Chapter 5: Your Shoes & Socks
In this chapter
- Shoe-fit & lacing
- Socks: Moisture-wicking socks and double-sock systems
- Polytetrafluoroethylene Patches (ENGO Patches)
1) SHOE-FIT & LACING
Shoe-fit should be your first and most basic blister prevention consideration. Too small and the extra pressure causes shear to become excessive with less repetitions. Too big and your foot slides around too much in the shoe, increasing the probability of blisters, deroofing of blisters, abrasions, bruises, black toenails, blisters under the toenails and even losing your toenails.³⁷ Keep these problems to a minimum with a shoe that fits perfectly. Optimal shoe-fit revolves around length, width and adjustability.
a) Length: The ‘rule of thumb’ is to have the width of your thumb between the end of your longest toe (usually the 2nd toe – but not always) and the end of the shoe upper. Measure this while you’re standing (because your foot will elongate a bit) and make sure your heel is right at the back of the shoe.
b) Width: You don’t want the side of your foot overhanging (bulging over) the side of your shoe. By the same token, you don’t want too much width and have your foot moving around too much. When you are trying on shoes, if they fit perfectly but the facings (the material the laces go through) are touching one another, the shoe is as tight as you can get it. You’ve got nowhere to go if your shoe stretches, it’s going to get too loose. Again, check this while you’re standing.
c) Adjustability: It’s not just about the length and width. You could have a perfectly sized shoe but if your laces are too tight or too loose, you’ve wasted your time. You might as well not have bothered getting the perfect sized shoes! The whole reason we have laces is to optimise shoe-fit at all times. Because it's only when the foot fits snuggly in the shoe that the shoe provides the support it’s designed to.
Consider the fact that shoe-fit changes during the course of your activity - foot volume will increase with longer duration walking and running, even in the exceptionally fit athlete. Over-hydration, topical in recent years following the work of Noakes¹³³ may contribute. So be prepared to adjust your laces as you go. Conversely, if your shoes are too loose and your foot feels like it’s sliding forward or your heel is lifting at the back, try this:
- first make sure you’ve used the last pair of eyelets – the holes your laces go through
- next try the lace-lock lacing technique (video below https://youtu.be/LXjOLWgWq9k)
- if that doesn’t work, check Ian’s Shoelace Site for a lacing technique that suits you
However, be aware that although shoe-fit has intuitive merit, it is not always enough to ensure blister protection.³¹ ⁷⁷ ⁸⁷
- Shoe-fit is your first blister prevention consideration.
- Lacing allows you to optimise shoe-fit at all times.
- Optimal shoe-fit may not be enough to ensure protection from blisters.
Socks provide relatively high friction to maintain traction for the foot inside the shoe. This is by design. But socks can be used in such a way as to lower friction, namely:
a) Moisture-wicking socks
b) Double-sock systems
a) Moisture-wicking socks – High skin moisture causes high skin friction³ ¹¹ ¹⁷ ²⁵ ⁷⁹ ⁸² ⁸⁴ ⁸⁶ and more likelihood of blisters. The initial goal of a sock is to absorb moisture (perspiration) to keep the skin dry. But because of the sheer volume of moisture encountered, absorption alone won’t always be adequate - the absorptive capacity of any sock can be exceeded. Some socks 'move' moisture away from the skin – from the skin side of the sock to the shoe side of a sock in a process known as wicking.⁸⁵ ⁸⁷ ¹¹³ The moisture can then evaporate through the shoe upper. This wicking function is achieved by sock manufacturers by using fibres in a way that sets up a moisture gradient to facilitate moisture movement in this direction.
Figure 18: Moisture-wicking moisture gradient
Socks have many functions and fibres are selected for their construction accordingly. Thermal insulation, cushioning, durability, quick drying and ability to maintain shape: these will demand certain fibre properties. In regard to their interaction with moisture, fibres are chosen according to their hydrophilic (absorbent or water-attracting) or hydrophobic (non-absorbent or water-repelling) nature. The former keeps moisture trapped against the skin while the latter repels it away from the skin. Richie¹²⁴ lists sock fibres from most hydrophilic to most hydrophobic:
Cotton --- Wool --- Acrylic --- Polyester --- Polypropylene
Cotton and wool are natural fibres (yarns). The others are synthetic fibres.
Cotton: Cotton is the most hydrophilic fibre used in sock construction. It is widely-known that cotton socks have no place in endurance activities where blisters are a common consequence. Moisture is trapped within the sock and against the skin, keeping the skin moist and clammy - just perfect for blister development.
Wool: Wool is a common fibre used in specialist hiking sock construction for its thermal insulation properties. On its own, wool is not a great fibre for sock construction because of its hydrophilic nature. But Richie⁸⁷ explains the premium Merino wool fibre is different: “Compared with traditional wool, Merino wool has a much finer core diameter of each fiber, 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 fiber and natural airspaces created by Merino wool have lead manufacturers to claim that this fiber is superior to any synthetic fiber for insulation and wicking.”
Acrylic: In 1990, Richie and Herring²⁷ assessed blister incidence in runners wearing either 100% acrylic socks (with 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). However ”one shortcoming of acrylic is its poor insulation. On hot surfaces in summer months, acrylic fiber socks can conduct heat and be undesirable.” (Richie⁸⁷)
Polyester: A common and well-known example of a polyester fibre is Coolmax. Van Tiggelen⁸⁵ found polyester socks to significantly reduce the incidence of blisters compared to a double-sock arrangement and compared to a standard military issue wool sock. Richie⁸⁷ explains “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.” And “… studies have shown that Coolmax and other polyester fibers have a 15% faster drying time compared to acrylic fibers”⁸⁷.
Polypropylene: Polypropylene fibres absorb very little moisture.
If you haven’t given much thought to your sock selection (like our hypothetical runner in Chapter 3) one of the easiest changes you can make to reduce your likelihood of blisters is to make sure there is no cotton fibre content in your socks and instead choose moisture-wicking sock. “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”.¹²⁴
Figure 19: Coolmax is a polyester fibre
Here’s something you may not have given much thought to. In considering the function of moisture-wicking socks, the shoes have to be recognised as part of the shoe/sock unit (or footwear system as described by Dyck³¹). To work adequately, a shoe with a breathable upper is required to allow the evaporation of moisture into the atmosphere.³¹ ⁸⁵ ⁹⁷ Water-proofing can prevent this evaporation and not surprisingly Bogerd et al¹⁰¹ found moisture vapour transmission rates (MVTR) through 4 different boot uppers to be far below that of sweating rates. On the other hand, the mesh upper of most running shoes will intuitively allow for much better MVTR.
Figure 20: The effectiveness of moisture-wicking socks is to some extent determined by the rate of evaporation of moisture through the shoe upper. Adapted from Richie and Herring (Ref 27)
b) Double sock systems – Double socks add a new interface into the equation. As well as the standard skin-sock and shoe-sock interface, you now have a sock-sock interface. Double sock systems come in two forms:
- Literally wearing two pairs of socks
- Socks that have two layers at certain locations within the sock (double layer socks)
Double-sock systems are all about reducing friction. The idea is to use different materials so that the sock-sock friction coefficient is lower than that of the other two interfaces. But double-socks can also provide a moisture-wicking function by using a hydrophobic material against the skin and a hydrophilic material on the outer.³² ³⁹ ⁸⁵ And the thick nap of the outer sock can act to absorb some shear.³⁹ Research²¹ ³² ³⁹ has demonstrated an improved blister prevention function with double socks as opposed to single socks. Although Van Tiggelen⁸⁵ found a single polyester sock to reduce the incidence of blisters better than a double-sock arrangement. Knapik³⁹ compared 3 sock systems (summarised below):
Figure 22: Double socks
Figure 21: Knapik’s research on double socks (Ref 39)
Van Tiggelen⁸⁵ provides this insight: “On a theoretical base, the inner sock provided the wicking capacity whereas the outer sock absorbs the moisture. On a practical basis, the recruits reported the formation of folds in the inner sock causing unequal pressure zones on the foot. Hence although the properties of this sock system are beneficial to the wearer, the application of the system could counteract its benefit by creating hot spots on the foot.” A very close-fitting inner sock is required to prevent this. And the fibre content of each sock is paramount - it's not as easy as throwing any two pairs of socks together. Interestingly, how often do you see two pairs of socks for sale at the sports shop, sold specifically to be worn as a double sock system? More common (but still fairly uncommon) are double socks - a single pair of socks with two layers within their construction.
Toe-socks can be thought of as double socks for interdigital areas. But this sock-sock interface is actually the same material. The crux of double-sock systems is to use different fibre content to ensure lower sock-sock friction. Alternatively, benefit may come from the increased cushioning bulk between the toes. Unfortunately, apart from some favourable results following a survey of runners after the 2012 Jungle Marathon,¹²⁹ research is lacking when it comes to toe-socks.
- Moisture-wicking socks aim to minimise friction at the skin-sock interface.
- Double-socks introduce an additional interface (sock-sock) where friction is theoretically kept lowest.
- The absorptive capacity of the sock and breathability of the shoe upper determine the success of these measures.
3) CUSHIONED INSOLES
Insoles can reduce blister formation in 2 ways:
a) cushioning reduces peak pressure
b) cushioning materials absorb shear (shear modulus)
a) Peak Pressure - Compared to other parts of our body, the pressure placed on our feet is unmatched. While mainly the weightbearing surface, there is a high force of contact between the shoe and the heel, toes and other parts of the foot even while standing. Start running, jumping, accelerating, decelerating and changing direction and we’re looking at even higher forces. This pressure contributes to blister-causing shear by way of the coefficient of friction. Pressure is not the cause of blisters, it is a contributing factor.⁷³ You'll see how with this experiment:
- STEP 1: Press the tip of your right index finger firmly on the back of your left hand.
- STEP 2: Wobble it back and forth but keep it stuck to the same bit of skin. Notice how your skin stretches? This is shear and this is what causes blisters.
- STEP 3: Wobble back and forth again but this time press softly with your finger tip. Notice how there is less shear. Low pressure allows your fingertip to slide across the skin before shear becomes excessive. High pressure sees your fingertip remain stuck to your hand for longer which causes a lot more shear.
- STEP 4: Put a drop of oil on the back of your hand (reduce friction) and wobble your fingertip back and forth again - with really firm pressure. Press as hard as you can! No matter how much you compress the skin against the bones underneath, there's only a tiny bit of shear! You really have to do this to believe it. And it's showing you how important friction is to blister development.
Cushioning reduces pressure peak by increasing surface area – by spreading the load over a larger area. Look at the diagram below. Fixed volume gels do this best as the gel is displaced laterally to form a cradle at the edges of the bony prominence.
Figure 24: Cushioning spreads load over a larger area - adapted from Carlson (Ref 73)
b) Shear Modulus - Due to its thickness and cellular composition, a cushioned insole has the ability to absorb shear. When the material itself undergoes shear, it means the skin doesn’t have to (or at least, less-so). This is called the material’s shear modulus. A low shear modulus indicates the ability of the material to absorb more shear. Silicone gel has a very low shear modulus and has the ability to absorb more shear than standard insole and cushioning materials.⁵³ ⁹⁴
What The Research Shows for Cushioning and Blisters
Not all insoles have the same ability to reduce pressure and absorb shear. Two insole materials that have been tested in regard to blister prevention are Spenco (a closed-cell neoprene polymer rubber) and Poron (a cellular polyurethane).⁵ ²³ The statistics from the Smith study (reported by Knapik³⁷) showed Spenco performed better at reducing blister and callus incidence (table below). However a further study found an un-named cellular polyurethane foam slightly increased blister incidence.¹⁰⁶
Figure 25: Spenco insoles provided the best blister prevention (Ref 37)
For cushioning to be effective, its properties must be a good match to the job at hand. If it’s too soft, the cushioning material will simply flatten and not reduce the peak pressure at all. And if shear modulus is excessively low, braking and propulsive mechanisms are compromised, reducing the mechanical efficiencies of gait. “Cushioning degrades control and energy efficiency. Ideally, cushioning should be used sparingly ...”.⁷³ This is one of the limitations of gels used under the sole of the foot.
One thing to be aware of is insole materials like Spenco, Poron and silicone gels tend to exhibit high friction⁴⁴ ⁴⁸ ⁵⁶ ¹²⁵ as seen in the graph below. Again, this is by design because the foot needs friction for traction. But it does provide a conundrum for blister-sufferers.
Figure 26: Coefficient of friction data in dry and moist conditions - adapted from Payette (Ref 125)
- Cushioning materials reduce peak pressure by spreading load over a larger area.
- Cushioning materials can also absorb shear via their shear modulus.
- More cushioning is not always better as it can affect shoe fit and reduce functional efficiency.
Foot orthoses (orthotics) can be used to change the magnitude and timing of vertical and parallel forces on the foot. This means orthotics can be used to prevent blister-causing shear.
Below is a table outlining possible biomechanical factors that may contribute to blister-causing shear and the potential orthotic prescription variables, musculoskeletal therapies and other interventions that can be used to alter foot function to minimise shear. While this information is mainly for the benefit of podiatrists and sports medicine professionals, you can see there is a huge potential for us to minimise blister-causing shear with the use of orthotics and other means, where it is applicable. Athletes should not use this as specific practical advice for their blisters. If you have ongoing blister issues, consult an expert in foot function for the best biomechanical advice and treatment relevant to you!
Figure 27: An example of biomechanical causes and treatments for foot blisters according to anatomical location. This is general advice only and should not to be seen as specific advice for your blisters!
- Podiatrists have an advanced understanding of foot biomechanics to help prevent blisters.
- Foot orthotics alter pressure and friction and therefore have a direct impact on soft tissue shear.
5) POLYTETRAFLUOROETHYLENE SHOE PATCHES (ENGO PATCHES)
Polytetrafluoroethylene (PTFE) is an ultra-low friction material. Teflon® is an example of a PTFE material. ENGO Blister Prevention Patches is another. ENGO Patches are adhesive patches that stick to your shoe or insole where high friction causes blisters. Research shows two things about the friction properties of these patches:
a) a coefficient of friction of around 0.16 which is very low compared to other in-shoe materials⁵⁶
b) the low coefficient of friction is maintained in the presence of moisture⁵⁶ ⁸⁹ ¹²⁵
Figure 28: ENGO’s coefficient of friction remains low in the presence of moisture - Adapted from Carlson (Ref 56)
Figure 29: More coefficient of friction data - adapted from Payette (Ref 125)
Most attempts at reducing friction are focused on the skin (the skin-sock interface) – like powders, antiperspirants, lubricants and moisture-wicking socks. But friction reduction can take place on the other side of the sock: the shoe-sock interface. The advantage is that sweat is not a constant threat to adhesion. So as long as the shoe isn't water-logged, the patches stay in place for an extended period. In fact both the adhesive and the PTFE (blue) surface are very hardy, lasting around 500kms of wear. On a personal note, the best thing I find with ENGO patches is once they're in the shoe, you can simply forget about blister prevention. Now please exercise a healthy degree of scepticism here as this is purely my own personal experience - and I have a commercial association with ENGO Patches. But if you're blister prone like me and taking preventative measures on a daily basis is a necessity, you'll understand what a bane it can be (in regard to time, cost and just having to be organised, among other things). Once these patches are in the shoe, you just get your shoes on and go - there's no fluffing about with taping or applying products to your skin each time or soaking your feet the day before. And there's no mess.
We've discussed the importance of friction - it is necessary for traction and the mechanical efficiencies of gait (propulsion, changing direction etc). So where friction is too high and causing blisters, it only needs to be minimised at that location (not the whole foot). This is called Targeted Friction Management. Unlike many other friction-reducing strategies, ENGO Patches allow the targeted management of friction: they minimise ‘bad’ friction in discrete areas to avoid blisters whilst maintaining ‘good’ friction elsewhere to maintain necessary traction - meaning biomechanical function is unchanged and functional efficiency is maintained.
Figure 31: ENGO patches applied to the insole/orthotic
Considering the very low coefficient of friction of under 0.20, it's very likely friction will be reduced (more than most other strategies considering the COF values above) to below the blister threshold of most people, regardless of how heavy perspiration or environmental moisture is. Informal blister case studies show favourable results for athletes of American football, soccer, volleyball and basketball sporting codes.¹²⁷ And PTFE has been found to significantly reduce the incidence of another shear injury, diabetic foot ulcers.¹⁰³
- Polytetrafluoroethylene (ENGO Patches) reduce friction at the shoe-sock interface.
- PTFE has very low friction properties that are unaffected by moisture.
- ENGO Patches allow the targeted management of friction.
- Disclaimer: We have an association with ENGO Blister Prevention Patches. To find out more visit www.blisterprevention.com.au/about-us