It’s the obvious question. Your clients want to know – what is the best sock for blister prevention? I’m sure you want to know. Heck, I want to know!
Sock fibers and moisture management is something I’ve been looking at closely lately. Not just with the paper that Doug and I have recently had published. But also in selecting our range of Injinji toesocks in the Blister Prevention shop.
In this month’s edition of Blister Prevention Office Hours, we’re going to look closer at:
- Cotton, wool and CoolMax fibers
- What is moisture wicking
- Does it actually happen happen in shoe
- And is it the main thing in preventing blisters
Below is an abbreviated transcript of the live Office Hours event. Please watch the video for a full replay. For references mentioned and further discussion of concepts discussed, please read our second paper in the Journal of Athletic Training 2023: https://doi.org/10.4085/1062-6050-0341.22
How socks prevent blisters
When we think about socks and how they prevent blisters we assume that it’s the way that they deal with moisture. Moisture comes from the skin as perspiration. that is wet that. It interacts with the sock. It moves through the sock. Then it evaporates into the in-shoe environment, before hopefully moving through the shoe to the outside environment.
When we look closely at how moisture wicking works at preventing blisters, we are basically saying that we are implementing a coefficient of friction (COF) reduction strategy. We’re trying to keep the skin dry because we know that very dry skin has a low friction level compared to moist skin. Dry skin that provides a low COF is inferring a more slippery surface between skin and sock. Remember the whole aim of blister prevention is to, as the bones move back and forth in the within the foot, to allow the skin to move more in synch with the bone. The problem is, high friction levels (COF) that eventuate due to perspiration means the skin doesn’t move with the bone – it stays in stationary contact with the sock and shoe, resulting in larger shear deformation magnitudes.
When it comes to moisture management we instantly think of hydrophilic and hydrophobic. We all know that cotton is a hydrophilic fiber. It holds moisture. CoolMax is a hydrophobic material – it repels water. And interestingly wool is a bit of both. It’s quite the paradox, wool!
With regard to how sock fibers interact with moisture, there’s absorption and adsorption. ABsorption is the amount of water that is absorbed into the actual material. ADsorption refers to
how the surface fiber reacts to the water molecules – they either hold on to it or not. Hydrophilic materials will hold that moisture on the surface of the fiber. Whereas hydrophobic fibers will just let it move easily along the fiber – it won’t hold on to it.
Fibres under the microscope
(Recommend watching the video for visual demonstration)
- Wool is characterized by scales. And the larger micron wools have a medulla. Finer wools like Merino do not.
- Synthetics are symmetrical and smooth.
- Cotton is a twisty “U” shape with a long narrow lumen or air space that is where the water is absorbed into the fibre.
The CoolMax sock fiber is a scalloped oval in cross-section. This fiber geometry increases the surface area by 20%. So whatever way it interacts with water, it does it 20% more because there’s 20% more surface area. CoolMax fibers are hydrophobic – they repel water. So water will basically just kind of sit on the surface there and is very happy to move back and forth there’s nothing keeping it stuck to that same spot. It doesn’t know whether we want it to move to the left or to the right along its fiber. So you have to realize that the the reason it moves is presumably according to whether there’s a moisture gradient. So it’s going to move from an area of high moisture content to an area of low moisture content.
Cotton is a hydrophilic fiber. It holds a lot of water and therefore it inhibits moisture wicking. Cotton fibers absorb three times the moisture of synthetic acrylic fibers. Once it’s wet, cotton has a 10-fold greater drying time compared to synthetics.
What about bamboo? I don’t know as there’s nothing that I’ve been able to find. I would assume that it’s more like cotton because we know that the drying time for bamboo is just very long – it’s difficult to get bamboo socks to dry.
In terms of wool, we know that it is both hydrophilic and hydrophobic. Wool structure is round or oval and the large micron size has a medulla – an airspace in the middle of the fibre. So, if the ability to absorb water is a favourable property of wool, you would expect to see a non-Merino wool in sock manufacture. But it’s Merino we only ever hear about. Merino wool is a relatively fine Micron size, around about 19 to 21 microns. Whereas the coarse wool like around 29 microns, they have the potential to absorb more water. Air pockets in a fibre are also advantageous as air pockets aids water transportation.
Moisture regain
Sock fibre research focuses on several moisture-management measurements. One is moisture regain, which describes the ability of the sock material to attract and hold on to moisture. In terms of overall moisture regain, wool is the most hygroscopic. Hygroscopic just means that the combination of absorption and adsorption. Wool holds the most moisture. So it’s interesting that we think that wool is good but cotton is bad. We’ve been told all these, years that moisture wicking is good because it doesn’t hold moisture. Cotton is bad because it holds moisture. And yet we know that wool fibre is popular and advantageous, yet it holds onto so much water, like cotton does, and yet we see cotton as a bad sock fibre. There’s a real disconnect there and I have to admit, I don’t have my head around it – I’m not sure anyone does, in terms of socks, sports and blisters. Cotton is the next most hygroscopic. It holds three times that of acrylic and 14 times that of CoolMax.
Swelling
Swelling of fibres is another thing they look at. The absorption of water will cause swelling of the sock fibers, this reduces air space within the sock and therefore it inhibits the movement of moisture away from the skin surface. So swelling is seen as a bad thing because it gets rid of all the air pockets and the air spaces can aid in moisture transport. Cotton fibers will increase volume and swell by 44 to 49 percent when immersed in water. Think back to that lumen in the the cotton fiber. It’s flat and it doesn’t take up much space. But it has the ability to fill with water and really swell that cotton fiber. Another study found that when moisture is applied, cotton fibers swell 45 percent, wool fibers 35 and acrylic 5 percent
Moisture-wicking
For moisture-wicking to occur, we need that moisture gradient. Moisture is going to move from an area of high moisture to low moisture. Rossi studied the moisture storage and moisture movement properties of three common sock fiber materials. Polypropylene showed the best capacity to wick moisture from the inside of the sock to the outside. The next best was woolen socks and the next best after that was nylon. Nylon, which is present in just about every sock to some degree, that showed minimal wicking capacity with large amounts of moisture retained on the inner side of the sock. Nylon isn’t used for moisture wicking it’s used to add stretchability and durability to the sock. But some socks have quite a lot of nylon in them.
Density and construction
Socks can be categorized by their their thickness and their construction technique. The density of fibers or construction of the sock can influence moisture wicking capacity. The denser the weave pattern
or the thicker the padding, this may enhance moisture movement through the sock by reducing compaction and preserving that air space between the fiber networks. So we are thinking that thicker socks are better than thin socks. The reason we think this is thanks to the work of Herring and Richie back in the 1990s. They conducted two studies looking at blister incidence and different sock materials and construction. They took 35 long distance runners and compared blister incidence in padded socks of identical construction but different materials. The socks were either 100% cotton or 100% acrylic socks. In this study they were both thick (there’s a follow-up study where they were both thin). So in this study where they were both thick. They found there were twice as many blisters with the cotton socks and those blisters were three times the size.
This suggested that the acrylic fibers were beneficial over the cotton fibers. The authors proposed that the results were explained by a lower friction force on the skin surface due to superior moisture management of the acrylic fibers. So that’s a reasonable thing to assume – the acrylic fibers dealt with the moisture better as it moved the moisture away from the skin keeping the skin drier and therefore more slippery such that it can move in sync with the bone at an earlier point in tim. In essence, a coefficient of friction (COF) reduction strategy.
However in their follow-up study, Herring and Richie found no difference in blister frequency when comparing the 100% cotton to the 100% acrylic socks, but this time remember the socks were thin. So when the socks were thin, the benefit of acrylic over cotton was lost. They speculated that the thick densely padded construction enhanced wicking capacity, which is a reasonable thing to assume. However, alternatively, it could be that the socks reduced pressure so because they were padded they reduced peak pressure and they can also potentially absorb a bit of shear. As you’ll know from the blister prevention mechanisms chart, reducing pressure and absorbing shear are two blister prevention mechanisms.
Pressure
So the next question is, do socks really reduce plantar pressure? Well, yes they do. Howarth and Rome studies the plantar shock attenuation provided over 72 hours in five types of athletic socks compared
to Barefoot. Both the padded wool and padded acrylic socks significantly reduced pressure underfoot, compared to barefoot. Cotton socks, double cotton socks and padded towelling weren’t any different to barefoot.
Interesting research in waterproof boots
Separately, Bogerd and co-workers conducted a field study of 37 military recruits who were marching for four consecutive days in Gore-Tex lined military boots. So these are boots are waterproof which means water doesn’t get in but similarly water doesn’t get out – so sweat is going to be maintained within the shoe. Two socks were compared. The first sock was a blend – it was a 50% Merino wool 30% polypropylene blend. The other sock was a 99% polypropylene sock. The study were designed to measure the moisture content on the skin surface of the feet as well as the moisture content retained by the sock. So we had subjective and objective measures.
The subjective results – the recruits rated the wool/polypropylene blend sock to be cooler less damp and more comfortable. Which is interesting because you would think that the 99% polypropylene sock, being more synthetic and more hydrophobic would have maximized moisture wicking function, because it’s hydrophobic. And yet these soldiers rated their skin was less damp with the wool blend sock. And for the objective results. Well, the wool blend socks kept the surface of the foot drier than the polypropylene sock. Again, the 99% polypropylene didn’t mean drier skin in two locations anyway: the top of the foot and the back of the heel. There was no difference in moisture content under the entire plantar surface of the foot. Now, if you think about an athletic shoe with its mesh upper, which is the opposite to what this study used (a waterproof boot), moisture will have an easier time evacuate the shoe through the mesh upper, compared to the waterproofing lined boot.
Moisture-management at different anatomical sites
But if you think about the plantar surface, it’s a different situation entirely. Any moisture that finds it’s way to the outer of the sock, is going to be absorbed by the insole. It doesn’t have easy access to any ventilated area of the shoe to enable evaporation. At some stage, you could get a situation where ths insole is wet enough to negate the moisture gradient. So there’s no further movement of moisture, or at least moisture movement is limited. That may be why there was no difference at the plantar surface. Another way to look at this information is, potentially socks can work to different degrees for different anatomical locations. I’m a big proponent of dealing with blister prevention specific to anatomical locations. The wool blend sock absorbed three times the moisture of the polypropylene sock. The author speculated that the superior moisture storage benefits of the wool blend sock outweighed the wicking capacity of a polyprop sock inside a closed boot where moisture evaporation is compromised. It would just be so interesting to repeat this study with a modern day athletic shoe. So, to reduce moisture content on the skin surface the absorptive capacity of a sock becomes most important when the footwear has resistance to evaporation.
Knit structures
(Recommend watching the video for visual demonstration)
Looking at the construction techniques athletic sock manufacturers generally use one of two knit structures that is terry and simple jersey. When you look at the different methods used, you soon realise things get really complicated. This is a study by Baussan and colleagues. They looked at six weave structures including two brands of simple jersey, two brands of double jersey, duffel jersey and terry jersey. They measured friction (horizontal force) and pressure (vertical force). The results showed that terry jersey socks generate less friction and absorb more energy than other textile structures investigated, provided that the terries are compact, homogeneous and orientated in the main sliding direction of the foot. It really makes a difference which way the terry loops are slanting. They found that in some of the socks, the the coefficient of friction was double that when going the opposite way to the terry loops compared to going with the direction. So, when it comes to sock performance, it’s not just about the fibre used, it’s also about how the sock fabric is constructed. Just to compound the complication, when you knit a fabric, it has a front side and a back side. They look different and therefore their properties will be different. At the end of the day, you can’t be blamed for scratching your head and thinking well how the hell do socks prevent blisters?
Sock fibers and moisture management – could wool be best?
Lately, I’ve been looking closer at wool. We’re actually stocking Blister Wool in the Blister Prevention online store. This is loose packed wool that you kind of place on your foot while you don your socks. The people I’ve spoken to that have used wool in this way, it seems that the like Merino, the fine wools don’t work so well. You need a coarse wool for it to be able to work in an athletic situation. Remember, course wool is the one that has the medulla so it has the potential to absorb more moisture. When you think about wool, not only do you start with specific Micron sizes that have different properties in itself, then the wool is cleaned and washed so that removes all the lanolin. Then they they don’t fully dry it they semi-dry it and they add some oil back in just to make it more usable and manageable. Then it goes through a whole bunch of combs which straightens it and puts it into slivers. It’s spun and that can be done in different ways. It’s weaved into fabric, that can be done in different ways. And there are some other processes like shrink proofing. And then it’s plaited and combined with other materials to form the final fabric that the sock is made out of. So good luck trying to say to your patient “this is the best sock”. I kind of think it’s impossible. But to the best of our understanding, thick is better than thin.
How I educate on socks for blisters
The way I educate my patients in terms of choosing a sock for blister prevention is, just get something that’s comfortable and that you’re happy with and there are more honed in and specific ways that we can deal with blisters if you’re still getting blisters. I’d never really expect a sock to be the be all and end all in terms of blister prevention. I’m sorry to say that we can’t really hang our hat on anything in terms of socks. It’s important to know what we don’t know.
Sock research limitations
It’s important to realize that lab studies do not always replicate sock performance in a real life situation. That is during actual physical activity and inside of footwear. A lot of sock research is done in dry conditions – just because it’s easy to standardize which doesn’t reflect the in-shoe environment. And there’s been precious few blister incident studies the Herring and Richie study from the 1990s is really the most significant that we’ve had.
It’s not just about moisture-wicking
So, socks don’t necessarily just work by one mechanism. It’s not all about reducing the coefficient of friction. In fact, I would suggest that it has very little to do with with how this is achieved through moisture management. Because the kind of skin that has a low friction level is bone dry and once you’ve had your shoes on for a little while your feet aren’t bone dry your skin isn’t bone dry – it’s a little bit damp and clammy. Then if you add a hot day, you’re exercising and you’ve been in those shoes for a long time, yeah I just can’t see that it is all about moisture management and how that affects a lower coefficient of friction.
Pre Q&A question
Carmel: I’m confused. I don’t know what to recommend. I had an AFL player in recently. He’s getting severe blistering under the first MPJ. He’s found that the double socks have helped. He’s doing a thin bamboo sock and then his footy sock on top, and that’s working quite well. We got onto your your ENGO patches, so they’re on his insole. But I’m a bit confused now because bamboo you’re saying is like cotton.
Rebecca: Well we don’t actually know. But the main thing that we know about bamboo is the drying time is LONG! It holds a lot of moisture so that’s there’s the potential for that to be a bad thing. But we don’t know. Essentially, I would never hold all my hopes on a sock. I would zero in on that anatomical location and understand how the bone is moving. Because the intraepidermal mechanical fatigue of the blister injury is driven by bone movement in the presence of high friction levels. The Engo Patches will have reduced the coefficient of friction more than any other intervention.
While the bones continue to move back and forth within the foot with every step, the skin is in stationary contact with the sock and the shoe, thanks to the inevitable high COF. That’s the value of the Engo patches – they reduce that friction Force no matter how much moisture there is and they will let the socks slide against the Engo patch just for that anatomical location. That will mean the skin can move in synch with the bone at an earlier point, and that reduces the the magnitude of the individual shear distortions. And if blisters continue, start thinking about the mechanics of that 1st MPJ. That’s where you’ll get the best result.
Wrapping up
So, wrapping up, it is complicated! But we’ve had a quick look at the properties of sock fibers. I’ve had a quick look at some research on fiber performance and blister incidence. We’ve had a quick look at the Paradox of wool, being both hydrophobic and hydrophilic, and how this might be a good combination for a sock material. Why moisture wicking might not be the solitary aim of a sock. And why it’s difficult to pinpoint the exact best sock for blister prevention.
