Receiving accurate sensory information is critical for effective motor control in any physical activity. It is especially true for sports activities that require stable standing and accurate ball control, such as baseball, golf, and tennis. The influence of footwear, such as orthotic insoles, shoe inserts, and ankle braces, on motor control has been investigated by many researchers and has been shown to contribute to physical performance.1–3 Socks are standard equipment in most forms of physical activity; however, limited scientific attention has been paid to the potential benefits of socks on physical performance.
Five-toed socks are unique in the United States, especially among athletes; however, in Japan 5-toed socks are being worn by athletes, as well as physical labor workers, and are gaining popularity because of the perceived sensation and balance improvements that result from their wear. The 5-toed socks are designed to fit each individual toe, mimicking being barefoot, and are to be worn inside shoes. It has been proposed that the individual toe sockets improve sensitivity and perception of the ground and also provide better balance by allowing stronger toe gripping.
In theory, the potential benefits of wearing the 5-toed socks could be achieved when the toe sockets provide a novel tactile sensation between the toes, which could increase sensory inputs to the central nervous system (CNS) through the somatosensory system (which plays an essential role in postural control). Afferent inputs coming from the mechanoreceptors of skin, joint, tendon, and muscles are constantly processed by the somatosensory system,4 which provides constant neuromuscular adjustments to maintain equilibrium and proper posture.5 Providing a novel cutaneous stimulation around the toes could influence the somatosensory system by offering more sensory information around the toe and foot; therefore, the CNS may be able to provide more efficient efferent output for better motor control.
Orthotic insoles, shoe inserts, and ankle braces have been hypothesized to influence the neuromuscular control system by enhancing the proprioceptive1,6 and cutaneous1–3 afferent inputs to the CNS. The tactile sensation of the plantar surface of the foot has been known to play an important role in human balance.2,3,7 Corbin et al3 observed improved static postural control during bilateral stance when using textured insoles. They stated that this improvement was observed because of the hyperesthesia of the plantar surfaces of the feet, resulting in hyperstimulation of the plantar afferent mechanoreceptors to contribute to the maintenance of postural control.3 Jerosch et al6 reported that ankle orthoses positively influenced participants with both healthy ankles and ankle instability during a single-limb balance test and ankle joint angle reproduction test. Improvement of ankle joint proprioception with the application of ankle taping and use of commercially available ankle braces was suggested by Heit et al.1 They stated that enhanced stimulation of cutaneous nerve receptors and joint mechanoreceptors by athletic tape or ankle brace application may result in earlier and improved muscular contractions during joint position sense testing.1
Similar to the devices discussed above, 5-toed socks may be beneficial in improving balance by increasing cutaneous receptor activation of the toes. In the United States, 5-toed socks are available commercially, but little attention has been paid to whether athletic abilities are enhanced with these socks. However, in Japan athletic versions of the socks are marketed in an attempt to enhance athletic abilities. Although orthotics and ankle braces are commonly used for pathological conditions, such as foot and ankle injuries, 5-toed socks have become popular in Japan for healthy, active individuals to enhance their athletic ability by increasing their postural stability.
Measurement of center of pressure velocity (COPV) has been used commonly to quantify static postural control changes with the introduction of various external foot devices, such as orthotics8 and ankle braces9; however, sensitivity in detecting static postural control with a center of pressure (COP)-based approach has been questioned by several researchers.10–12 Time-to-boundary (TTB) is an approach used to measure static postural control by estimating the time it takes for the COP to reach the boundary of the base of support if the COP were to stay on its instantaneous trajectory and velocity.12
The TTB measurement allows for determination of the spatiotemporal characteristics of postural control by considering not only the COP velocity excursions, but also the location on the foot where the excursions are occurring.13,14 Time-to-boundary has been shown to detect deficits in static postural control with neuromuscular impairments associated with aging12 and Parkinson’s disease.15 The investigation by Hertel et al13 found that the TTB measurement appears to have greater sensitivity in detecting postural control deficits associated with chronic ankle instability compared with more traditional COP-based analyses. Since then, TTB analysis has been used to quantify static postural control with those who have foot and ankle pathology, particularly in the chronic ankle instability literature.14,16,17 Time-to-boundary analysis has also been used to quantify the effect of postural control improvements.
McKeon et al17 reported that 4-week balance training has significantly improved static postural control, determined by both TTB and COP-based analyses of individuals who have chronic ankle instability during single-leg trials when their eyes are closed. However, they mentioned that TTB analysis may be more sensitive in detecting improvements in static postural control as they found the significance in 4 of 6 TTB measures, whereas only 1 of 7 COP-based measures demonstrated significant differences.17
Although 5-toed socks have been worn by physically active individuals in hopes of improving balance and performance, no scientific research has been conducted to determine whether the socks influence static postural control. Therefore, the primary purpose of this study is to assess the effect of 5-toed socks on static postural control during single-limb stance with eyes open and closed. The secondary purpose is to compare the influence of the socks on postural control using two methods of analysis (COPV and TTB) to determine which is more effective in detecting the influence of this intervention. It was hypothesized that postural control would be improved with the 5-toed socks by enhancing sensory feedback from the mechanoreceptors of the toes and foot into the CNS. Our corollary hypothesis is that TTB measures of postural control would be more sensitive to the influence of the socks.
Twelve men (mean age, 25.2±2.9 years; mean height, 177.0±8.6 cm; mean weight, 79.1±15.2 kg) and 9 women (mean age, 25.9±2.4 years; mean height, 162.5±2.4 cm; mean weight, 67.3±10.3 kg) from the university community volunteered to participate in the study. The participants were physically active young adults, defined as individuals between the ages of 18 and 30 who participated in at least 30 minutes of moderate or high intensity physical activities, such as walking and jogging, more than twice per week. The participants were excluded if they had a history of knee or hip musculoskeletal injury or surgery; fracture or dislocation of the testing ankle or leg; neurological problems; or vestibular disorders, concussions within the past 6 months, or any other conditions that may influence postural control.
A force plate (Bertec NC-4060; Bertec, Corp., Columbus, Ohio) integrated with MotionMonitor software (Innovative Sports Training, Inc, Chicago, Ill) was used to collect COP data during the single-limb balance test. MATLAB software (The Mathworks Inc, Natick, MA) was used to calculate the COPV and the TTB in both the anteroposterior and mediolateral directions. Regular seamless running socks (ASICS Corp., Kobe, Japan) and 5-toed seamless running socks (ASICS Corp.) were used for the testing. Both regular and 5-toed seamless running socks were made by the same fabric type (a combination of polyester, cotton, nylon, and polyurethane), with no cushioning or gripping materials in either sock. The design of the toe was the only difference between the two socks. One of 3 sock sizes—small (22 to 24 cm), medium (24 to 26 cm), and large (26 to 28 cm)—was selected based on the participant’s foot size.
The primary focus of this study was to determine the differences in static postural control in physically active young adults during single-limb balance testing between 3 conditions: wearing 5-toed socks, wearing regular socks, and not wearing socks (Figure 1). Therefore, participants were asked to complete 3 individual testing sessions to measure static postural control under these 3 conditions, separated by approximately 1 week, in the laboratory. The order of the 3 sock conditions was randomized.
Figure 1. Sock conditions with single-limb balance tests. (A) 5-toed sock condition. (B) Regular sock condition. (C) No sock condition.
At the beginning of the first session, the details of the study were provided to each participant orally and in written form. Prior to testing, each participant read and signed an informed consent form that was approved by the university’s biomedical institutional review board. During the testing session, the participants’ age, height, and gender were recorded. The participants were asked to wear clothing items (eg, shorts) that did not cross the knee to minimize cutaneous sensation in the lower limbs, other than the applied testing socks. The test limb was the limb the participant chose to stand on while kicking a ball.
Static postural control was assessed on the force plate with the participant in a single-limb stance with their eyes open and closed with their hands on the iliac crests. The participants were instructed to stand as still as possible for 15 seconds while COP data were collected. During the eyes open test, participants were asked to focus on a large “X” on the wall that was 3.5 meters in front of them and 1.5 meter from the floor.
At the beginning of each testing session, the sock condition was revealed. After assuming the designated sock condition on the testing limb, participants performed a total of 6 single-limb balance trials (3 with their eyes open and 3 with their eyes closed), with a 1-minute rest between the trials. The participants were instructed to keep the non-test limb off of the ground in a comfortable position without touching the test limb (Figure 2). If the participants hopped or touched the ground with the non-test leg in an attempt to gain stability, the trial was discarded and repeated.
Figure 2. Single-limb stance trial. The participants were instructed to keep the non-test limb off the ground in a comfortable position without touching the test limb.
The TTB and COPV in both the anteroposterior and mediolateral directions were calculated using the method previously described by Hertel et al.13 To calculate the TTB and COPV, the testing foot was placed exactly in the center of the force plate to separate the anteroposterior and mediolateral components of COP.13 The COP data were sampled at 50 Hz; therefore, 750 data points were calculated during the 15 seconds of each trial. All trials were filtered by a fourth order Butterworth filter with a cutoff frequency of 10 Hz. A MATLAB custom software program was used to process COP data to calculate the TTB and COPV variables.
In terms of the TTB variables, each TTB sequence included a series of peaks and valleys.13 The valleys were called TTB minima (the smallest value in TTB), which indicated the turning points of the COP to maintain single-limb balance over the base of the support.14 These minima were indications of where the participant was making a correction in the path of COP to maintain his or her balance.13 On the basis of the TTB minima, the TTB absolute minimum (the smallest of the minima), mean of the TTB minima (measurement of TTB magnitude), and standard deviation of TTB minima (measurement of TTB variability) were calculated separately for the mediolateral and anteroposterior directions.14,16 The standard deviation of TTB minima has been proposed to detect the degree of available freedom that the sensorimotor system can use during a postural task.16 A higher standard deviation of TTB minima indicates a higher degree of freedom that is available within the sensorimotor system to accomplish postural stability.17
The means and standard deviations from the testing trials were used for the statistical analyses. The TTB dependent variables, calculated for both anteroposterior and mediolateral directions, were the TTB absolute minimum, mean of the TTB minima, and standard deviation of TTB minima.14,16 The COP dependent variables were COPV in both the anteroposterior and mediolateral directions. Each of the dependent variables were calculated separately from the eyes open and eyes closed trials.16 Therefore, there were 8 dependent variables for eyes open and 8 for eyes closed. The independent variable was sock condition (wearing 5-toed socks, wearing regular socks, and not wearing socks).
For each dependent variable, a one-way repeated measure analysis of variance was performed. The significance level was set a priori at P < .05. Effect size was also calculated using Cohen’s d18 to determine the magnitude of difference in postural control. Ninety-five percent confidence intervals were calculated around each effect size. The magnitude of effect sizes was determined based on Cohen’s guidelines.18 An effect size less than 0.4 was small, 0.41 to 0.7 was moderate, and greater than 0.7 was large.18
There was no significant difference in the main effect among 6 different sock conditions for any of the dependent measures with the eyes open or the eyes closed trials (P > .11). Means and standard deviations of the sock conditions with corresponding F and P values are reported in Tables 1 and 2. The calculated effect sizes for all comparisons (a total of 45) ranged from 0.00 to 0.55. Although 3 of these comparisons were shown to have moderate effects, all calculated effect sizes had associated 95% confidence intervals crossing zero, negating any potential magnitude of the differences.
Table 1: Group Means (±SD) of Time-to-Boundary (TTB) and Center of Pressure Velocity (COPV) Measures of Postural Control for Eyes Open Trials (n=21)
Table 2: Group Means (±SD) of Time-to-Boundary (TTB) and Center of Pressure Velocity (COPV) Measures of Postural Control for Eyes Closed trials (n=21)
The primary purpose of this study was to examine the influence of 5-toed socks on static postural control during single-limb stance with eyes open and closed. The results support the conclusion that the 5-toed socks did not improve static postural control among physically active young adults during eyes open and eyes closed trials in either the anteroposterior or the mediolateral directions. This finding was consistent with both the TTB and COPV analyses.
It was proposed that toes wrapped individually with the 5-toed socks increases proprioceptive and cutaneous inputs by enhancing tactile sensations and providing pressure to the skin between the toes, improving sensory feedback from the mechanoreceptors of the toes and foot into the CNS. Although this could have had the potential benefit of improving postural control, our results did not support this theory.
Several studies have reported improved postural control with enhanced tactile sensation from the legs and feet during standing.3,19–21 Menz et al21 have demonstrated that passive electrical tactile stimulation to the lower limbs (knee, calf, and ankle) improved postural control in healthy adults. Corbin et al3 observed significant static postural control improvement by using a textured insole with healthy participants, whereas Maki et al19 found that the electrical stimulation applied to the sole of the foot significantly improved the postural control in both healthy young adults and older participants. Our chosen intervention (the 5-toed socks) may not have provided substantial enough tactile stimulation around the toes to have a significant influence on postural control. To influence postural control, the skin may need to be stimulated via a different method.
The importance of the tactile sensation of the sole of the foot on balance has been recognized by previous authors.7,22–24 Some of these studies have shown that diminished somatosensory feedback from the sole of the foot in patients with diabetic peripheral neuropathy,24 as well as anesthetizing23 and cooling22 the sole of the foot in healthy participants, caused a significant decline in postural control. Further studies may warrant using the 5-toed socks in an attempt to improve tactile sensation on the sole of the foot, as well as in the toes. In fact, there is a different type of 5-toed sock not used in this study that has a non-slipping material comprised of multiple rubber bits on the sole of the foot; it may provide enhanced tactile sensation to the sole of the foot and improve balance. In addition to the multiple rubber bits on the sole of the foot, different fabrics, tensions, or structures in the design of the sock to further stimulate cutaneous receptors may also be an important factor to consider in future studies.
The secondary purpose of this study was to compare the effect of the socks on static postural control using two methods of analysis (COPV and TTB) to determine which method may be used to detect the influence of the 5-toed socks more efficiently. However, both measurements did not show the influence of this intervention. It is possible that assessing static postural control with single-limb balance testing may not be the most sensitive or applicable measure of the effectiveness of 5-toed socks in physically active young adults. In the standing position, the function of the toes could play a less important role when compared with performing dynamic activity, such as jumping and landing. Because the 5-toed socks have been used among athletes and physical labor workers due to the perceived sensation and postural control improvements resulting from their wear, using a dynamic postural control measurement could be a better way to examine the effectiveness of the socks.
Improved dynamic postural control with an external intervention device was reported by Shaw et al.25 They reported improved dynamic postural control measured by a jump-landing task with an ankle bracing condition after a fatigue protocol was provided to Division I college volleyball players. Although the 5-toed socks have become popular in Japan, people who choose to use these socks may have considered themselves in need of a device to improve their postural control because of some preexisting conditions, such as ankle or knee injuries, or a self-perception of poor balance. Using dynamic postural control tasks in pathological participants may need to be considered to detect the influence of the socks.
A dynamic postural control deficit in individuals with chronic ankle instability has been reported in many studies.26–29 Ross et al29 demonstrated dynamic postural control deficit in participants with chronic ankle instability during a jump-landing task. Gribble et al26 observed dynamic postural deficits in participants with chronic ankle instability during the prefatigued stage of the Star Excursion Balance Test, and they also reported that the observed deficit was amplified with fatigue protocol. Future research should use a dynamic postural control task in pathological participants, such as individuals with chronic ankle instability. In addition, the interaction of potentially enhanced cutaneous information from the socks during the fatigue stage may be warranted in this line of inquiry.
Limited scientific information is available regarding the functional role of the toes in healthy individuals during physical activities. However, some studies have suggested the importance of the toe function in postural control. Tanaka et al30 reported that the great toe tactile sensitivity and postural control in elderly participants were significantly worse than in the young healthy participants. In the same study, the authors also reported significantly increased great toe pressure in the elderly participants when standing on a soft surface compared with that of healthy participants.30 This increased great toe pressure in the elderly participants is most likely a compensate mechanism to gain postural stability because of age-related degeneration of the tactile sensitivity and motor function.
The body sway and toe pressure during dynamic postural control tasks was quantified by the same group in healthy young adults using a single-leg balance testing on a moving force plate.31 They reported that postural sway was more significantly correlated with the peak anterior and posterior sway component than with lateral sway, and that the peak pressure value of the great toe was significantly greater than the sum of the peak values of the other 4 toes for both feet.31 This study has indicated that the motor control of the great toes may play an essential role in maintaining postural stability. Duckworth et al32 have reported that the great toe pressure during standing was greater than the pressure created by the 5 metatarsal heads and the heel, suggesting the importance of the great toe function on postural stability. Although functions of the toes and plantar cutaneous tactile sensation of the foot are known to be highly influential factors in accomplishing postural tasks, influences of the tactile stimulation around toes on its neuromuscular functions and on postural control are unknown.
In the current study, all of the participants were tested for their static balance without wearing shoes. We have chosen this method as an initial investigation to eliminate as many external influences as possible to detect the influence of the 5-toed socks. However, the condition of wearing both the 5-toed socks and shoes may demonstrate different results on the postural control. This condition should be examined in a further study because most individuals wear socks and shoes during their physical activities.
Implications for Clinical Practice
To our knowledge, this is the first study to evaluate the effects of 5-toed socks on static postural control. We chose to study physically active young adults for this initial investigation to establish what affects, if any, these socks may have on postural control. It appears that the socks are not altering static postural control in these individuals, who are assumed to have an intact, properly functioning postural control system. However, it is not yet known what influence the 5-toed socks may have on postural control in individuals who have altered postural control (ie, from pathology or fatigue).
In addition, our current study used a global measure (static postural control) to assess the influence of a locally applied intervention at the foot. Therefore, we have not determined whether alterations in afferent inputs from the foot that the socks are proposed to provide are influencing aspects of the neuromuscular control system, specifically muscle activation patterns, which contribute to maintenance of postural control. As we continue to investigate the effectiveness of these sock interventions, it will be important to measure what, if any, changes in foot and ankle muscle activation are occurring, especially in populations with compromised postural control.
Single-limb balance testing using TTB and COPV revealed that the 5-toed sock condition did not influence static postural control among physically active young adults. It remains unclear whether assessing static postural control with TTB and COPV analysis is the most sensitive or applicable measure to evaluate effectiveness of 5-toed socks. Therefore, investigation using measures of dynamic postural control and muscle activation patterns may be warranted. Further study on populations with compromised postural control is also needed to determine whether the socks are effective at overcoming deficits in postural control.
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- Nigg B, Nurse M, Stefanyshyn D. Shoe inserts and orthotics for sport and physical activities. Med Sci Sports Exerc. 1999;31(suppl 7):S421–S428. doi:10.1097/00005768-199907001-00003 [CrossRef]
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- Riemann B, Lephart S. The sensorimotor system, part I: The physiologic basis of functional joint stability. J Athl Train. 2002;37:71–79.
- Tropp H, Odenrick P. Postural control in single-limb stance. J Orthop Res. 1988;6:833–839. doi:10.1002/jor.1100060607 [CrossRef]
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- Baier M, Hopf T. Ankle orthoses effect on single-limb standing balance in athletes with functional ankle instability. Arch Phys Med Rehabil. 1998;79:939–944. doi:10.1016/S0003-9993(98)90091-0 [CrossRef]
- Holme E, Magnusson S, Becher K, Bieler T, Aagaard P, Kjaer M. The effect of supervised rehabilitation on strength, postural sway, position sense and re-injury risk after acute ankle ligament sprain. Scand J Med Sci Sports. 1999;9:104–109. doi:10.1111/j.1600-0838.1999.tb00217.x [CrossRef]
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Group Means (±SD) of Time-to-Boundary (TTB) and Center of Pressure Velocity (COPV) Measures of Postural Control for Eyes Open Trials (n=21)
|DEPENDENT VARIABLE||5-TOED SOCK||REGULAR SOCK||NO SOCK||F||P|
|TTB mediolateral absolute minimum (sec)||1.34±0.40||1.39±0.48||1.44±0.80||0.32||.73|
|TTB anteroposterior absolute minimum (sec)||4.78±1.33||5.20±1.45||4.83±1.6||0.75||.48|
|TTB mediolateral mean of minimum (sec)||6.22±2.15||5.98±1.94||6.93±3.55||1.91||.17|
|TTB anteroposterior mean of minimum (sec)||18.04±4.90||19.8±5.15||18.79±7.51||0.83||.44|
|TTB mediolateral SD of minimum (sec)||5.08±1.60||4.75±1.62||6.17±3.94||2.33||.11|
|TTB anteroposterior SD of minimum (sec)||11.76±3.61||13.10±4.25||12.97±6.29||0.98||.38|
|COPV mediolateral (cm/s)||0.65±0.18||0.63±0.14||0.59±0.24||2.02||.15|
|COPV anteroposterior (cm/s)||0.57±0.15||0.50±0.10||0.55±0.17||1.96||.16|
Group Means (±SD) of Time-to-Boundary (TTB) and Center of Pressure Velocity (COPV) Measures of Postural Control for Eyes Closed trials (n=21)
|DEPENDENT VARIABLE||5-TOED SOCK||REGULAR SOCK||NO SOCK||F||P|
|TTB mediolateral absolute minimum (sec)||0.58±0.17||0.66±0.26||0.63±0.24||1.77||.19|
|TTB anteroposterior absolute minimum (sec)||1.98±0.63||2.06±0.60||2.00±0.72||0.2||.82|
|TTB mediolateral mean of minimum (sec)||2.9±1.59||2.87±1.34||3.26±1.63||1.3||.29|
|TTB anteroposterior mean of minimum (sec)||7.9±1.99||8.16±2.39||7.9±2.29||0.24||.78|
|TTB mediolateral SD of minimum (sec)||2.89±2.64||2.64±1.58||3.2±1.74||0.81||.45|
|TTB anteroposterior SD of minimum (sec)||5.54±1.45||5.70±2.00||6.48±3.42||1.03||.37|
|COPV mediolateral (cm/s)||1.36±0.34||1.33±0.38||1.23±0.42||2.1||.14|
|COPV anteroposterior (cm/s)||1.26±0.41||1.20±0.31||1.25±0.36||0.46||.64|