With regard to moisture management on the surface of the foot, the terms “hydrophobic” (repel moisture) and “hydrophilic” (retain moisture) are utilized in describing sock fiber performance. In general, cotton fibers and most wool fibers are considered hydrophilic, while synthetic fibers are hydrophobic. The response of socks to exposure to moisture is important from both a comfort and clinical standpoint.

Moisture can accumulate in the shoe of the athlete from three different sources: the foot itself, the legs and trunk of the athlete, and the outside environment. The foot contains eccrine sweat glands which are innervated by cholinergic fibers activated by the sympathetic nervous system. The palms and soles are unique in having the highest density of eccrine sweat glands in the body: 2000 glands/cm², compared to a density of only 100 glands/cm² in the rest of the body [11].

The production of moisture from the sweat glands of the feet during vigorous physical activity is estimated to be as much as 200 cm³/h [12]. The production of moisture from the remainder of the body during exercise can exceed 1 L/h [12]. The sum total of moisture potentially collecting in the shoe of the athlete during exercise will quickly exceed the absorptive capacity of any sock. Therefore, in order to keep moisture content at a minimal 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 [13].

Cotton fibers are hydrophilic and absorb three times the moisture as synthetic acrylic fibers which are commonly used in athletic hosiery [14]. Once wet, cotton socks retain moisture and have a tenfold greater drying time compared to synthetic fiber socks [15]. In sedentary activity, cotton socks may be preferable to acrylic socks, given the low moisture output of the feet, and the better absorptive capacity of these hydrophilic fibers.

However, during vigorous activity, the absorptive capacity of any sock will be exceeded, and only a wicking gradient will allow movement of moisture from the foot surface to the shoe for evaporation to the outside environment. Hydrophilic fibers such as cotton have a 2.4 times greater resistance to moisture transport [15]. This may be related to absorption of fluid and swelling within the fibers themselves. When wet, acrylic fibers swell 5% while wool fibers swell 35% and cotton fibers swell 45% [16]. Swelling of fibers is related also to a loss of shape and conformability to the foot. Cotton socks tend to bunch and elongate when wet, while synthetic fiber socks are more likely to retain shape, cushion, and resiliency in these conditions.

While wicking properties of sock fibers vary considerably, the ability of a sock to keep the surface of the foot dry relies on several variables. In an athletic shoe, there may be less resistance to moisture transport and evaporation thru the upper material, a high top boot such as worn during hiking or worn by military personnel will not allow evacuation of moisture to the outside environment. This was shown in a study by Bogerd et al. where moisture levels were measured on the feet of soldiers after marching 6.5 km in standard military boots [17]. Socks composed of a blend of 50% Merino wool and 33% polypropylene kept the surface of the foot drier than socks composed of 99% of polypropylene. According to previous research, polypropylene would be expected to wick better than wool [18]. However, the wool/polypropylene blended socks absorbed more moisture than the pure polypropylene socks and also kept the surface of the foot drier in marching soldiers wearing military boots. When the footwear does not permit evacuation of moisture to the outside environment, absorption rather than wicking may be the desirable feature of sock fiber composition.

The common fibers used in the manufacture of specialized athletic hosiery are listed in Table 7.1. The majority of fibers used in the construction of athletic hosiery are from synthetic sources. This is because synthetic fibers have been engineered to have physical properties which are desirable for athletic performance: water resistance, wicking, thermal insulation, wind resistance, antimicrobial resistance, reduced weight, cushion and resiliency, and reduced coefficient of friction. Other important features of athletic socks include durability, maintenance of shape when wet, machine washable, quick drying, and odor resistance. Although cotton fiber socks do not fulfill these functions, other natural fibers may perform just as well as some synthetic fibers.

Wool, being a natural fiber, is hydrophilic but may not have all of the undesirable features of cotton fibers when used for high performance sock construction. Specialized wool yarns known as Merino Wool have been developed which have many of the characteristics of synthetic fibers. Compared to 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 air spaces created by Merino wool have lead manufacturers to claim that this fiber is superior to any synthetic fiber for insulation and wicking.

The most popular synthetic fibers utilized 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. Coolmax fibers have a four-channel geometric configuration to enhance surface area and moisture movement. As a result, studies have shown that Coolmax and other polyester fibers have a 15% faster drying time compared to acrylic fibers. Both acrylic and polyester remain soft with multiple machine washings, resist wrinkles and stains, and retain their shape with moisture exposure. One shortcoming of acrylic is poor insulation. On hot surfaces in summer months, acrylic fiber socks can conduct heat and be undesirable. Hollow core polyester or Coolmax socks may be preferred in these conditions.

Insulating fibers have been developed for cold climate sporting conditions. Thermolite and Hollofil are examples of hollow core fibers designed to trap air and provide an insulating layer for trapping heat against the skin of the foot. Wool fibers have this same “air-trapping” framework which has made wool a fiber of choice for cold climates for decades. Newer fibers such as Outlast have a chemical property to store and release heat, depending upon the skin temperature. Silver impregnated X-static fibers have a natural heat retaining capacity. X-static claims that 95% of body heat is reflected back to the skin by the silver fibers within the sock.

X-static is also one of the newer types of sock fibers which have antimicrobial properties. Other fibers marketed with antimicrobial claims include Microsafe, Innova, Cupron, and Biofresh. The benefits of antimicrobial fibers for sock construction are discussed later.

During walking and running impact shock occurs over a relatively short period of time as ground reaction forces are transmitted into the foot and then dissipated throughout the body [23]. Impact shock has been attributed to be a contributing factor to a wide range of pathologies including degenerative joint disease, soft tissue injuries and low back pain, plantar fasciitis and Achilles tendinitis [24].

Artificial shock absorbers such as footwear and insoles have been studied to determine ability to protect the body from impact stress [25, 26]. More recently, researchers have begun studying the potential of specialized hosiery products to attenuate shock and provide benefit in the reduction of impact-related injuries.

Howarth and Rome studied five different sock constructions to determine if any reduction of impact shock could be measured during treadmill walking [27]. A wool cushion sole sock as well as an acrylic cushion sole sock significantly reduced impact shock compared to either a standard cotton sock, double-layer cotton sock, and a cotton sock with a terry pile weave. The authors concluded that certain fibers such as wool and acrylic are better suited to reduce impact shock as long as denser padding is provided in a cushion sole construction.

Blackmore et al. used an impact testing system which simulated heel strike forces during running to measure the reduction in impact shock provided by socks and shoes [28]. Eight different sock designs demonstrated a reduction of peak impact force by up to 20%, a delay in onset of loading by up to 33% and a reduction of loading rate by up to 47%. Reductions in impact shock also occurred with the sock-shoe condition, but to a lesser degree owing to the superior cushioning effect of the shoe alone. The sock which achieved the best shock attenuation had the thickest construction and was composed primarily of Merino wool. The researchers concluded that socks composed primarily of cotton fibers have the least capacity for reducing impact shock and plantar pressure.

Plantar pressure measurements study the foot-ground interface and can provide insight into the role of hosiery to protect the integument and underlying skeletal structures of the foot. Veves et al. conducted several studies of plantar pressure dissipation during barefoot walking on an optimal pedobarograph of specialized padded (ThorLo) hosiery. These densely padded socks showed a 30% reduction of peak plantar pressures during walking in diabetic patients with peripheral neuropathy. Less padded, sport socks also demonstrated significant pressure reduction of 15% which was maintained after 6 months of continuous use [3].

Donaghue et al. also studied padded (ThorLo) hosiery using in-shoe pressure measurements on diabetic patients [4]. Padded hosiery demonstrated a significant 10.7% reduction of peak plantar pressure inside the shoe when padded hosiery was compared to conventional socks. More recently, Garrow et al. utilized in-shoe pressure testing of specialized double-layer acrylic hosiery in diabetic patients [5]. A 10.2% reduction of peak forefoot pressure was measured compared to conventional socks.

Studies of friction blisters and hosiery utilized subjects more representative of athletic patients rather than diabetic subjects with neuropathy. Friction blisters are considered the most common skin injury in sport [29]. Because the sequela of these blisters can result in infection and disability, the subject of blister prevention has been of keen interest particularly in the United States Military.

Herring and Richie conducted a prospective, randomized cross-over study of 35 long-distance runners wearing padded socks composed of either acrylic fibers or cotton fibers [7]. The runners wearing acrylic socks experienced half as many blisters as those wearing cotton socks. The subjects wearing acrylic fiber socks perceived that their feet were dryer compared to wearing cotton socks. Previous studies had shown that moisture content on the skin surface increased frictional force and tendency to form blisters.