Relevance of foot sweat to blister formation? <\/h2>\n\n\n\n
Quite simply, even mild to moderate hydration levels of the feet are known to increase skin friction and the likelihood of blisters. This is a snapshot of research and papers that support that. One that I’ll draw your attention to is if you were on last month’s call, we looked at the Hashmi study<\/a> where they produce blisters on the posterior heel. The same set of researchers basically replicated that study, but this time they had one foot in a bucket of water, and one foot not (it remained dry), and they applied the same repeated shear load to both. What they found was that the rate of temperature change in response to that repeated shear loading was significantly greater on the hydrated skin compared to dry skin. From their previous research, we know that this temperature increase can be used to indicate imminent blister formation. So, the wetter your skin (as opposed to dry), the sooner you’ll get blisters with the same amount of shear load.<\/p>\n\n\n\n In terms of mechanics, where does friction come in? Friction is the force that exists between the interfaces or the materials external to the skin surface. So we’re talking at the skin-sock and sock-shoe interfaces. So if friction force is high, it keeps everything stuck together while the bones are moving back and forth in the foot as part of normal mechanics. So the bones are moving back and forth, the skin staying still, and all the soft tissues in between allow that to happen by undergoing shear deformation<\/a>. If those fiction levels are high, the skin stays still for longer; and given the bone has to move a certain amount, the magnitude of shear deformation is going to be larger when that friction force is high. But if we can reduce that friction force, then as that bone moves the exact same amount, then the skin is able to move in sync with the bone at an earlier moment in that individual shear distortion, thereby reducing the magnitude of that shear distortion. Foot sweat is what increases friction force, compared to dry.<\/p>\n\n\n\n Sweat gland comparison, from Baker, L. (2019). Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature. 6(3), p 211-259<\/p>\n<\/div><\/div>\n\n\n\n There are some sweat glands that are irrelevant to the feet, so just quickly:<\/p>\n\n\n\n The sweat glands relevant to the feet are the eccrine sweat glands:<\/p>\n\n\n\n If we have a real quick look at the sweat gland cells, they can be basically divided into the duct (the straight bit that takes the sweat to the surface of the skin); and the secretory coil (the big ball of wool at the bottom).<\/p>\n\n\n\n The bits that I’ve circled in blue are the duct cells. Their primary function is the reabsorption of sodium and chloride ions as the sweat passes up towards the skin, so that the sweat that is finally secreted on the skin is hypotonic.<\/p>\n\n\n\n And if we look at the secretary coil cells, there are three types of cells. There are clear cells – they’re the ones that actually produce the sweat. There are dark cells – we don’t really know what they do. And there are myoepithelial cells (the clue is in the name) – they have a contractile function.<\/p>\n\n\n\n Eccrine sweat glands primarily respond to thermal stimuli; particularly increased body core temperature, but also skin temperature and increased skin blood flow; so basically, thermoregulation. Thermal sweating is mediated predominantly by sympathetic cholinergic simulated stimulation. So we’ve got this sympathetic post-ganglionic neuron; it gets to the clear cells; it releases acetylcholine and that binds to the muscarinic receptor on the clear cell. So it’s the release of the acetylcholine that kick-starts sweat production.<\/p>\n\n\n\n Eccrine glands also secrete sweat in response to adrenergic stimulation but to a much lesser extent. So that’s basically the fright or flight response. In addition, eccrine sweat glands respond to non-thermal stimuli related to exercise; this is in reference to the fact that we know people that are fit and exercise a lot, they sweat more and sooner than people that don’t exercise a lot. So you can say that their thermoregulation. is more effective.<\/p>\n\n\n\n So if you had to say right now which part of the foot sweats more – the dorsal surface or the plantar surface, which would you say? I\u2019d say the plantar surface.<\/p>\n\n\n\n Well, you might be in for a surprise\u2026<\/p>\n\n\n\n Carlson mentioned a quick pilot study that they did in his 2006 paper. They got fresh socks from three runners and the moisture content was 15%, 23% and 31%. So he suggested that if we’re interested in what’s going on in the shoe in terms of the moisture from foot sweat in the sock, we should be looking at a moisture content of the sock of about 25 to 30%. The graph on the right is where they got a cotton sock and they gradually increase the water content from zero to 100%; and you can see the differences in the coefficient of friction on the y axis here.<\/p>\n\n\n\n If we look at what happens as the sock gets wetter, PTFE (which is the Engo patches or Shearban) the COF remains relatively constant no matter how wet the sock gets, with a coefficient of friction of about 0.2. The yellow line is Poron. It increases sharply to about that 25%-30% mark, then at levels off; and then it increases again at around 70%. Plastazote is relatively stable from around 0.45 to 0.5. And Spenco increases for the first half and then kind of levels off for the second half. Now, we know Spenco has a good blister prevention effect – that research has been done in terms of blisters and calluses. So from this data, you would probably question whether it’s main mechanism is coefficient of friction reduction. Probably I would think it’s more likely to be something else and my best guess would be shear absorption and maybe even pressure management.<\/p>\n\n\n\n How much do the feet sweat? Here are some values from the research:<\/p>\n\n\n\n The first two are from 2006 to 2007. The others are from a subsequent paper that was similar to the first two, and we’ll talk about in a minute (see references).<\/p>\n\n\n\n From what I understand, these Taylor (2006) and Fogarty (2007) papers are the first two that really looked closely at regional differences on the feet. Both of them find that the dorsal aspects sweats a lot more than the plantar aspect; around about 70:30. Nigel Taylor is from the University of Wollongong and he and his team have done a lot of work in this area, publishing a lot of data. And the Fogerty is a UK study. The heating method was one-legged cycling. So what they did was that both feet were \u201cnude\u201d or uncovered. One foot was doing the cycling and the other one had all these capsules on it for measurement.<\/p>\n\n\n\n Let’s have a look at what they actually found in terms of sweat rates. The top graph is the average across the whole trial; the bottom one is the last 15 minutes. Either way. they’re kind of similar. So the dark black bar which is the biggest, that’s the forehead. So first we can tell that the feet sweat less than the forehead. Second thing that is interesting is the last little ones are for the sole of the foot capsule. The others were the dorsal and dorsolateral capsules. So, there you go.<\/p>\n\n\n\n Now, I was keen to just see if there was more information. Because I found it hard to believe that the plantar surface doesn’t sweat more than the dorsal surface. So if you read more research, we’ve got this combined paper. The one on the right with the same capsules is from Nigel Taylor and his crew at the University of Wollongong and you can see that they had more capsules on the feet (11 all up). And the one on the left was from the UK and they used absorbent patches that stuck to the skin. You can see that it covers about 90% of the foot.<\/p>\n\n\n\n The idea was that they wanted to pool the data. So although they were using different methodologies, they found a way to make it so the results could be pooled and be more meaningful due to the larger number of subjects.<\/p>\n\n\n\n The one on the left was absorbent patches collecting the foot sweat. The subjects were running at different intensities. And for the study on the right, there were two heating mechanisms: passive heating at rest; they wore what I imagine is something like a wetsuit where they pump hot water into it to heat the skin. And then after they did that, they performed that one-legged cycling thing again. Two differences between the two studies to note are, the one on the left were wearing shoes, and the one on the right was barefoot.<\/p>\n\n\n\n Results from both laboratories are in agreement:<\/p>\n\n\n\n Another finding was that wearing shoes (absorbent patches) limited the increase in foot sweat production, while the sweat rate of uncovered feet (sweat capsules) kept increasing with work and thermal load. This is what they call hidromeiosis<\/a>. Essentially the skin swells due to the excessive hydration and that closes off the sweat ducts and blocks sweat secretion.<\/p>\n\n\n\n This is a different study. Fourteen females performed low intensity running with nude feet, and low and high intensity running with shoes. Foot sweat rates were measured at 14 regions using the absorbent patches and what they found was the barefeet sweated 22% more than those that were in shoes – that\u2019s another example of hidromeiosis.<\/p>\n\n\n\n The highest foot sweat rates were observed at the medial ankle and dorsal regions. Lowest sweat rates were at the toes. And interestingly, and significantly for anyone who is having a hard time believing the plantar surface of the foot sweats less than the dorsum \u201cPerceptions of wetness and foot discomfort did not correspond with regions of high sweat production or low skin temperature but rather seemed dominated by tactile interactions caused by foot movement within the shoe.\u201d So even though the dorsal surface is sweating more, you just feel it more on the plantar surface.<\/p>\n\n\n\n Another study: this one was an non-thermal stimulus. Subjects performed mental arithmetic in a temperature controlled room, with psychological sweating evaluated from nine sites. They used the ventilated capsules for this one. The results showed sweating was evident from three glabrous sites (glabrous means not hairy ie: the plantar\/palmar surface). However, significant sweating was evident from two non-glabrous (hairy surfaces). There is thought that psychological stimulates sweat only on the glabrous skin, but these results refute that: \u201cThese observations further refute the proposition that psychological sweating in humans is restricted to glabrous (plantar and palmar) skin surfaces.<\/p>\n\n\n\n One interesting thing that was mentioned was that \u201cMost of the variability in regional and whole body sweating rates within and between individuals is due to differences in sweat secretion rate per gland, rather than the total number of active sweat glands\u201d. So that kind of makes sense because we’ve got high density sweat glands on the plantar surface and yet that’s not the part that sweats more.<\/p>\n\n\n\n Hyperhidrosis is a disorder of excessive sweating for no good reason. It’s not people that sweat a lot when they exercise; and it’s not people that sweat a lot when they’re stressed out. It’s people that sweat profusely in a resting state with no thermal or psychological stimuli. That is the definition of hyperhydrosis.<\/p>\n\n\n\n So, it’s a disorder of excessive sweating beyond what the body uses for homeostatic temperature regulation. It’s due to the overstimulation of cholinergic receptors on eccrine sweat glands. The acetylcholine negative feedback loop is likely impaired. And interestingly, eccrine sweat glands appear normal in size and number but the sympathetic ganglia are usually larger, in people with hyperhidrosis. We can obviously use socks to manage foot sweat. Moisture wicking<\/strong> obviously makes intuitive sense. So if there is a moisture wicking function, it takes moisture away from the skin surface, so it can be evacuated into the shoe environment and then through the shoe into the atmosphere. Which moisture wicking sock works best? Who knows. Does moisture-wicking work better than a wool sock? Who knows. We don’t know; the research isn’t there<\/a>. Merino wool<\/strong> is used in a lot of socks. We know that wool has the ability to absorb moisture within the fibre, which moisture wicking synthetic fibres don’t.<\/p>\n\n\n\nSweat gland anatomy and physiology<\/h2>\n\n\n\n
![\"Sweat](\"https:\/\/pro.blister-prevention.com\/wp-content\/uploads\/2024\/04\/Sweat-glands-comparison.jpg\")
<\/figure>Apocrine sweat glands (not on the foot)<\/h3>\n\n\n\n
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Apoeccrine sweat glands (not on the foot)<\/h3>\n\n\n\n
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Sebaceous glands (not on the foot)<\/h3>\n\n\n\n
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Eccrine sweat glands<\/h2>\n\n\n\n
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Sweat gland cells<\/h2>\n\n\n\n
A quick summary so far:<\/h2>\n\n\n\n
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How much does each foot sweat?<\/h2>\n\n\n\n
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Foot sweat rate: regional differences<\/h2>\n\n\n\n
Let’s have a look at some pictures from the Taylor study. They used these sweat capsules on 5 areas of the feet:<\/p>\n\n\n\n\n
Results<\/h3>\n\n\n\n
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Hyperhidrosis<\/h2>\n\n\n\n
Hyperhidrosis can be classified as either primary or secondary. Primary presents earlier in life; the aetiology remains unknown and; genetic factors are thought to be involved, but it’s not that well understood. Secondary hyperhidrosis is usually the effect of medications or systemic disorders.<\/p>\n\n\n\nSome interesting facts about hyperhydrosis:<\/h3>\n\n\n\n
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Moisture Management \/ Treatment<\/h2>\n\n\n\n