When you apply an ice pack or a heating pad to your skin, what happens to the temperature of deeper tissues: muscle and bone, and ligaments and joints? What’s the temperature change at one centimetre, or three? Or even the core body temperature? How do those changes compare to the effects of saunas, fevers, exercise?
Our bodies have amazing environment control systems. We’re extremely good at cooling down when we’re hot, and heating up when we’re cold, and protecting the most important tissues from the sharpest temperature changes. For instance, if you have a hot bath, you can raise your core temperature a little — a mild artificial fever — but it has to be really hot, and the effect doesn’t last long.1 And no tolerable spot heating or cooling has any significant effect on core temperature.
Testing the Effect of Heat
There’s surprisingly little hard data on this topic, but just enough to get a good sense of it. Taking tissue temperatures is not rocket science.
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 thin meat thermometer (don’t worry, they were anaesthetized).2
They found an average increase of 3.8˚C at a depth of one centimetre, and .78˚ at three centimetres. I think it’s safe to extrapolate a warming of a couple degrees at 2cm, and barely anything at 4cm.
Do those kind of temperature increases mean anything?
Yes and no. The 3.8˚C increase in the muscle shallows is noteworthy (the same change in core body temperature would be a serious fever). But the .78˚ change at 3cm depth — and a lot of muscle is that deep3 — is clinically trivial, well within the range of healthy variations in core body temperature4 — not counting the effect that exercise would have, which can easily add another degree or two.5
I think this data clearly shows that superficial heating is an easy way to modestly increase tissue temperature up to a couple centimetres — a majority of the volume of most muscles — for whatever that’s worth (see above).
Location, location, location: hands, feet and joints are a lot easier to heat up or cool down
Joints are less able to adapt to regulate their temperature, and can be cooled or heated much more easily. In two different experiments with dozens of healthy or arthritic knees, Oosterveld et al demonstrated that hot wax (ligno-paraffin) could raise deep knee temperatures by 1.7–3.5˚C.6 Even the low end of that range is probably a clinically significant number: that could have some impact on joint physiology.
And now finally we get to ice: the effect of cooling was even more dramatic, dropping the temperature as much as 9˚C. Brrr! If that was core temperature, you’d be in trouble — although not as bad as being 9˚ too hot.7
Hands and feet are chock-a-block with joints and they are thinner anatomy, and so they are the easiest places in the body to warm up. Borrell et al showed that they should be heated to 9˚(hands) and 5˚(feet) at a depth of a half centimetre,8 and it’s reasonable to assume that the heating at greater depths is greater than it is for thick muscles (because of all the joints).
Fat is a factor
Adipose tissue is both a great insulator and, of course, adds thickness. Many areas naturally have more fat just under the skin, even in very lean people. The penetrating power of heating and cooling is modest even without fat. Almost any amount of fat over the target tissue will render superficial heating or cooling pointless.9
But that’s relatively obvious, and it gets worse: even just being unfit (not obviously fat) could be a deal-breaker. There can also be amazing differences in the amount of fat people have between muscles (think about “marbling” of fat in a steak). So a superficially skinny person may have a much higher percentage of fat filling up their large muscle compartments. These muscle compartments would definitely be much harder to heat up.
Different methods of heating and cooling
While they were at it, Borrell et al also showed that a dry superficial heating method is actually superior to ultrasound or diathermy. They trialed three kinds of superficial heating:
- immersion in hot wax
- hot water
- “dry whirlpool,” a novel method of dry heating called Fluidotherapy
Dry heat is more easily tolerated, so the hope was that it would be more effective, and it was: dry heat on the hands and feet raised tissue temperature at a half centimetre depth to 9˚ and 5˚ respectively, somewhat more than hot wax or water. The authors makes the case these methods are superior to the “penetrating” heat of microwaves or ultrasound because they can be easily applied to larger surface areas. And so they concluded
that surface heating modalities are much more effective in producing elevated temperatures than is ultrasound therapy or diathermy at depths of up to 1.2cm.
Surprising and interesting! Most people would assume the opposite, I’m sure.
This video is only barely relevant, but it is really interesting. What fantastic cameras we have these days!
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 — Cited Petrofsky 2009, regarding the effect of adipose tissue.
2016 — Corrections and clarifications, and new information about body temperature in baths, more information about fat as an insulator, and an interesting video.
- I haven’t found any formal data on this, but I have personally tested my temperature many times before, during, and after taking hot baths, to try to get a sense of how much you can tinker with your core body temperature. After 5-10 minutes in a bath so hot it’s barely tolerable, I’ve seen my own temperature get up to 38.5˚C, a degree or two over the top of the normal range (more below on what the normal range is). It falls back to something saner really fast, within minutes.
- 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 ❐
- Three centimetres is quite deep — some people might not even have that triceps muscles that thick! Any important temperature change at that depth would affect the majority of most muscles in the human body. But a few key areas would be too thick to touch with superficial heat: the deepest muscle tissue in the back, buttocks, and thighs. And the temperature depth at 3cm was not important.
- Normal body temperature, measured orally, is usually reported as 36.8±0.5°C, for a total range of 1˚C. However, a 2002 review reported a normal range twice as large, 36.7±1°C.
- Intense exertion can raise core body temperatures to dangerous levels. Heat exhaustion can occur even in cool weather, starts around 38˚, and transitions to “heat stroke” around 40˚ — which is always a medical emergency.
- Oosterveld FG, Rasker JJ. Effects of local heat and cold treatment on surface and articular temperature of arthritic knees. Arthritis Rheum. 1994 Nov;37(11):1578–82. PubMed #7980668 ❐
One of two similar experiments (Oosterveld 1992 and Oosterveld 1994) testing heating and cooling methods on knees, with or without arthritis. This test measured the effect of ice chips, ligno-paraffin (hot wax), nitrogen-cold air, and placebo short wave diathermy on 39 arthritic knees.
All methods changed the temperature inside healthy knees, by as much as several degrees. Cooling had a greater impact than heating, with temperatures inside the joint dropping as much as 6.4˚C with ice chips (similar but different from the 1992 experiment). Hot wax increased intrarticular temperature by 1.7˚C in this experiment (compared to 3.5˚C in the 1992 test).
- The body tolerates chilling better than overheating. It only takes a ~3˚ boost for heat stroke to kick in. Hypothermia begins after a drop of ~2˚, but after even a 4˚ drop you’re still in the “awake and shivering” phase. This isn’t healthy, but it’s not a medical emergency yet. The “drowsy and not shivering” phase starts around 28˚, which is a full 9˚ below normal.
- 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 ❐ “Temperature rises of capsules in the hand and foot were consistently higher than temperature rises in muscle at the same depth and that the temperature rises in the foot were higher than those in the hand. The former is probably due to blood flow variations between muscle and capsule; the latter because foot skin temperatures are normally two or three degrees lower than hand temperatures.”
- Petrofsky JS, Laymon M. Heat transfer to deep tissue: the effect of body fat and heating modality. J Med Eng Technol. 2009;33(5):337–48. PubMed #19440919 ❐ “If the body fat exceeded 25% of the subject's weight, 20 min of immersion was not enough to either warm the muscle or cool it down substantially. … Body fat plays a major role … in controlling the movement of heat across the limb.”