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.
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.
Draper et al. heated subjects triceps muscles with hot packs for 15 minutes & then checked their temperature with a needle probe — like a very thin meat thermometer.
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.
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).
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).
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.
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:
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!
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.
— Cited Petrofsky 2009, regarding the effect of adipose tissue.
— Corrections and clarifications, and new information about body temperature in baths, more information about fat as an insulator, and an interesting video.
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).BACK TO TEXT