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How deep does the heat of a heating pad go? 

 •  • by Paul Ingraham
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Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

What happens to the temperature of the muscle underneath a heating pad? 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).18 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 assume there was virtually no effect at 4cm, and at least a couple degrees at 2cm.

So … how much is that? Do those kind of increases mean anything?

Three centimetres is deep — some people might not even have triceps muscles that thick! Any temperature change at that depth would affect the majority of any muscle. Some areas would obviously be too thick to touch with superficial heat: the deepest muscle tissue in the back, buttocks, and thighs especially. But muscle in many other areas is much more shallow.

The 3.8˚C increase in the muscle shallows is substantial: the same change in core body temperature would be a serious fever. But the .78˚ change at 3cm depth? That’s trivial, well within the range of healthy variations in core body temperature. In other words, exercise would probably increase all tissue temperature, at any depth, by at least the same amount.

This post is a new excerpt from a much longer featured article about heating.