The Who, What, Why & How Of Double-Socks
No doubt you've heard of double socks or double-layer socks). In this article, we’ll answer:
What they are exactly
Where you get them from?
Who uses double socks?
2 Types of Double Socks
Double socks systems come in two forms:
Literally wearing 2 pairs of socks
Socks that have two layers at certain locations within the sock (double-layer socks)
To understand double socks, you need to know about interfaces
An interface is where two surfaces meet. When wearing shoes & socks there are two interfaces (see image below): the skin-sock interface (black line) and the sock-shoe interface (blue line). Each interface has a certain level of friction. This measurement is the coefficient of friction (COF) which is a number usually less than 1. In-shoe friction levels generally range from 0.40 - 0.90 (Carlson, 2009). When blisters are a problem, there is high friction at both of the interfaces. One way to prevent blisters is to reduce the COF at one of these interfaces. Examples would be lubricants (skin-sock) and ENGO Patches (shoe-sock).
A third interface
But there is another way. You can create an extra interface by wearing another pair of socks, creating a sock-sock interface. If the sock-sock interface COF is lower than that of the other two interfaces, the two socks will slide across one another before there is movement elsewhere. That will reduce blister-causing shear.
Double Socks Research
Wearing two pairs of socks has been shown to somewhat reduce blister incidence in a military population when compared to the standard-issue military sock (Knapik et al, 1996). The following two results are quoted from Richie (1997):
Study 1 (1992) In a study of 357 marine recruits on Parris Island, South Carolina in 1992, the use of a CoolMax® liner with a heavily padded terry design outer sock using a wool/polypropylene blend significantly reduced blisters compared to a single layer sock (40% vs. 69%). Adding a CoolMax® liner to the standard sock significantly reduced sick call visits (24.4% standard vs. 9.4% standard with liner).
Study 2 (1993) Another study of 1,079 soldiers in 1993 tested five sock systems on blister frequency and acceptability by soldiers. Synthetic fiber socks significantly outperformed the standard wool sock. Adding a CoolMax® liner to the wool sock significantly reduced blisters. When comparing single, extra-thick acrylic padded socks to double layer sock-systems, the double-layer system was superior owing to the shielding of the open terry loops from the skin surface and the movement interface created by the double-layer system.
In spite of these favourable research results, there is little information available about specific successful two-sock combinations. It's likely that individuals are experimenting with their own combinations in a trial and error approach. My understanding is the ideal sock combination is a close-fitting thin hydrophobic (water-repelling) inner sock, with a thicker hydrophilic (water absorbant) or moisture-wicking outer sock that maintains its bulk in spite of moisture. This sets up a moisture-wicking function as well as serving as a third interface. Richie's outstanding discussion on fibre properties states the descending order of sock fibre hydrophillic ranking is:
Cotton - Wool - Acrylic - CoolMax® [Polyester] - Polypropylene
Here's a summary of three commercially available double socks I have purchased and my own experience (for what it's worth):
WrightSocks: These are socks with two layers built in to them. Everytime I put them on, the inner sock tends to fold over on itself in a couple of places, enough to think that it's going to cause a problem. But that doesn't last long and has never been an issue while I'm walking or running. My foot seems to move around too much in the beginning as the two layers easily glide over one another. But again, this doesn't last long - presumably a bit of sweat increases friction levels enough to lessen the relative movement. The ones I've used have the fibre content below.
Lightweight: InnerLayer (70% Polyester 30% Nylon); OuterLayer (68% Polyester 28% Nylon 4% Lycra)
Midweight: InnerLayer (70% Polyester 26% Nylon 4% Lycra); OuterLayer (68% Polyester 24% Nylon 8% Lycra)
Fortisocks (no longer in production): A one size fits all thin short tubular sock designed to be the inner sock layer. The closest alternative is the good old ladies nylon knee-high stocking cut shorter and worn under your standard sock.
ArmaSkin: Also an inner sock layer, the ArmaSkin sock is different to other double sock inners I've seen. They feel very unusual because the sock sticks to you. I'm sure that's because of the silicone content - silicone has a high friction level. Their fibre content is 87% polyester 16% Spandex 5% silicone.
You don't pull these on like normal socks - they don't slide against the skin. You need to "roll" them on. Although I got the smallest size (Ladies 2-8) they were a bit big and the heel doesn't contour exactly right. It didn't really both me though. And the big bulky seams surprisingly didn't bother me (except the one under my heel that ordinarily wouldn't have been there if you had the right sized foot).
I am left wondering how the silicone inner side deals with sweat. I've heard of variable levels of satisfaction from runners and bushwalkers. Ultra168 had a few readers trial them to get feedback - worth a read. As did The Bushwalking Blog here.
Knowing what I know about interfaces, ArmaSkin socks work by creating an artificially high skin-sock friction level, so the sock grips to the skin. This is good because it will save your skin from abrasion. But you better hope the friction level between the two socks is very low (there's got to be low friction somewhere) or there will be too much gripping at all interfaces with the result being the internal rubbing we call shear that causes blisters. I have a feeling ArmaSkin sock provides this better than the others, but that's just a guess.
Unfortunately, the manufacturers are yet to test the friction properties of the ArmaSkin sock (no sock manufacturer has to my knowledge). I’d love to know as I’m sure these have a really low friction level.
Overall, double-socks aren't enough to stop me from getting blisters (my blister issues are at the back of my heel and edge blisters at my big toe knuckle). But I seem to be blister-prone. Different people need different levels of protection.
Double Socks or Not Really?
I have heard of socks being referred to as double socks but which in my opinion aren't. They consist of different materials on the inner compared to the outer side of the sock, most likely serving a moisture-wicking function as decsribed earlier. However, the layers are not separated. There's a discussion about it amongst bushwalkers at http://bushwalk.com/forum/viewtopic.php?f=15&t=11323&mobile=on. The way double socks work is by having the two sock layers moving relative to each other so they must be separate layers.
Toe-socks such as Injinji can be thought of as a form of double sock system for interdigital areas and may theoretically reduce the incidence of interdigital blisters. That is assuming the COF remains sufficiently low. This sock-sock interface is actually the same material, whereas the principal of double sock systems is to use the effect of different sock materials or construction for friction reducing properties. However, blister incidence may be reduced as a result of the increased bulk between the toes acting as cushioning to reduce pressure peaks.
Who's Wearing Double Socks?
My informal conversations with blister-sufferers and athletic footwear retailers indicates it is a method used more by hikers than it is runners and other athletes. Double-socking is talked about a lot on various forums as the concept is well known. I’m not sure it is used widely though, but it’s hard to tell for sure. There are many that do double-sock and get the blister prevention results they need.
A Final Thought
Assuming a double sock system reduces friction significantly, it does so all over the foot. This global approach to friction management is not without its potential problems. My own thought is, assuming vigorous or long-lasting exercise, this effect will soon be over-ridden due to perspiration such that higher friction exists at this interface. At that point, whether the friction level remains low enough to maintain blister-free friction conditions will depend on the individual. If you continue to get blisters, an additional strategy will be required.