1 Introduction

Shoes and foot functions are important for safety, efficiency, and performance during walking, running, and other sports activities. Many studies have investigated standing balance and gait motion with respect to various shoe types.

Masai Barefoot Technology (MBT) shoes are a typical example of rollover shoes. The MBT shoe is an unstable shoe with a rounded sole design and an integrated soft pad sensor under the rear foot, thereby producing instability in both the anterioposterior (AP) and the mediolateral (ML) directions.¹ Previous studies showed that postural sway was greater while standing in the unstable shoe than barefoot and in the stable control shoe.^1,2 In addition, the muscle activity while standing in the unstable shoe increased compared to the stable control shoe.¹

Kinematic analysis of gait showed that the ankle, knee, and hip angles during heel contact differed between cushioned soles and bare feet.³ Lieberman et al. showed that the average impact loading rate for shod rearfoot strike runners was seven times lower than that for barefoot runners.³ Researchers studying MBT have reported a loss of normal ankle plantar flexion in the early stance.^4,5 Furthermore, there are differences in muscle activity depending on shoe conditions. The maximum electromyography (EMG) values for tibialis anterior activity at initial contact differed between rollover and MBT shoes. Meanwhile, the other shoes⁴ and rollover footwear increased the EMG profile for the soleus muscle.⁴

Compared to barefoot walking, the boost sports shoe significantly increased the peak amplitude of the gastrocnemius medialis muscle during the midstance phase.^6,7

Running in shoes with stiffer midsoles reduced peak eversion and eversion excursion during a running motion.⁸ Different mechanical responses also reported ground reaction force and eversion angle when different running shoe soles were used.⁹ Excessive pronation has been linked to stress fractures, plantar fasciitis, and lower limb pain among runners, attributable to heightened rearfoot eversion at the peak and increased excursion eversion.^10,11 Conversely, foot pronation during running is greater in rearfoot motion control shoe conditions than in barefoot conditions, and there are some concerns about the effect of shoes.⁹ Regarding the relationship between shoes and disability, most running injuries are thought to be caused by excessive motion or impact shocks during stances.⁸ Motion control shoes that can control foot pronation or supination reduce the risk of knee injury in low-arched runners.⁸ In addition, overpronation has been associated with running-related injuries (lower leg pain and medial tibial stress syndrome).¹²

Athletes use shoes with various functions; shoes with cushioning and thick soles are increasingly used as running shoes. Recently, many running shoes with improved structures and functions have been developed to improve performance, described as “thick-soled shoes.” These shoes are designed using elastic materials in a large heel to absorb some of the transient force and spread the impulse over time.³ These running shoes are not exclusive to athletes who specialize in running; they are often worn by athletes in other disciplines during training and warm-ups. Running is one of the movements included in various sports, and we considered the versatile potential of thick-soled shoes. In sports activities, forward movement and balance are important^13–17; however, the relationship between shoes with cushioning and thick soles and postural stability is unclear. If there is a difference in static postural control between different running shoe constructions, we believed that the results could help in shoe selection during training and warm-up.

In the present study, we investigated the differences in center of pressure (COP) trajectories during one-leg standing barefoot, with shoes with thin soles and small heel-to-toe drops, and with shoes with thick soles and large heel-to-toe drops in healthy participants. We hypothesized that the COP would be displaced forward during one-leg standing and that COP displacement toward the front and back would be greater with thick-soled shoes than with the bare feet and thin-soled shoes.

2 Material and methods

In total, 21 male university students participated in this study (age 19.5 ± 0.5 years, height 172.3 ± 5.5 cm, weight 67.1 ± 8.4 kg). The inclusion criteria were 1) healthy male university students and 2) shoe size of 26.0–27.5 cm. The exclusion criteria were: 1) history of lower limb surgery, 2) any trauma or disease of the lower limb at the time of measurement, and 3) difficulty understanding the content of this study. All participants signed consent forms to participate after being informed about the study. This study was conducted with ethical considerations in accordance with the Declaration of Helsinki and with the approval of the Research Ethics Review Committee of Sendai Seiyo Gakuin Junior College (Approval No. 0203).

The task involved standing on one leg with the dominant foot for 30 s, and the sway of the COP was measured using a grab coder (G-620; ANIMA Co., Tokyo, Japan). The arms of the participants stayed on the chest, and their feet were placed slightly apart on marks drawn on a force platform. They were asked to remain as stable as possible throughout the trial. COP displacements were recorded at a frequency of 100 Hz. Three successful trials for each condition were included in the analysis. The leg with which a participant kicked the ball was defined as the dominant leg.

The following parameters were obtained from the COP sway measurements: total locus length, locus length per second, AP locus length, ML locus length, AP locus length per second, ML locus length per second, maximum amplitude in the AP direction, maximum amplitude in the ML direction, rectangular area, area of sway, and locus length per area of sway. Total locus length is the length of the total movement of the COP during the measurement. Locus length per second was calculated as the total locus length per unit time [locus length per second = total locus length/30 (s)]. The AP locus length indicates the total distance moved by the COP in the AP direction during measurement, highlighting the COP's heel-to-toe movement. ML locus length is the displacement extent in the mediolateral direction, illustrating the COP's side-to-side movement. AP locus length per second was calculated as AP locus length per unit time [AP locus length per second = AP locus length/30 (s)]. ML locus length per second was calculated as ML locus length per unit time [ML locus length per second = ML locus length/30 (s)]. Maximum amplitude in the AP direction is the peak displacement in the AP direction, representing the maximum extent of front-to-back movement. Maximum amplitude in the ML direction is the peak displacement in the ML direction, indicating the maximum extent of side-to-side movement.

The rectangular area is the area encompassed by the COP sway trajectory boundaries, indicating the overall spatial extent of the COP sway. The area of sway is the overall area covered by the COP sway trajectory, providing a comprehensive measure of COP sway dispersion. Locus length per unit area was calculated by dividing the total locus length by the outer area of COP movement per unit time. This indicates how much the center of gravity has moved per unit area (locus length per unit area = total locus length/outer area of COP movement per unit time) [Fig. 1].

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Fig. 1 Representative COP displacement. A: Total locus length, B: AP locus length, C: ML locus length, D: Rectangular area, E: Area of sway. Total locus length is the length of the total movement of the COP during the measurement time. AP locus length is displacement extent in the anteroposterior direction. ML locus length is displacement extent in the mediolateral direction. Rectangular area is the area encompassed by the COP sway trajectory boundaries. Area of sway is the overall area covered by the COP sway trajectory.

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Three shoe-wearing states, including bare feet, thin-soled shoes, and thick-soled shoes thin-soled shoes (New Balance HANZO S v2; New Balance Athletics, Inc., Boston, MA, USA), and thick-soled shoes (NIKE ZOOM FLY3; Nike, Inc., Beaverton, OR, USA), with eyes closed and open in each condition. In the present study, we selected the NIKE ZOOM FLY3, which has the largest market share among thick-soled shoes, and the New Balance HANZO S v2, one of the thin-soled shoes with the smallest wheel-toe drop among marathon running shoes in 2021. The thin-soled shoes weighed 170 g, had a heel-to-toe drop of 4 mm, an outsole of aggressive rubber lugs mounted over the forefoot (carbon rubber), and a midsole of single-density Revlite Foam (TPU). The upper material was engineered mesh, synthetic suede, and nubuck overlays. The thick-soled shoe had a heel-to-toe drop of 8 mm and weighed 272 g. Its outsole was hard rubber under the heel, with softer blown rubber under the forefoot. The midsole was a single-density act foam with an internal carbon plate, and the upper material was VaporWeave (TPU and TPE) [Fig. 2]. In the thin and thick sole conditions, socks were not worn; shoes were worn barefoot, and laces were used to ensure fitting.

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Fig. 2 A: Thin-soled shoes (New Balance HANZO S v2; New Balance Athletics, Inc., Boston, MA, USA) The thin-soled shoes weighed 170 g, had an offset of 4 mm, an outsole of aggressive rubber lugs mounted over the forefoot (carbon rubber), and a midsole of single-density Revlite Foam (TPU). The upper material was engineered mesh, synthetic suede, and nubuck overlays. The thick-soled shoe had an offset of 8 mm, and it weighed 272 g. Its outsole was hard rubber under the heel, with softer blown rubber under the forefoot. The midsole was a single-density act foam with an internal carbon plate, and the upper material was VaporWeave (TPU and TPE). https://www.solereview.com/new-balance-hanzo-s-v2-review/. https://www.runningshoesguru.com/2019/08/nike-zoom-fly-3-review/. B: Thick-soled shoes (NIKE ZOOM FLY3; Nike, Inc., Beaverton, OR, USA).

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The order of each condition was randomized. The testing requirements were randomly selected. All measurements for each participant were performed on a single day. For statistical analysis, two-way analysis of variance was performed after testing for normal distribution, and further comparisons between the open and closed eye conditions and Tukey's multiple comparison tests between shoe conditions were performed. The significance level was set at 5%. SPSS (version 27.0; IBM Corp., Armonk, NY, USA) was used for the analysis.

3 Results

The dominant leg of 18 participants the right leg and that of three participants was the left leg. Table 1 shows the mean and standard deviation for the COP sway measurements in one-leg standing position for 30 s and the comparison between bare feet, thin-soled shoe, and thick-soled shoes. Two-way analysis of variance showed significant differences between the open and closed eye conditions for all items of the COP sway measurement. The COP sway was greater in the closed eye state than in the open eye state in the barefoot, thin-soled shoe, and thick-soled shoe conditions. In addition, significant differences were observed between the shoe-wearing states in AP locus length, AP locus length per second, rectangular area, area of sway, and maximum amplitude in the AP direction. In the multiple comparison results, the AP locus length, AP locus length per second, and maximum amplitude in the AP direction were significantly larger with thick-soled shoes than with bare feet under the closed eye state. The locus length per unit area was significantly smaller with the thick-soled shoes than with the bare feet. Other items did not differ significantly between the shoe-wearing state.

4 Discussion

This study showed that under the closed eye state with the thick-soled shoes, the AP locus length, AP locus length per second, and maximum amplitude in the AP direction increased, whereas the locus length per unit area decreased. In the present study, the COP sway in the AP direction during the one-legged stance was higher with thick-soled shoes than with bare feet in the closed eye state.

Landry et al.¹ compared COP sway and muscle activity between unstable MBT shoes and barefoot and stable control shoes. The results showed that COP excursion ranges in the AP and ML directions for the unstable MBT shoe were larger than those for the other footwear states. The unstable MBT shoe produced larger intensities of muscle activity than the other two footwear states. Therefore, the thick-soled shoe used in this study and the MBT shoe are more unstable than bare feet or other shoes. The thick-soled shoe used in this study had a thicker sole and a larger heel-to-toe drop than bare feet and thin-soled shoes, resulting in a more unstable sole structure and greater COP movement in the AP direction.

However, no difference in COP movement was observed in the open eyes state due to differences in shoe-wearing states. The differences between the shoe-wearing states did not differ in the open eyes state. Meanwhile, they differed in the closed eyes state, possibly due to an association between sensory input and instability caused by the difference in soles.

Postural control coordinates the musculoskeletal, visual, somatosensory, and vestibular systems; when one system is inconsistent, another works more.¹⁸ In the closed eyes state, visual input was blocked and reliance on vestibular and somatosensory cues was higher. When the eyes are closed and the visual system cannot compensate, the ankle joint must respond in multiple planes and ranges.¹⁹ Goodworth et al. showed that there was no significant difference between open and closed eyes states in the bipedal stance.²⁰ However, in the case of one-leg standing position in this study, the thicker sole and greater heel-to-toe drop of the shoes made the posture more unstable, and thus the shift in COP was greater than in barefoot or thin-soled shoe conditions. While standing on an unstable surface, information derived from the somatosensory system becomes ambiguous because changes in the length of muscles in the lower extremity are not congruous with changes in body orientation relative to gravity.^21,22 The sensory input from the contact surface with the floor might take longer to induce a reaction due to the long distance from the ground to the sole caused by increased sole thickness.

The heel-to-toe drop size determines the point of contact and the contact area with the floor. With MBT and cushioned shoes, the COP is more anterior during initial contact when walking and more posterior in the terminal stance phase than in control footwear.⁴ The COP excursions were significantly greater for the unstable test shoe than for the control shoe in the AP and ML directions.^5,23 Forghany et al. suggested that the activity between the tibialis anterior and medial gastrocnemius and soleus increased when wearing MBT and rollover footwear and might increase ankle stability and joint loading during the early stance phase.⁴

In other words, thick-soled shoes are considered to have an unstable support surface under static standing conditions. Due to their shape, the large heel-to-toe drop is believed to have caused the back-and-forth movement of the COP, resulting in a larger COP oscillation. The heel-to-toe drop of the sports shoes was designed to move the COP smoothly to the forefoot. For running shoes, in particular, the efficiency of the COP can be shifted from heel contact to take-off, which impacts athletic performance. The increased COP sway in the AP direction during the one-leg stance obtained in this study is believed to decrease static stability.

The results of the present study and those mentioned above suggest that the increased COP sway during static postural control when wearing thick-soled shoes may indirectly increase the load on joints, muscles, and surrounding tissues in sports situations. It is necessary to pay attention to the use of shoes with thick soles and large heel-to-toe drops, especially in situations where static stability is required. Conversely, the distance of forward-backward and side-to-side movements during the standing test was greater with MBT shoes. Thus, MBT shoes may be effective training devices for muscle strength, stability, and proprioception.⁵ Similarly, the thick-soled shoes used in this study may also be effective for balance training.

The present study showed no difference in COP movement parameters in the open eyes state due to visual compensation. With thick-soled shoes, closing the eyes and excluding visual input increases the COP shift in the AP direction when allocation to the somatosensory system becomes relatively large and the ability to control fine posture is thought to be reduced. However, the results of this study suggest that visual compensation was sufficient to produce a significant difference in COP movement. However, the test in this study was conducted in static standing, and the results may have more effects in dynamic postural control situations, where more sensory input and higher balance ability are required.

There was no difference in COP movement to the medial or lateral side between the conditions. There is a difference in COP movement to the left and right in vernal control shoes. Although there were differences in the size of the heel-to-toe drop, sole material, and upper material between the two shoes used in this study and bare feet, we believe that the effect on left-right COP control was small.

This study had several limitations. It included only healthy male college students and could not consider sports experience or sex differences. Foot shape and alignment may affect gait and standing balance; however, since foot shape and alignment were not assessed in this study, their effects cannot be considered. For the measurement test, only static stability was measured; therefore, the effect on dynamic stability was unclear. In the future, increasing the number of participants, targeting athletes, and testing dynamic balance tasks would be useful. Further, usual shoe wear may affect center-of-gravity sway under different shoe conditions. This point was not investigated in this study and is one of the study's limitations.

5 Conclusions

Thick-soled shoes caused greater COP movement in the AP direction in the static one-leg standing position than did bare feet. Our findings suggest that the condition with thick-soled shoes was more unstable in static environments. The thick-soled shoe used in this study had a thicker sole and a larger heel-to-toe drop than did the bare feet and thin-soled shoes, resulting in a more unstable sole structure and greater COP movement in the AP direction. The results of this study could help determine whether athletes choose shoes with more unstable soles when training, warming up, or competing or shoe tails that allow for more forward and backward center of gravity movement.

Author contributions

Yasuhiro Endo: Conceptualization, Methodology, Validation, Formal analysis, Writing-Original Draft, Project administration.

Masashi Miura: Resources, Writing-Review & Editing.

Funding

This study was supported by Grants-in-Aid for Research from the Sendai Seiyo Gakuin College to Yasuhiro Endo.

Informed consent

All participants signed consent forms to participate in this study after being informed about the study.

Institutional ethical committee approval

This study was conducted with ethical considerations in accordance with the Declaration of Helsinki and with the approval of the Research Ethics Review Committee of Sendai Seiyo Gakuin Junior College (Approval No. 0203).