distinction between feeling damaged and being damaged. Several kinds of pain exist in a gray area between the two.
The best example is probably muscle strains versus muscle “knots.” A strain is an actual rip in the muscle — physical damage, while a “knot” or trigger point (TrP) is an irritable patch of tissue, probably a micro-spasm (more about this below). The difference between a minor strain and a bad trigger point can be subtle.6 And yet you want to heat the knot, not the strain! Fortunately, if the strain is mild enough that you can’t tell if it’s a strain, it’s probably not a big deal one way or another: just try some heat and see what happens.
But there are many situations where this kind of ambiguity is a challenge, like low back pain.7
If you suspect a muscle strain, but you’re not sure, here’s a good tutorial for helping you sort it out: Save Yourself from Muscle Strain! If back pain is your issue and you’re not sure what’s causing it, get thee to another tutorial: Save Yourself from Low Back Pain! And I have an article specifically devoted to helping people understand why heating back pain is usually better than icing it: (Almost) Never Use Ice on Low Back Pain!
The hot water bottle: The classic original source of therapeutic heat.
How does heat therapy work?
The next several sections explore different mechanisms and details of how heating might be helpful to people in pain:
- Heat is reassuring, and reassurance is analgesic. (This is applied neurology, not just a psychological effect.)
- Heat can penetrate a few centimetres into tissue, and cells and biochemistry speed up when the tissue temperature rises, which might have therapeutic implications. For instance …
- Heat may have an effect on the common painful phenomenon known as “trigger points.”
- Heat may help soreness after exercise.
- Tiger Balm and similar products are “spicy” not warm, but they might tinker usefully with sensation: a neurological distraction.
Heat is reassuring, and reassurance is analgesic
Our comfort zone is a warm place. And so, almost no matter what kind of pain you have …
To reduce pain, we need to reduce credible evidence of danger & increase credible evidence of safety.
~ Lorimer Moseley. Explainer: what is pain and what is happening when we feel it? TheConversation.com.
The brain probably interprets a safe source of warmth as good “credible evidence of safety,” for basic psychological reasons related to the environmental and social conditions we evolved in. Cold kills! Hypothermia has been at or near the top of the list of threats to our safety throughout all of prehistory and most of history.8 And we also associate warmth with contact and intimacy — another powerful goodness.
So a nice controlled source of warmth is probably just about the most basic reassuring thing there is. And that’s always good for pain.9 You might mistake this for a psychological effect, and it is in a sense, but it’s more useful to look at it as “applied neurology”: leveraging what we know about how pain neurology works. It’s more akin to triggering a reflex than a mind game.
Some like it hot
How hot is too hot? It depends on your tolerance. Some like it a lot hotter than others. Comic by Jake Likes Onions
Heating depth: how far does it go?
Roughly a degree celcius or two at a couple centimetres depth, give or take, depending on how and where it’s done.
Scientists have tested this. For instance, in 1998, Draper et al heated subjects triceps muscles with hot packs for fifteen minutes, and then checked their temperature with a needle probe — like a very thin meat thermometer (don’t worry, they were anaesthetized).10 They found an average increase of 3.8˚C at a depth of one centimetre, and .78˚ at three centimetres.
I think this data clearly shows that superficial heating is an easy way to modestly increase tissue temperature up to a couple centimetres — which is most of the volume of most muscles — for whatever that’s worth. The 3.8˚C increase in the muscle shallows is something, but .78˚ change at 3cm depth — and a lot of muscle is that deep — is clinically trivial, well within the range of healthy variations in core body temperature.
Heat for trigger points
A trigger point is a small patch of acutely sensitive soft tissue, a poorly understood little patch of sensory misery. They are probably contracted, stagnant, swampy11 sections of muscle tissue — but that’s just a theory.12
However they work, no one doubts that these sensitive spots in muscle are common. They can cause anything from stiffness and dull aching to show-stopping agony. The pain often spreads in confusing patterns, and they grow like weeds around other painful problems and injuries, making them interesting and tricky and incredibly common.
And heat seems to be a particularly good therapy for trigger points. Never formally tested! But plausible.
Pure speculation about why heat might be good for trigger points
Trigger points are probably aggravated by stress, and (as noted above) being warm is a pleasant and comforting sensation, as long as we aren’t overheated to begin with. But it probably goes beyond that …
Relaxation reduces resting muscle tone. You can have “tight” muscles without actually being in frank spasm. There are many degrees of muscle tone between deep relaxation and a charlie horse. Many otherwise healthy people live in a state of uncomfortably high muscle tone, their muscles always a little clenched and exhausted, probably with some specific areas even worse from awkward working postures. This state is inherently uncomfortable, like being tired from exercise — but without the endorphins — and it may be fertile ground for trigger points. If so, any reduction in muscle tone may be quite helpful.
Heat for soreness after exercise
Perhaps the most popular therapeutic reason to get into a hot tub is to try to beat the pain of DOMS (delayed-onset muscle soreness) — that nasty 24–28 hours of muscle pain you get after unfamiliar exercise. But DOMS is mostly invincible. Researchers have proven over and over again that there really is no effective treatment for it.
Except, maybe, heat?
For a long time, “warm underwater jet massage” — hot tub jets — was the only treatment that had ever shown any potential, in a 1995 study.13 But it was a small and flawed piece of research — and most people know from personal experience that a soak in a hot tub may “take the edge off,” but hardly constitutes a miracle cure for DOMS. I mostly ignored that evidence.
In 2006, we got some good science news: more persuasive data, surprisingly good results in treating DOMS in the low back with a “heat wrap,” a wearable device that applies heat for hours at a time.14 Another small study, but this time more persuasive one. Eureka? Proof needs more data, but this makes it well worth trying heat on your sore muscles after exercise. Happy heating!
Is Tiger Balm hot?
Ointments and balms like Tiger Balm15, RUB A535, and Deep Heat are not really hot, but they do have two things in common with heat — they feel hot, and they cause superficial capillaries to open up as wide as they can.
All these products contain a chemical irritant or rubefacient (and there’s your word-of-the-day). Rubefacients feel hot because they give you a mild chemical burn! Capsaicin is the most classic rubefacient, the active ingredient in most of these products, and it’s literally spicy: it’s the same stuff that makes chilli peppers spicy, on your tongue or your back?
So rubefaciants piss off your skin, in other words.
And that is the only sense in which a rubefacient is heating anything. The product name “Deep Heat” always makes me roll my eyes, because a more truthful name would be Shallow Irritation! “Chemicals that are mild skin irritants may make a patient feel warm,” wrote Borrell et al, “but they will not produce any in vivo temperature rises or any of the physiological effects of heat.”16 The effect is just too superficial.
And is a mild chemical burn helpful in any way? The main rationale for a rubefacient is that it’s a counter-irritant — a neurological distraction from your pain, which is the more general and well-known principle of diffuse noxious inhibitory control. Counter-irritation is a real thing, but it’s not a powerful thing. You could call it another tool for the pain treatment toolbox, but it’s not an impressive tool, maybe just like a small screwdriver or a tack hammer.
Or maybe a crowbar? Capsaicin has actually gotten a lot of high fives and thumps on the back from science.17 It seems likely that some people, for unclear reasons, respond to them much better than others, which boosts the worth-a-try factor.
Some of the effects of capsaicin are more exotic than expected, too — like actually making nerve endings shrivel and retreat.18 Interesting stuff.
Note that many rubefacients also have touted other medicinal and pain-killing ingredients and mechanisms. For instance, garlic is supposed to be good for all kinds of things, salicylates are definitely medicinal in some applications. But of course any such benefits are just bonus prizes that have nothing to do with heating or even rubefaciants, and it’s a rogue’s gallery of mostly unexciting possibilities.
Local heating versus systemic heating
There are many ways to heat yourself up, but two broad categories: local and systemic.
Local heating means specific heating: applying a hot water bottle, heating pad, heated gel pack or bean bag to a specific place on the body.
Systemic heating means raising the entire body temperature with a bath or jacuzzi, steam bath, or piping hot shower — basically creating an artificial fever!
For most local heating, I recommend seeking out a large-sized (14 × 27) Thermophore, the Rolls Royce of heating pads: large, heavy, thick, and moist. Yes, moist — they produce a moist heat with “a special tightly-woven fleece blend cover which retains moisture from the air.” If you leave a Thermophore on a plastic surface, there will be beads of water under it five minutes later. The moisture captured from the air by the Thermophore conducts heat far more effectively than a dry heating pad. Luxurious! Both genuine Thermophores (roughly USD $50-100) and cheaper knock-offs (and probably nearly as good) can be ordered online from Relaxus.com, and are also available in some medical supply stores. Of course, Amazon.com has them, too.
Full-body heating in hot tubs, showers, saunas and steam rooms is also often helpful factor with conditions where emotional stress and/or knots in your muscles are the cause of your troubles, or a significant complicating factor — low back pain is the most obvious example. See Hot Baths for Injury & Pain for tips on getting the most out of your bathtub or Jacuzzi. Believe it or not, many people do not really know how to take a bath! There are several simple tips that can definitely wring more therapeutic value out of the experience. (Adding Epsom salts to a bath is not one of them, however — it’s popular, but probably useless.24)
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About Paul Ingraham
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.
What’s new in this article?
2016 — Added evidence of efficacy of NSAIDs for common injuries.
— A series of substantial upgrades, details unlogged.
- Garra G, Singer AJ, Leno R, et al. Heat or cold packs for neck and back strain: a randomized controlled trial of efficacy. Acad Emerg Med. 2010 May;17(5):484–9. PubMed #20536800. ❐ This 2010 study showed quite clearly that both ice packs and hot packs were only a little bit beneficial for neck and back pain, and about equally so. But a small therapeutic effect is still valuable, and there are almost certainly situations where it works even better — after all, these were people with acute pain bad enough that they went to the hospital. They may have been a bit beyond the help of a hot pack! BACK TO TEXT
- PS Ingraham. Masking Symptoms Is Under-Rated: Masking symptoms,” especially with medications, is often maligned. But sometimes symptoms need masking! ❐ PainScience.com. 843 words. “Masking symptoms,” especially with medications, is often maligned because it doesn’t “treat the root cause.” But masking symptoms can be a good idea, and it should not be eschewed just because it doesn’t have a real healing effect … because there are very few real healing effects! “Healing” is mainly about removing impediments to natural recovery, like stress on tissues. It’s not dictated by some mythical power to speed healing, but by a strong understanding of the nature of the problem and what pisses it off and impedes recovery. Focus on facilitating natural recovery, and don’t knock a little “symptom relief” along the way. BACK TO TEXT
Muscle tension can cause a surprising amount of pain. A leg or foot cramp is a common and extreme example, where no one has any doubt of the cause of pain. But imagine a muscle spasm much less strong, but lasting for days and days — or years! Although superficially a simple concept, there are actually several physiological mechanisms by which muscle can become shortened and painful for a long time, some well understood and straightforward (spasticity from neurological diseases), while others are quite mysterious (like spasms peole are born with, as in torticollis or wry neck).
One excellent example of neuropathic spasticity is the “MS hug”: a symptom of multiple sclerosis that feels like a painfully tight band around the chest, often experienced long before diagnosis. Although the feeling of constriction is the classic symptom, many patients also just experience widespread pain in the chest wall.BACK TO TEXT
- Several drugs make muscles hurt, but the most notorious are the statins (cholesterol-reducing drugs) and bisphosphonates (for osteoporosis). In some people, these act like muscle poisons. BACK TO TEXT
- Here’s an example of what can happen when you heat an inflamed injury: When I was still in school, and my father had not yet learned to call me before asking a doctor about his aches and pains, he went to a drop-in clinic following a traumatic knee injury. The physician on duty prescribed heat! That was wrong, and the results spoke loud and clear: my dad’s knee swelled dramatically, outrageously, causing severe pain and immobility. BACK TO TEXT
- Some rips aren’t as sudden and obvious as you might expect, and trigger points can flare up faster than you expect. They also probably occur together: a TrP may be why the muscle tore in the first place, or it could crop up in the aftermath, or it could simply feel like a tear. BACK TO TEXT
- In low back pain — infamous for being frustratingly chronic — strains and trigger points are both common, but there are also many other potential sources of pain. So it’s much more difficult to know if heat is a good idea than it is with, say, thigh pain. BACK TO TEXT
- Obviously heat kills too, but much, much less frequently. Humans can survive almost any amount of heat as long as we can get some shade and water and don’t have to overexert ourselves. Cold kills relatively effortlessly! BACK TO TEXT
- PS Ingraham. Pain is Weird: Pain science reveals a volatile, misleading sensation that is often more than just a symptom, and sometimes worse than whatever started it. ❐ PainScience.com. 12992 words. Modern pain science shows that pain is a volatile, complex sensation that is thoroughly tuned by the brain and often overprotectively exaggerated, so much so that sensitization often becomes more serious and chronic than the original problem. All of this has complicated “all in your head” implications: if the brain controls all pain, does that mean that we can think pain away? Probably not, but we do have some “neurological leverage” of great value — we can influence pain, if we understand it. BACK TO TEXT
- Draper DO, Harris ST, Schulthies S, et al. Hot-Pack and 1-MHz Ultrasound Treatments Have an Additive Effect on Muscle Temperature Increase. J Athl Train. 1998 Jan;33(1):21–4. PubMed #16558479. ❐ PainSci #54111. ❐ BACK TO TEXT
- Shah JP, Danoff JV, Desai MJ, et al. Biochemicals associated with pain and inflammation are elevated in sites near to and remote from active myofascial trigger points. Arch Phys Med Rehabil. 2008;89(1):16–23. PubMed #18164325. ❐
This significant paper demonstrates that the biochemical milieu of trigger points is acidic and contains many pain-causing metabolites. For much more information about this, see Toxic Muscle Knots.BACK TO TEXT
- “Trigger points” are controversial, although published expert opinions criticizing the concept remain relatively scarce. This article does not get into the controversy, but it’s important to acknowledge it. The bottom line is that people do suffer from a painful phenomenon (very sensitive spots), and no one disputes that — they just dispute what to call it, how it works, and how to treat it! If you want to learn more about the controversies, see: Trigger Point Doubts: Do muscle knots exist? Exploring controversies about the existence and nature of so-called “trigger points” and myofascial pain syndrome BACK TO TEXT
- Viitasalo JT, Niemela K, Kaappola R, et al. Warm underwater water-jet massage improves recovery from intense physical exercise. Eur J Appl Physiol Occup Physiol. 1995;71(5):431–8. PubMed #8565975. ❐ BACK TO TEXT
- Mayer JM, Mooney V, Matheson LN, et al. Continuous low-level heat wrap therapy for the prevention and early phase treatment of delayed-onset muscle soreness of the low back: a randomized controlled trial. Arch Phys Med Rehabil. 2006 Oct;87(10):1310–7. PubMed #17023239. ❐ BACK TO TEXT
- Ironically, tigers really hate Tiger Balm. BACK TO TEXT
- Borrell RM, Parker R, Henley EJ, Masley D, Repinecz M. Comparison of in vivo temperatures produced by hydrotherapy, paraffin wax treatment, and Fluidotherapy. Phys Ther. 1980 Oct;60(10):1273–6. PubMed #7443789. ❐ BACK TO TEXT
- Derry S, Sven-Rice A, Cole P, Tan T, Moore RA. Topical capsaicin (high concentration) for chronic neuropathic pain in adults. Cochrane Database Syst Rev. 2013;2:CD007393. PubMed #23450576. ❐ BACK TO TEXT
- Kennedy WR, Vanhove GF, Lu SP, et al. A Randomized, Controlled, Open-Label Study of the Long-Term Effects of NGX-4010, a High-Concentration Capsaicin Patch, on Epidermal Nerve Fiber Density and Sensory Function in Healthy Volunteers. J Pain. 2010 Jun;11(6):579–587. PubMed #20400377. ❐ BACK TO TEXT
- There are four kinds: acetaminophen/paracetamol (Tylenol, Panadol), plus three non-steroidal anti-inflammatories (NSAIDs): aspirin (Bayer, Bufferin), ibuprofen (Advil, Motrin), and naproxen (Aleve, Naprosyn).
BACK TO TEXT
- Science Based Pharmacy [Internet]. Gavura S. How risky are NSAIDS?; 2015 Jul 25 [cited 16 Aug 18]. BACK TO TEXT
- Hung KK, Graham CA, Lo RS, et al. Oral paracetamol and/or ibuprofen for treating pain after soft tissue injuries: Single centre double-blind, randomised controlled clinical trial. PLoS One. 2018;13(2):e0192043. PubMed #29408866. ❐
In a test of ibuprofen vs paracetamol for hundreds of soft tissue injuries, there was no significant difference in the modest benefits or side effects, contrary to some past evidence and the widely held medical belief that ibuprofen is better for injuries (inflammation). It’s possible that placebo is powering the effects of both medications, and that's why they were equal, but it's impossible to know without a control group for this study. Note that the superiority of these medications is probably variable and not as clearly established as you might think.BACK TO TEXT
- FDA.gov [Internet]. Acetaminophen and Liver Injury: Q & A for Consumers; 2009 Jun 4 [cited 16 Aug 31].
“This drug is generally considered safe when used according to the directions on its labeling. But taking more than the recommended amount can cause liver damage, ranging from abnormalities in liver function blood tests, to acute liver failure, and even death.”BACK TO TEXT
- Machado GC, Maher CG, Ferreira PH, et al. Efficacy and safety of paracetamol for spinal pain and osteoarthritis: systematic review and meta-analysis of randomised placebo controlled trials. BMJ. 2015;350:h1225. PubMed #25828856. ❐ PainSci #54220. ❐ BACK TO TEXT
- Epsom salt in your bath is cheap and harmless and it makes the water feel “silkier,” but it’s unlikely that it has a therapeutic effect on aches and pains. Magnesium supplementation might be helpful for some patients with some kinds of pain, but not many others, and it’s doubtful that it can soak through the skin, and it definitely doesn’t “detox” anything. The soothing heat of a nice bath is probably the main source of health benefits. The case for the healing powers of Epsom salt is mostly made by people selling the stuff, or recommending it as casually and imprecisely as an old wives’ tale. See Does Epsom Salt Work? The science of Epsom salt bathing for recovery from muscle pain, soreness, or injury. BACK TO TEXT