The most common question podiatrists (and people in general) ask me is, “What is the best athletic sock to recommend for blister prevention?” My answer always seems insufficient. That’s because not only has the subject not received adequate “real life” research, but the functions and features of sports performance socks is a complicated subject. This diagram is my attempt at providing a visual summary of the key concepts identified through Dr Doug Richie’s literature review in preparation for submitting our manuscript to the Journal of Athletic Training in 2022 (published January 2024).
Introduction
Socks have the potential to prevent blisters by reducing moisture content on the surface of the foot, thereby reducing the COF. The sock fibers best suited for moisture reduction also have properties which can prevent blisters via additional mechanisms. Baussan et al describe the properties of sock fibers and sock construction which can affect friction blister rates: moisture regain, swelling properties, water transport, heat transfer and friction coefficient [1]. Moisture regain is a quantitative parameter which describes the ability of the sock material to attract and hold on to moisture. Hygroscopy describes the overall moisture absorption or adsorption capacity of the sock material. The ability of socks to absorb moisture is largely dependent upon the water attraction, i.e. hydrophilic or upon the water repelling, i.e. hydrophobic properties of the surface of the fibers. The moisture absorptive capacity and surface properties of the sock fibers have a significant influence on a process known as wicking which is the movement of moisture out of the sock to the outside ambient environment [2].
Cotton is a hydrophilic fiber which inhibits moisture-wicking ability. Cotton fibers absorb three times the moisture as synthetic acrylic fibers [3]. Once wet, cotton has a ten-fold greater drying time compared to synthetic fibers [4]. Conversely, synthetic fibers such as acrylic, polypropylene and polyester are hydrophobic and facilitate wicking by transporting moisture along the fiber surfaces [3]. A specialized polyester fiber known as Coolmax has a scalloped oval cross-sectional fiber geometry designed to increase its surface area by 20% to facilitate moisture transport [5]. When comparing synthetic fibers, polyester fibers (Coolmax®) have a 15% faster drying time compared to acrylic fibers [5].
In terms of overall moisture regain, wool is the most hygroscopic of all sock fibers, with an ability to carry between 30-50% of its weight in a moist environment [6]. This is due to a greater capacity of wool for both absorption and adsorption of moisture within and along the surface of the fibers. Cotton fibers rank second in overall moisture regain, retaining three times the moisture of acrylic and fourteen times the moisture of Coolmax [5]. The absorption of water will cause swelling of the sock fibers. The swelling of sock fibers reduces air space within the sock and thus inhibits the movement of moisture away from the skin surface. Cotton fibers will increase volume and swell by 44-49% when immersed in water [7]. Another study shows that when moisture is applied, cotton fibers swell 45%, wool fibers swell 35% and acrylic fibers swell 5% [8].
Moisture transport
Rossi studied the moisture storage and moisture movement properties of three common sock fiber materials [9]. Polypropylene socks showed the best capacity to wick moisture from the inner side of the sock to the outer side. Woolen socks showed moderate wicking capacity which was slower than polypropylene. Polyamide (nylon) socks showed minimal wicking capacity with large amounts of moisture retained on the inner side of the sock.
Sock density
The density of fibers or construction of the sock can influence the moisture wicking capacity [4]. Denser weave patterns or thicker padding may enhance moisture movement thru the sock by reducing compaction and preserving air-space between the fiber networks [4].
2 Longitudinal double-blind studies comparing socks
Herring and Richie [10] looked at 35 long-distance runners and compared blister incidence in padded socks of identical construction but different materials – either 100% cotton or 100% acrylic fibers. There were twice as many blisters in the cotton sock group and they were three times the size, suggesting acrylic fibers were beneficial over cotton fibers in athletic socks. The authors proposed that the results were explained by lower friction force on the skin surface due to superior moisture-management of acrylic. However, in their follow-up study, Herring and Richie implemented socks with reduced padding, contrary to the dense padding in the first study, and found no difference in blister frequency when comparing cotton and acrylic fiber socks [11]. The authors concluded that the superior blister prevention capacity of acrylic fibers over cotton fibers depends upon sock construction. They speculated that the wicking capacity of acrylic fibers is enhanced by denser padding within the sock enabling better moisture movement from the skin surface. Alternatively, Herring and Richie proposed that a sock’s ability to prevent blisters could depend upon some other mechanism related to its thickness, such as pressure-reduction or shear absorption.
Pressure reduction
In regard to pressure reduction, athletic hosiery has been found to dissipate pressure against the skin of the foot, dependent on the fiber composition as well as the thickness or density of the fibers in the construction of the sock. Howarth and Rome studied the plantar shock attenuation provided over 72 hours by 5 types of athletic socks compared to barefoot, including: cotton socks; wool cushion sole sports socks; acrylic cushion sole hiking socks; double layer cotton socks; and toweling cushion sole sports socks [12]. Only the wool cushion sole sports sock and the acrylic cushion sole hiking sock demonstrated a significantly increased shock attenuation compared to barefoot walking. The cotton sock, double layer cotton sock and the toweling cushion sole sock did not. Other studies of padded hosiery have demonstrated reduced peak plantar pressures in the forefoot in patients with rheumatoid arthritis and diabetic neuropathy [13–17].
Coefficient of friction (COF)
While socks can affect moisture management to reduce COF, the inherent frictional properties of the sock itself should also be considered [18]. A study was undertaken to determine if PTFE (polytetrafluoroethylene, Teflon®) could reduce friction blisters when incorporated into the construction of an athletic sock at the heel, forefoot and toe area. In this study, blister incidence in a subject group of 77 university students participating in aerobics classes over 4 weeks showed no significant protective effect from the PTFE sock [19]. Dai et al used a 3-D finite element model to simulate the foot-sock-insole interfaces and investigate the effects of wearing socks with different combinations of frictional properties on plantar foot contact [20]. They found that wearing socks with low friction against the foot skin was found to be more effective in reducing plantar shear force than a sock with low friction against the insole.
Knapik recognizes the multiple mechanisms by which socks may reduce blister formation when he surmises “Besides moisture reduction, thick socks that maintain their bulk during sweat and compression may effectively absorb Ff, thus reducing blister probability.” [18]. This underscores the fact that socks can be part of three strategies which can reduce the risk of foot blisters: COF reduction, pressure reduction and shear absorption.
Friction coefficient is largely dependent upon sock construction
Friction coefficient is largely dependent upon sock construction. Baussan and co-workers studied the coefficient of friction of two common knitted structures used in athletic socks, namely terry and simple jersey, also known as flat-knit [1]. Jersey fabric is knit, not woven and has a smooth surface on one side and piles on the other. Terry fabric is also knitted and features distinct loops and soft piles of yarn on one side, with a smooth surface on the other side. The limitation of the investigation by Baussan et al was that only cotton fiber construction was tested and this is not representative of the preferred fibers for today’s modern athletic socks. Also, only dynamic friction force was measured in this study which is less important than static friction force is when considering the pathomechanics of friction blisters in the feet. Finally, friction was measured only in the dry condition of the sock fabrics which does not replicate the actual moist environment inside the shoe of an active individual. Notwithstanding, Baussan et al found that the friction and shock absorption experiments showed that terry construction generates less friction and absorb more energy than simple jersey or flat knit construction socks. The authors stated that this study supported the previous research of Howarth and Rome showing that higher density terry construction have greater capacity for shock absorption [12]. Also, Herring and Richie demonstrated in their two studies that socks with high density terry padded construction appeared to reduce blister rates via superior cushioning and better moisture management compared to standard flat knit construction [10,11]. Finally, Baussan et al point out that COF of any sock material is significantly affected by moisture [1]. In fact, the COF can be increased by a factor of 2 when any sock fiber is exposed to high moisture [21].
Subsequently, two other investigations of sock coefficient of friction were conducted which contradicted the findings of Bussan et al. Van Amber and co-workers measured both static and dynamic coefficient of friction at the sock-skin interface, comparing three fibers: fine wool, mid-micron wool and acrylic [22]. In this study, fabric structure played a more significant role than sock fiber type in determining friction force and coefficient of friction at the sock-skin interface. Single jersey fabrics had the lowest coefficient of static and dynamic friction compared to half-terry and terry fabric structure. Acrylic fabrics consistently exhibited the highest frictional forces and coefficient of friction under wet and dry conditions. All fabrics and fibers showed increased friction with increased moisture. The most important effect of fiber was on the static frictional force and coefficient of static friction of damp fabrics, with fabrics composed of fine wool exhibiting lowest friction, and acrylic fabrics the highest. The authors also measured fabric deformation, a factor which increased friction force. Compared to fabrics with wool fibers, acrylic fabrics deformed more under weight when wet which could account for the difference in friction force between the two fibers. Also, half-terry and terry fabrics underwent more deformation than single jersey fabrics which also may explain the difference in friction force between these fabrics.
DeBois and co-workers also found that sock knit structure is more important than sock fiber composition in terms of friction force at the sock-skin interface [23]. This study was limited to measuring dynamic friction force in the dry condition only, and found that terry knit structures consistently produced higher frictional force than their corresponding single knit jersey structures. The authors also found that higher density of the yarn in the terry fabrics, ie. loops per square inch, influenced frictional force with high density yarn having the highest friction force. The authors speculate that denser terry loops have more surface area to contact the skin and thus greater potential to resist skin sliding. Also, the experimental apparatus used in all three studies cited above resulted in compression of the terry piles by the friction measurement probe. As the piles compressed, an increased contact angle occurred between the probe and the sock interface increasing the sliding resistance or friction force. In other words, the terry piles appeared to undergo deformation with application of a friction probe which might be similar to what happens when a bone prominence contacts the terry loops of a sock inside the shoe. Finally, terry loops have a directional orientation which may increase friction with forward sliding while decreasing friction with backwards sliding. Indeed, Bussan found conflicting results with friction force in terry knit structures depending upon which direction the measurement apparatus was moved across the fabric [1].
Important consideration
While these studies of friction force at the sock-skin interface offer insight into how fabric structure and sock fibers may affect coefficient of friction, conclusions about how these factors relate to blister formation in the feet should be made cautiously. Laboratory studies vary in methodology and none fully replicate the in-vivo condition of a sock worn by a person inside a shoe. While laboratory studies suggest that fabric structure is more important than fiber composition in terms of friction force, other factors such as wicking, thermal dissipation and pressure reduction by socks must also be considered.
Moisture management in occlusive footwear situations
Wicking capacity of socks demonstrated in laboratory studies is not always replicated in studies of sock performance during actual physical activity inside of footwear. Without exposure of the entire sock to the outside ambient environment, moisture absorptive capacity of the sock may be more important than wicking in order to keep the skin of the foot dry. Sweat production in the foot has been estimated to range between 381 to 447 grams per hour which can often times overwhelm the simple wicking capacity of the sock fibers [24,25].
Bogerd and co-workers conducted a field study of 37 military recruits who were marching over a period of four consecutive days [26]. This study was designed to measure moisture content on the skin surface of the feet of the participants as well as moisture content retained by the socks after marching. Also, the participant’s perception of skin temperature, overall dampness, friction and comfort was measured by questionnaire. Inexplicably, these parameters were all proposed by the authors to be critical to the formation of friction blisters on the feet, yet actual documentation of blister events was not carried out. Of the two socks tested, a 50% Merino wool and 33% polypropylene blend was rated to be cooler, less damp, and more comfortable than a 99% polypropylene sock. Surprisingly, in this study of soldiers wearing prototype military boots equipped with a GORE TEX membrane, the wool blend socks kept the surface of the foot drier than the polypropylene sock in two foot locations (dorsal metatarsals and posterior calcaneus) while the entire plantar surface of the foot showed no difference in moisture content when comparing the two different socks. In this study, the wool blend sock absorbed 2.9 times the moisture of the polypropylene sock. The authors speculated that the superior moisture storage benefits of the wool blend sock outweighed the wicking capacity of a polypropylene sock inside a closed boot where moisture evaporation is compromised. Thus, to reduce moisture content on the skin surface, the absorptive capacity of a sock becomes most important when the footwear has resistance to vapor evaporation [26].
Thermal conductivity
Finally, the thermal conductive properties of sock fibers are important considerations for blister prevention. Reducing or evacuating heat from the skin surface depends upon the thermal conductivity of the sock fibers. Cotton fibers have low thermal conductivity of 0,07W/m/K. Polyester has average thermal conductivity of 0.14W/m/K and polyamide (nylon) has a high thermal conductivity of 0.25 W/mK but has 6-fold greater moisture regain than polyester [3].
References
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