The Effect of Natural and Synthetic Fibres
Skin friction is higher when your skin is moist compared to when it’s dry. So keeping the skin dry is one consideration in preventing blisters. That’s why blister-prone athletes choose their socks carefully. Or at least they should, because not all socks are created equal. It comes down to fibres.
A tough ask
Maintaining dry skin is a tough ask considering how much the feet perspire – especially when it’s hot and especially when we exercise. The soles of the feet (along with the palms of the hands) have the highest density of eccrine sweat glands on the body – the feet can produce in excess of a cup of sweat on an average day. That’s a lot of moisture for the sock to deal with. And that doesn’t count hot/humid weather, heavy exercise or environmental sources of moisture (like rain, dew, creek crossings, drink stations etc).
The initial goal of a sock is to absorb this moisture. But because of the sheer volume of moisture encountered, no material can be expected to keep the skin dry by absorption alone.
"The sum total of moisture potentially collecting in the shoe of an athlete during exercise will quickly exceed the absorptive capacity of any sock. Therefore, in order to keep moisture content at a minimum level on the surface of the foot during exercise, a sock must “move” moisture away to the shoe upper for evaporation. This process is known as wicking" (Richie, 2013).
Podiatrist and researcher Doug Richie DPM on sock fibres
There's a lot of hype surrounding the topic of athletic sock construction. I must admit I do not possess the expertise in fibre and material properties. Much of the information to follow comes from American Podiatrist Doug Richie. He has been at the forefront of sock research and education since the 1990s, is an authority on the subject and a trusted source of factual information. Before we go into this any further, please consider this insight from him from a recent article. It makes reference to diabetic socks but is similarly relevant to blister prone individuals and really any athletic or endurance activity.
"A review of the medical literature shows a clear superiority of synthetic fibers over cotton fibers in providing health benefits for people with foot pathology. These studies have documented that acrylic fibers and polypropylene fibers can provide better protection from impact, pressure, shear, and moisture accumulation compared to cotton fibers Yet, the medical marketplace continues to feature “diabetic socks” composed of cotton fibers. Many authorities and professional organizations continue to provide consumer information to the general public which includes the choice of white cotton socks as being preferred for healthy feet … There is no regulation of claims made by manufacturers of footwear and foot products sold to the general public. These products are sold in retail pharmacies, department stores, home/health stores and, most importantly, over the Internet. A visit to any of these vendor outlets will reveal a plethora of products and remedies for foot conditions which promise therapeutic benefits without any scientific verification."
Now let me attempt to explain how sock fibres interact differently with moisture. And how this knowledge relates to sock selection if you’re blister prone. But first, what is moisture-wicking?
What is moisture wicking in socks?
Moisture wicking is the process of moving moisture from the skin side of the sock, through the sock, to the shoe side of the sock in an attempt to keep the skin dry. Moisture-wicking socks can be made of a single fibre type or combination of materials (blends) with the ability to achieve a moisture gradient that encourages to transport of moisture.
As moisture reaches the outside of the sock, it needs to leave the sock. To be truly effective, this moisture needs to be evaporated to the outside of the shoe. So shoe upper breathability helps moisture-wicking socks function for longer duration and in wetter conditions.
Sock fibres: Hydrophilic or hydrophobic
There are natural and synthetic sock fibres. Natural fibres include cotton and wool. Most athletic socks are synthetic – fibres that have been engineered with one or more favourable characteristics to suit the in-shoe application. These include acrylic, polyester and polypropylene. Favourable characteristics include thermal insulation, cushioning, durability, quick drying, ability to maintain shape … and in regard to the topic of this article, its interaction with water. That is, their hydrophilic or hydrophobic nature and wicking ability.
Hydrophilic fibres are water-attracting (absorbent)
Hydrophobic fibres are water repelling (non-absorbent)
Cotton is the most hydrophilic fibre used in sock construction - it absorbs a lot of water! This is not good if you’re blister prone and taking part in activities where blisters are likely. The moisture held by the sock is trapped within the sock and against the skin, keeping it moist and clammy (ie: higher skin friction, easier to produce blisters). One of the easiest changes you can make is to wear something other than cotton socks. The next most absorbent fibre is wool. Richie (2010) lists sock fibres from most hydrophilic to most hydrophobic:
Cotton --- Wool --- Acrylic --- Polyester --- Polypropylene
On its own, wool is not a great fibre for sock construction because of its hydrophilic nature. But Richie explains the premium Merino wool fibre is different. It is a common fibre used in specialist hiking sock construction for its thermal insulation properties.
"Compared with traditional wool, Merino wool has a much finer core diameter of each fibre, giving a softer feel and more air space for moisture movement. Merino wool has fewer tendencies for skin itch, which is common with regular wool socks and apparel. The finer fibre and natural airspaces created by Merino wool have lead manufacturers to claim that this fibre is superior to any synthetic fibre for insulation and wicking."
Synthetic fibres used for their moisture wicking properties in sock construction include:
Acrylic: In 1990, Richie and Herring assessed blister incidence in runners wearing either 100% acrylic socks (padded construction) or 100% cotton socks. The padded acrylic socks out-performed cotton in regard to both blister incidence and blister size (acrylic sock wearers experienced half as many blisters and of those blisters that did occur, they were one-third the size of those of cotton socks).
Polyester: An example of a polyester fibre is Coolmax. Coolmax polyester fibres have a four-channel configuration that increases surface area and moisture movement. The best image I could find of this fibre and its properties is below.
Polypropylene: Polypropylene fibres absorb next to no moisture at all.
One of the main aims of synthetic fibres is to be more hydrophobic (water-repelling / less absorbant) than the natural fibres. The following three snippets about synthetic fibre performance are from publications of Richie :
"The most popular synthetic fibers utilised in athletic hosiery are acrylic and polyester. Both acrylic and polyester fibers are hydrophobic and have superior wicking properties and reduced drying time than cotton."
"Cotton fiber retains three times the moisture of acrylic and fourteen times the moisture of CoolMax®. When exposed to ambient air, socks composed of cotton retain moisture ten times longer than acrylic."
"One shortcoming of acrylic is its poor insulation. On hot surfaces in summer months, acrylic fiber socks can conduct heat and be undersirable. Hollow core polyester or Coolmax socks may be preferred in these conditions."
You might have heard of Coolmax - it's a polyester fibre
"… studies have shown that Coolmax and other polyester fibers have a 15% faster drying time compared to acrylic fibers" (Richie, 2010).
Coolmax is a well-known sock fibre. Coolmax is not a brand of sock - it is simply a fibre that sock manufacturers purchase along with other fibres to use to construct their socks. Socks can be 100% Coolmax fibre. But mostly it is used in combination with other fibres. This video from the Thorlo website shows they use Coolmax in their Xperia sock for its moisture-wicking ability. The table below shows how Coolmax is just one example of a polyester fibre, there are several others:
Combining fibres in sock construction
Socks can be constructed of one single fibre (eg: 100% cotton socks), or a combination (blend) of fibres. Synthetic fibres predominate in specialised socks though wool continues to be a common component, particularly where thermal insulation is a priority.
Fibres can be combined either in defined layers or evenly intertwined (image below). I’m not sure if one arrangement is better than the other or if this is important to sock performance, but to explain further:
When there are defined layers, the hydrophobic synthetic layer is against the skin and the hydrophilic natural (usually wool) layer is on the shoe side. This arrangement sets up the moisture gradient required to wick moisture away from the skin. The layers are joined, unlike double-socks where the layers are separated and move relative to one another.
When fibres are evenly intertwined (see images below) the moisture gradient is present throughout the entire sock.
If you're prone to blisters or participate in activities where blisters are common (distance running, hiking, tennis) you should recognize the effect of moisture on skin friction. It's one factor that makes blisters more likely. The take home messages are:
Cotton is not a good option for athletes or blister prone people.
100% wool may not be a good option unless it is Merino wool or where thermal insulation is a priority.
Moisture-wicking is a favourable property for athletes, the blister prone and where moisture is a significant factor.
Synthetic fibres improve the moisture wicking ability of natural fibres. A well-known fibre added to socks for its moisture wicking ability is Coolmax.
Written by Rebecca Rushton
Rebecca is an Australian podiatrist with over 20 years experience. She has spent a lifetime dealing with her own blister prone feet in her sporting and everyday life. Rebecca specialises in helping athletes and sports medicine professionals figure out how to manage foot blisters with ease. And for kicks, she enjoys providing blister care at multiday ultramarathon events.
Rebecca is the founder of Blister Prevention and author of "The Blister Prone Athlete's Guide To Preventing Foot Blisters".
Richie, DH. 2013. Therapeutic hosiery: An essential component of footwear for the pathologic foot. Podiatry Management. Oct: 155-62.
Richie, D. 2010. How to manage friction blisters. Podiatry Today. 23 (6): 42-48.
Richie, D. In Werd, MB and Knight, EL. 2010. Athletic Footwear and Orthoses in Sports Medicine. Springer, New York: 72-75.
Richie, DH. 1997. Socks: Hosiery - Essential Equipment for the Athlete. AAPSM.
Herring, KM and Richie, DH. 1990. Friction blisters and sock fiber composition. A double-blind study. Journal of the American Podiatric Medical Association. 80 (2): 63-71.
Herring, KM and Richie, DH. 1993. Comparison of cotton and acrylic socks using generic cushion sole design for runners. Journal of the American Podiatric Medical Association. 83: 515-22.