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Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing Tomohiro Demura a,∗ , Shin-ichi Demura b , Masanobu Uchiyama c , Tamotsu Kitabayashi d , Kenji Takahashi e a
Jin-ai Women’s College, Fukui, Fukui, Japan Kanazawa University, Kanazawa, Ishikawa, Japan c Akita Prefectural University, Shimoshinjo-Nakano, Akita, Japan d Tokyo University of Science, Shinjyuku-ku, Tokyo, Japan e Teikyo Heisei University, Ichihara, Chiba, Japan b
a r t i c l e
i n f o
Article history: Received 5 April 2014 Received in revised form 16 February 2015 Accepted 19 February 2015 Keywords: Shoes Static standing Center of pressure
a b s t r a c t Shoes with curved rocker bottom soles may induce an unstable standing posture. This study was aimed to mainly examine the effect of such shoes on the center of pressure (COP) during static standing. Ten healthy young male adults had their COP measured during static standing with four types of shoe conditions (Stretch Walker® : SW (shoes with curved rocker bottom soles), Masai Barefoot Technology®: MBT (similar to SW in form and material), more conventionally soled shoes with a typical toe-spring: MCS, and bare feet: BF) for 60 s. The mean path length and mean velocity of Y (front-back) axis were significantly greater when wearing the MBT than when wearing the SW, and when wearing the SW than when BF or when wearing the MCS. In addition, mean velocity of X (left-right) axis, area surrounding root mean square, root mean square, and root mean square of Y-axis were significantly greater when wearing the MBT than when wearing the SW, MCS, or when BF. In conclusion, when wearing the MBT or SW with rounded sole, static standing posture becomes unstable because of their characteristics as compared with wearing MCS or when BF, but the MBT has a larger sway in the front-back direction than the SW. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction According to Nigg et al. [1], posture stability is very important for walking and it is necessary to wear appropriate shoes that support gait and enforce leg strength for enhanced walking stability. Shoes used for walking or exercise are generally designed to aim at stability of posture and gait. However, it is feared that long-period use of such shoes induces a decrease in leg strength [2,3]. Recently, training devices which aimed at enhancement or decrease-prevention of leg strength have been developed. A wobble board is used as a representative device. It has been reported to improve proprioception of the ankles and knees in addition to increasing leg strength [2,3]. Shoes with rounded and soft soles made with special materials, called Masai Barefoot Technology (MBT), are based on a similar idea. It was reported that wearing these shoes significantly changes gait [1] and provides special stimulation to the leg muscles and leg joints [4]. In addition,
∗ Corresponding author. Tel.: +81 776 56 1133. E-mail address: [email protected] (T. Demura).
it was reported that when wearing these shoes, the sway of center of pressure (COP) during static standing increases in the anterior–posterior and lateral directions [1]. Furthermore, in Japan, “Stretch Walker” (SW) shoes with a curved rocker bottom sole and a soft cushion material in the heel portion of the outsole and forepart have been developed and are currently available on the market. Demura et al. [5] reported that wearing these shoes change gait properties. The SW has characteristics similar to the MBT but has greater stability because the heel portions are somewhat flat (Figs. 1 and 2). Hence, it was hypothesized that the COP differs when standing while wearing SW and MBT. It has been reported that static standing posture becomes more unstable because of a curved rocker bottom sole and a soft heel pad when wearing unstable shoes as represented by MBT [1,6–8]. In addition, Lord et al. [9] reported that COP during static standing differed with different types of shoes; however, Wilson et al. [10] and Brenton-Rule et al. [11] reported no significant difference. In short, contradictory results have been reported. Wearing shoes with very special soles such as MBT affects COP during static standing; however, few studies have examined how the degree of round
http://dx.doi.org/10.1016/j.foot.2015.02.004 0958-2592/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
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2
Fig. 1. Stretch Walker (Nosaka, Japan). Fig. 3. More Conventionally soled Shoes with a typical toe-spring (MCS).
2.2. Materials 2.2.1. Shoe conditions In this study, three types of shoes were used to examine the effect of rounded soles in the anterior–posterior direction on gait (Figs. 1–3). Shoe with sizes from 24 to 29 cm (US size: 6–11) at intervals of approximately 0.5 cm were prepared, and the subjects selected preferred shoe size after trying on the shoes.
Fig. 2. Masai Barefoot Technology (MBT, Switzerland).
sole angle affects COP during static standing. Hence, the above, in short, the shoes with the different sole round angle would be necessary to examine. The degree of the round angle of the sole is related to the stability of standing posture and the muscle activity of the legs while walking. In addition, it is assumed that the effect of the abovestated special soles differs between elderly and young individuals with different leg strengths. Shoes with curved rocker bottom soles have been developed for walking. However, a large body sway, even during static standing in elderly individuals with poor leg strength or leg muscle fatigue after walking, entails a risk of falling when wearing the shoes. The results presented here will be useful for the optimal selection of shoes according to individual physical fitness levels. The results will also aid shoe selection for young individuals with the goals of strength training via gait for a short time, or of keeping a comfortable gait for a long time. This study is aimed to compare the COP during static standing with BF (bare feet) and while wearing SW, MBT, and MCS (more conventionally soled shoes with a typical toe-spring).
2. Materials and methods
2.2.1.1. Stretch Walker (SW; Nosaka Ltd., Japan). The “Stretch Walker® ” (SW) shoes manufactured by Nosaka, Ltd. were used for this study as the shoes with curved rocker bottom soles (Fig. 1). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 3.8 cm anteriorly and 3.0 cm posteriorly (Fig. 1). 2.2.1.2. Masai Barefoot Technology (MBT; Masai Marketing & Trading AG, Switzerland). The “Masai Barefoot Technology® ” (MBT) shoes in which the soles have an ungrounded area while standing upright, with surface area wider than SW, were used for this study as another pair of curved rocker bottom soles (Fig. 2). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 2.9 cm anteriorly and 4.3 cm posteriorly (Fig. 2). 2.2.1.3. More conventionally soled shoes with a typical toe-spring (MCS). We used more conventionally soled shoes with a typical toe-spring as the control (Fig. 3). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 2.3 cm anteriorly and 0.2 cm posteriorly (Fig. 3). 2.2.1.4. Barefeet (BF). Posture stability of subjects with BF was measured for the control condition. 2.2.2. Measurement of COP The measurement instrument was a stabilometer G5500 (Anima, Japan). This instrument can calculate the COP of vertical loads from the values of three vertical load sensors, which are located at the corners of an isosceles triangle on a level surface. The data sampling frequency was 20 Hz.
2.1. Subjects 2.3. Testing protocol Ten healthy young male adults without extremity disorders and with regular exercise habits participated in this study (age: 23.9 ± 3.6 years, height: 171.8 ± 4.1 cm, body mass: 67.6 ± 4.9 kg). Before the experiment, the purpose and procedure of this study were explained to all subjects in detail and an informed consent was obtained. However, subjects were not informed about the characteristics of the shoes. In addition, this study was approved by the Ethics Committee on Human Experimentation of the Faculty of Human Science, Kanazawa University (Ref. No. 2012-08).
The measurement procedure followed a method prescribed in the standardized stabilometry test [12]. Subjects maintained a static upright posture with feet together (Romberg posture) for 1 min, and then they were instructed to watch an achromatic target placed at eye level. The measurements began after the subjects’ posture and eyes were stable. The subjects performed each condition three times with one minute rest in a seated position after each trial; the trial order was set randomly.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
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Table 1 Basic statistics of COP parameters in each shoe condition and results of one-way ANOVA.
Mean path length (cm/s) Mean velocity of X-axes (cm/s) Mean velocity of Y-axes (cm/s) Area surrounding maximal amplitude rectangular (cm2 ) Root mean square (cm2 ) Root mean square of X-axis (cm) Root mean square of Y-axis (cm)
ANOVA
Shoes
n = 10 Parameters
Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD
SW
MBT
MCS
BF
F-value
p-Value
partial 2
Post-hoc Bonferroni
1.13 0.12 0.67 0.11 0.62 0.06 7.59 2.54 0.74 0.15 0.46 0.10 0.57 0.13
1.73 0.24 0.87 0.12 1.11 0.20 13.21 3.78 1.02 0.24 0.50 0.10 0.88 0.26
0.99 0.10 0.59 0.09 0.55 0.05 5.86 2.05 0.67 0.15 0.42 0.10 0.51 0.14
0.97 0.10 0.60 0.10 0.50 0.05 5.72 2.69 0.67 0.16 0.43 0.12 0.50 0.13
58.27
0.00
0.87
MBT > SW > MCS, BF
32.49
0.00
0.78
MBT > SW, MCS, BF
57.96
0.00
0.87
MBT > SW > MCS, BF
28.25
0.00
0.76
MBT > SW, MCS, BF
23.29
0.00
0.72
MBT > SW, MCS, BF
2.60
0.73
0.22
17.58
0.00
0.66
MBT > SW, MCS, BF
SW: wearing “Stretch Walker”, MBT: wearing “MBT”, MCS: wearing “More Conventionally soled Shoes with a typical toe-spring”, BF: “Bare Foot”.
2.4. Parameters Mean path length, mean velocity of X (left-right) and Y (frontback) axes, area surrounding maximal amplitude rectangular, root mean square, and root mean square of X (left-right) and Y (frontback) axes were used as evaluation parameters. Demura et al. [13] reported that these parameters have high reliability and can evaluate sway characteristics.
Table 2 Ratios of the other shoe conditions to BF condition and their effect size.
Mean path length (cm/s) Mean velocity of X-axes (cm/s) Mean velocity of Y-axes (cm/s)
2.5. Data analysis Mean differences among three conditions for parameters were tested using repeated one-way analysis of variance (ANOVA). When a result of ANOVA showed statistical significance, the Bonferroni method was selected for multiple comparisons. The statistical SPSS package ver. 11.0 (SPSS Japan) was used for data analysis. Effect size (ES) was calculated to examine the size of the mean difference. An ES of 0.2 or below and of 0.8 or above are generally interpreted as a small difference and a large difference, respectively. A probability level of 0.05 was indicative of statistical significance. 3. Results Table 1 shows the values of COP parameters in each condition and the results of one-way ANOVA. Mean path length and mean velocity of Y-axis showed significantly greater values in the MBT than in the SW, MCS, and BF, and in the SW than in the MCS and BF. In addition, mean velocity of X-axis, area surrounding maximal amplitude rectangular, root mean square, and root mean square of Y (front–back)-axis showed significantly greater values in the MBT than in the SW, MCS, and BF. Table 2 shows the ratio of the SW, MBT, MCS, and BF (SW/BF, MBT/BF, and MCS/BF) and effect size (ES). A significant difference was observed in the parameters except in the X direction for MBT and in the mean path length and mean velocity of Y-axis for SW, but not in all parameters for MCS. The effect size in parameters that showed a significant difference was very large (ES = 1.40–3.86), particularly in differences with MBT. 4. Discussion This study aimed to compare and examine the COP during static standing while wearing SW, MBT, and MCS, and when BF. MBT and SW with curved rocker bottom soles and a soft cushion material in the outsole have features different from common shoes.
Shoes
n = 10 to BF
Area surrounding maximal amplitude rectangular (cm2 ) Root mean square (cm2 ) Root mean square of X-axis (cm) Root mean square of Y-axis (cm)
Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES
SW
MBT
MCS
1.17 1.40* 1.12 0.65 1.22 2.00* 1.33 0.68 1.11 0.44 1.09 0.34 1.13 0.46
1.78 3.86* 1.43 2.24* 2.20 3.83* 2.31 2.16* 1.54 1.67* 1.18 0.66 1.74 1.71*
1.02 0.19 0.97 0.18 1.08 0.75 1.02 0.06 1.01 0.04 0.99 0.02 1.01 0.02
SW: wearing “Stretch Walker”, MBT: wearing “MBT”, MCS: wearing “More Conventionally soled Shoes with a typical toe-spring”, BF: “Bare Foot”, ratio: ratios of the other shoe conditions to “BF”, ES: effect size. * p < 0.05/6.
Nigg et al. [1] and Landry et al. [7] reported that gait significantly changes when wearing the MBT compared to when wearing walking shoes currently on the market and when special stimulation is provided to leg muscles and leg joints. Demura et al. [5] reported that in free walking while wearing the SW, because subjects could walk fast regardless of small changes in the range of motion of hip and knee joints, it is possible to walk efficiently. In short, wearing the MBT or SW affects the gait. In addition, because the MBT and SW have curved rocker bottom soles, unlike BF or MCS, it was hypothesized that unstable posture is further induced during static standing while wearing them; in short, they greatly affect body sway during standing. On an unstable stool, static standing is unstable [14] and body sway becomes larger. Wearing the MBT or SW with rounded soles is similar to standing on an unstable stool. Nigg et al. [1] and Landry et al. [7] reported that wearing the MBT compared with wearing commonly marketed walking shoes resulted in a larger body sway. The results of the present study are in agreement with the cited previous studies, while standing with the MBT or SW, sway velocity front–back (Y) direction was faster than while standing with MCS or BF (ES = 1.2, 2.0). Previous studies have reported increased muscle activity of tibialis anterior muscles [1] and medial gastrocnemius [15] in the standing position while wearing the MBT. In addition, Landry et al. [7] reported increased muscle activity in the standing position while wearing the MBT shoes compared with a stable
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
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control shoe, which persisted even after a 6-week accommodation period. Hence, wearing MBT or SW affects not only walking but also body sway in the front–back direction during static standing. Mean velocity in the left–right direction (ES = 1.6–2.2), mean velocity in the front–back direction (ES = 3.1–3.8), and sway area (ES = 1.7–2.2) were higher when wearing the MBT as compared with the SW. Nakayama [16] reported that while standing on a slope as compared with level ground, muscle activity of the rectus femoris, vastus medialis, and vastus lateralis muscles and the lateral head of the gastrocnemius muscle increases. Furthermore, the muscle activity’s tendency becomes stronger with increasing slope angle. He discussed that an increase of muscle activity depends on holding the standing position on the slope. MBT has a larger round angle and a smaller contact area than the SW (Figs. 1 and 2). Therefore, the posture assumed while wearing the MBT was more unstable and the body sway was larger than when wearing the SW. From the abovementioned discussion, when wearing shoes with curved rocker bottom soles, compared to barefeet or when wearing general shoes, static standing becomes unstable, particularly in the front–back direction, and the effect is greater in the MBT with steeper arc soles than the SW. In addition, because standing posture becomes unstable while wearing the SW or MBT, it is considered that muscle activity increases to maintain a stable posture. From the present results, a certain training effect on leg muscle groups may be expected by wearing either type of shoes. Originally, the MBT was designed based on a principle similar to that of the training device, i.e., the enhancement of leg strength and prevention of its deterioration. Although the sole of the SW is similar to that of the MBT, it has a relatively small round angle. Hence, it is highly likely that the MBT stimulates leg muscles more strongly even during static standing and has a larger training effect than the SW. However, the MBT increases the risk of falls in the elderly due to a higher level of posture instability caused by inferior leg strength. Therefore, from the safety perspective, the SW may be preferable to the MBT for the elderly. From the results of this study, the following point is clear: although static standing posture becomes unstable when wearing MBT or SW with rounded soles, the degree and content of instability differ. This may be useful knowledge for people with poor leg strength wearing these shoes. It may be noted that the results of this study broadly reflected the intention of the manufacturer that developed the SW. In the future, it will be necessary to examine electromyographs during standing and the training effect after wearing the MBT or
the SW for a certain period of time using a variety of subjects to clarify their training methods and quantify their effect. 5. Conclusion In conclusion, static standing posture becomes more unstable when wearing the MBT or SW with curved rocker bottom soles than when wearing more conventionally soled shoes with a typical toespring or when barefoot. In addition, the MBT has lower stability in the front–back direction than the SW. References [1] Nigg B, Hintzen S, Ferber R. Effect of an unstable shoe construction on lower extremity gait characteristics. Clin Biomech 2006;21(1):82–8. [2] Waddington G, Seward H, Wrigley T, Lacey N, Adams R. Comparing wobble board and jump-landing trainig effects on knee and ankle movement discrimination. J Sci Med Sport 2000;3(4):449–59. [3] Waddington G, Adams R. The effect of a 5-week Wobble-Board exercise intervention on ability to discriminate different degrees of ankle inversion, barefoot and wearing shoes: a study in healthy elderly. J Am Geriatr 2004;52(4):573–6. [4] Romkes J, Rudmann C, Brunner R. Changes in gait and EMG when walking with the Masai Barefoot Technique. Clin Biomech 2006;21(1):75–81. [5] Demura T, Demura S, Yamaji S, Yamada T, Kitabayashi T. Gait characteristics when walking with rounded soft sole shoes. Foot 2011;22:18–23. [6] Federolf PRL, Nigg B. The effect of footwear on postural control in bipedal quiet stance. Footwear Sci 2012;4:115–22. [7] Landry SC, Nigg BM, Tecante KE. Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait Posture 2010;32(2):215–9. [8] Nigg BFP, von Tscharner V, Nigg S. Unstable shoes: functional concepts and scientific evidence. Footwear Sci 2012;4:73–82. [9] Lord SR, Bashford GM, Howland A, Munroe BJ. Effects of shoe collar height and sole hardness on balance in older women. J Am Geriatr 1999;47(6):681–4. [10] Wilson ML, Rome K, Hodgson D, Bell P. Effect of textured foot orthotics on static and dynamic postural stability in middle-aged females. Gait Posture 2008;27(1):36–42. [11] Brenton-Rule A, Bassett S, Waish A, Rome K. The evaluation of walking footwear on postural stability in healthy older adults: an exploratory study. Clin Biomech 2011;26(8):885–7. [12] Yamamoto M, Yoshida T. Body tracking test (BTT)·Galvanic body sway test (GBST). Equilib Res 2011;70(3):135–44. [13] Demura S, Yamaji S, Noda M, Kitabayashi T, Nagasawa Y. Examination of parameters evaluation the center of foot pressure in static standing posture from the viewpoints of trial-to-trial reliability and interrelationships among parameters. Equilib Res 2001;60(1):44–55. [14] Ogaya S, Ikezoe T, Tsuboyama T, Ichihashi N. Postural control on a wobble board and stable surface of young and elderly people. Rigakuryoho Kagaku 2009;24(1):81–5. [15] Sousa A, Tavares J, Macedo R, Rodrigues A, Santos R. Influence of wearing an unstable shoe on thigh and leg muscle activity and venous response in upright standing. Appl Ergon 2012;43(5):933–9. [16] Nakayama Y. Influence of postural strategy on standing on a slope: electromyographic analysis. Rigakuryoho Kagaku 2009;24(6):817–20.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
ARTICLE IN PRESS
YFOOT-1366; No. of Pages 4
The Foot xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
The Foot journal homepage: www.elsevier.com/locate/foot
Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing Tomohiro Demura a,∗ , Shin-ichi Demura b , Masanobu Uchiyama c , Tamotsu Kitabayashi d , Kenji Takahashi e a
Jin-ai Women’s College, Fukui, Fukui, Japan Kanazawa University, Kanazawa, Ishikawa, Japan c Akita Prefectural University, Shimoshinjo-Nakano, Akita, Japan d Tokyo University of Science, Shinjyuku-ku, Tokyo, Japan e Teikyo Heisei University, Ichihara, Chiba, Japan b
a r t i c l e
i n f o
Article history: Received 5 April 2014 Received in revised form 16 February 2015 Accepted 19 February 2015 Keywords: Shoes Static standing Center of pressure
a b s t r a c t Shoes with curved rocker bottom soles may induce an unstable standing posture. This study was aimed to mainly examine the effect of such shoes on the center of pressure (COP) during static standing. Ten healthy young male adults had their COP measured during static standing with four types of shoe conditions (Stretch Walker® : SW (shoes with curved rocker bottom soles), Masai Barefoot Technology®: MBT (similar to SW in form and material), more conventionally soled shoes with a typical toe-spring: MCS, and bare feet: BF) for 60 s. The mean path length and mean velocity of Y (front-back) axis were significantly greater when wearing the MBT than when wearing the SW, and when wearing the SW than when BF or when wearing the MCS. In addition, mean velocity of X (left-right) axis, area surrounding root mean square, root mean square, and root mean square of Y-axis were significantly greater when wearing the MBT than when wearing the SW, MCS, or when BF. In conclusion, when wearing the MBT or SW with rounded sole, static standing posture becomes unstable because of their characteristics as compared with wearing MCS or when BF, but the MBT has a larger sway in the front-back direction than the SW. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction According to Nigg et al. [1], posture stability is very important for walking and it is necessary to wear appropriate shoes that support gait and enforce leg strength for enhanced walking stability. Shoes used for walking or exercise are generally designed to aim at stability of posture and gait. However, it is feared that long-period use of such shoes induces a decrease in leg strength [2,3]. Recently, training devices which aimed at enhancement or decrease-prevention of leg strength have been developed. A wobble board is used as a representative device. It has been reported to improve proprioception of the ankles and knees in addition to increasing leg strength [2,3]. Shoes with rounded and soft soles made with special materials, called Masai Barefoot Technology (MBT), are based on a similar idea. It was reported that wearing these shoes significantly changes gait [1] and provides special stimulation to the leg muscles and leg joints [4]. In addition,
∗ Corresponding author. Tel.: +81 776 56 1133. E-mail address: [email protected] (T. Demura).
it was reported that when wearing these shoes, the sway of center of pressure (COP) during static standing increases in the anterior–posterior and lateral directions [1]. Furthermore, in Japan, “Stretch Walker” (SW) shoes with a curved rocker bottom sole and a soft cushion material in the heel portion of the outsole and forepart have been developed and are currently available on the market. Demura et al. [5] reported that wearing these shoes change gait properties. The SW has characteristics similar to the MBT but has greater stability because the heel portions are somewhat flat (Figs. 1 and 2). Hence, it was hypothesized that the COP differs when standing while wearing SW and MBT. It has been reported that static standing posture becomes more unstable because of a curved rocker bottom sole and a soft heel pad when wearing unstable shoes as represented by MBT [1,6–8]. In addition, Lord et al. [9] reported that COP during static standing differed with different types of shoes; however, Wilson et al. [10] and Brenton-Rule et al. [11] reported no significant difference. In short, contradictory results have been reported. Wearing shoes with very special soles such as MBT affects COP during static standing; however, few studies have examined how the degree of round
http://dx.doi.org/10.1016/j.foot.2015.02.004 0958-2592/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
G Model YFOOT-1366; No. of Pages 4
ARTICLE IN PRESS T. Demura et al. / The Foot xxx (2015) xxx–xxx
2
Fig. 1. Stretch Walker (Nosaka, Japan). Fig. 3. More Conventionally soled Shoes with a typical toe-spring (MCS).
2.2. Materials 2.2.1. Shoe conditions In this study, three types of shoes were used to examine the effect of rounded soles in the anterior–posterior direction on gait (Figs. 1–3). Shoe with sizes from 24 to 29 cm (US size: 6–11) at intervals of approximately 0.5 cm were prepared, and the subjects selected preferred shoe size after trying on the shoes.
Fig. 2. Masai Barefoot Technology (MBT, Switzerland).
sole angle affects COP during static standing. Hence, the above, in short, the shoes with the different sole round angle would be necessary to examine. The degree of the round angle of the sole is related to the stability of standing posture and the muscle activity of the legs while walking. In addition, it is assumed that the effect of the abovestated special soles differs between elderly and young individuals with different leg strengths. Shoes with curved rocker bottom soles have been developed for walking. However, a large body sway, even during static standing in elderly individuals with poor leg strength or leg muscle fatigue after walking, entails a risk of falling when wearing the shoes. The results presented here will be useful for the optimal selection of shoes according to individual physical fitness levels. The results will also aid shoe selection for young individuals with the goals of strength training via gait for a short time, or of keeping a comfortable gait for a long time. This study is aimed to compare the COP during static standing with BF (bare feet) and while wearing SW, MBT, and MCS (more conventionally soled shoes with a typical toe-spring).
2. Materials and methods
2.2.1.1. Stretch Walker (SW; Nosaka Ltd., Japan). The “Stretch Walker® ” (SW) shoes manufactured by Nosaka, Ltd. were used for this study as the shoes with curved rocker bottom soles (Fig. 1). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 3.8 cm anteriorly and 3.0 cm posteriorly (Fig. 1). 2.2.1.2. Masai Barefoot Technology (MBT; Masai Marketing & Trading AG, Switzerland). The “Masai Barefoot Technology® ” (MBT) shoes in which the soles have an ungrounded area while standing upright, with surface area wider than SW, were used for this study as another pair of curved rocker bottom soles (Fig. 2). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 2.9 cm anteriorly and 4.3 cm posteriorly (Fig. 2). 2.2.1.3. More conventionally soled shoes with a typical toe-spring (MCS). We used more conventionally soled shoes with a typical toe-spring as the control (Fig. 3). In the case of a 26 cm (US size: 8) size shoe, the height from the ground was 2.3 cm anteriorly and 0.2 cm posteriorly (Fig. 3). 2.2.1.4. Barefeet (BF). Posture stability of subjects with BF was measured for the control condition. 2.2.2. Measurement of COP The measurement instrument was a stabilometer G5500 (Anima, Japan). This instrument can calculate the COP of vertical loads from the values of three vertical load sensors, which are located at the corners of an isosceles triangle on a level surface. The data sampling frequency was 20 Hz.
2.1. Subjects 2.3. Testing protocol Ten healthy young male adults without extremity disorders and with regular exercise habits participated in this study (age: 23.9 ± 3.6 years, height: 171.8 ± 4.1 cm, body mass: 67.6 ± 4.9 kg). Before the experiment, the purpose and procedure of this study were explained to all subjects in detail and an informed consent was obtained. However, subjects were not informed about the characteristics of the shoes. In addition, this study was approved by the Ethics Committee on Human Experimentation of the Faculty of Human Science, Kanazawa University (Ref. No. 2012-08).
The measurement procedure followed a method prescribed in the standardized stabilometry test [12]. Subjects maintained a static upright posture with feet together (Romberg posture) for 1 min, and then they were instructed to watch an achromatic target placed at eye level. The measurements began after the subjects’ posture and eyes were stable. The subjects performed each condition three times with one minute rest in a seated position after each trial; the trial order was set randomly.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
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ARTICLE IN PRESS
YFOOT-1366; No. of Pages 4
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Table 1 Basic statistics of COP parameters in each shoe condition and results of one-way ANOVA.
Mean path length (cm/s) Mean velocity of X-axes (cm/s) Mean velocity of Y-axes (cm/s) Area surrounding maximal amplitude rectangular (cm2 ) Root mean square (cm2 ) Root mean square of X-axis (cm) Root mean square of Y-axis (cm)
ANOVA
Shoes
n = 10 Parameters
Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD
SW
MBT
MCS
BF
F-value
p-Value
partial 2
Post-hoc Bonferroni
1.13 0.12 0.67 0.11 0.62 0.06 7.59 2.54 0.74 0.15 0.46 0.10 0.57 0.13
1.73 0.24 0.87 0.12 1.11 0.20 13.21 3.78 1.02 0.24 0.50 0.10 0.88 0.26
0.99 0.10 0.59 0.09 0.55 0.05 5.86 2.05 0.67 0.15 0.42 0.10 0.51 0.14
0.97 0.10 0.60 0.10 0.50 0.05 5.72 2.69 0.67 0.16 0.43 0.12 0.50 0.13
58.27
0.00
0.87
MBT > SW > MCS, BF
32.49
0.00
0.78
MBT > SW, MCS, BF
57.96
0.00
0.87
MBT > SW > MCS, BF
28.25
0.00
0.76
MBT > SW, MCS, BF
23.29
0.00
0.72
MBT > SW, MCS, BF
2.60
0.73
0.22
17.58
0.00
0.66
MBT > SW, MCS, BF
SW: wearing “Stretch Walker”, MBT: wearing “MBT”, MCS: wearing “More Conventionally soled Shoes with a typical toe-spring”, BF: “Bare Foot”.
2.4. Parameters Mean path length, mean velocity of X (left-right) and Y (frontback) axes, area surrounding maximal amplitude rectangular, root mean square, and root mean square of X (left-right) and Y (frontback) axes were used as evaluation parameters. Demura et al. [13] reported that these parameters have high reliability and can evaluate sway characteristics.
Table 2 Ratios of the other shoe conditions to BF condition and their effect size.
Mean path length (cm/s) Mean velocity of X-axes (cm/s) Mean velocity of Y-axes (cm/s)
2.5. Data analysis Mean differences among three conditions for parameters were tested using repeated one-way analysis of variance (ANOVA). When a result of ANOVA showed statistical significance, the Bonferroni method was selected for multiple comparisons. The statistical SPSS package ver. 11.0 (SPSS Japan) was used for data analysis. Effect size (ES) was calculated to examine the size of the mean difference. An ES of 0.2 or below and of 0.8 or above are generally interpreted as a small difference and a large difference, respectively. A probability level of 0.05 was indicative of statistical significance. 3. Results Table 1 shows the values of COP parameters in each condition and the results of one-way ANOVA. Mean path length and mean velocity of Y-axis showed significantly greater values in the MBT than in the SW, MCS, and BF, and in the SW than in the MCS and BF. In addition, mean velocity of X-axis, area surrounding maximal amplitude rectangular, root mean square, and root mean square of Y (front–back)-axis showed significantly greater values in the MBT than in the SW, MCS, and BF. Table 2 shows the ratio of the SW, MBT, MCS, and BF (SW/BF, MBT/BF, and MCS/BF) and effect size (ES). A significant difference was observed in the parameters except in the X direction for MBT and in the mean path length and mean velocity of Y-axis for SW, but not in all parameters for MCS. The effect size in parameters that showed a significant difference was very large (ES = 1.40–3.86), particularly in differences with MBT. 4. Discussion This study aimed to compare and examine the COP during static standing while wearing SW, MBT, and MCS, and when BF. MBT and SW with curved rocker bottom soles and a soft cushion material in the outsole have features different from common shoes.
Shoes
n = 10 to BF
Area surrounding maximal amplitude rectangular (cm2 ) Root mean square (cm2 ) Root mean square of X-axis (cm) Root mean square of Y-axis (cm)
Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES Ratio ES
SW
MBT
MCS
1.17 1.40* 1.12 0.65 1.22 2.00* 1.33 0.68 1.11 0.44 1.09 0.34 1.13 0.46
1.78 3.86* 1.43 2.24* 2.20 3.83* 2.31 2.16* 1.54 1.67* 1.18 0.66 1.74 1.71*
1.02 0.19 0.97 0.18 1.08 0.75 1.02 0.06 1.01 0.04 0.99 0.02 1.01 0.02
SW: wearing “Stretch Walker”, MBT: wearing “MBT”, MCS: wearing “More Conventionally soled Shoes with a typical toe-spring”, BF: “Bare Foot”, ratio: ratios of the other shoe conditions to “BF”, ES: effect size. * p < 0.05/6.
Nigg et al. [1] and Landry et al. [7] reported that gait significantly changes when wearing the MBT compared to when wearing walking shoes currently on the market and when special stimulation is provided to leg muscles and leg joints. Demura et al. [5] reported that in free walking while wearing the SW, because subjects could walk fast regardless of small changes in the range of motion of hip and knee joints, it is possible to walk efficiently. In short, wearing the MBT or SW affects the gait. In addition, because the MBT and SW have curved rocker bottom soles, unlike BF or MCS, it was hypothesized that unstable posture is further induced during static standing while wearing them; in short, they greatly affect body sway during standing. On an unstable stool, static standing is unstable [14] and body sway becomes larger. Wearing the MBT or SW with rounded soles is similar to standing on an unstable stool. Nigg et al. [1] and Landry et al. [7] reported that wearing the MBT compared with wearing commonly marketed walking shoes resulted in a larger body sway. The results of the present study are in agreement with the cited previous studies, while standing with the MBT or SW, sway velocity front–back (Y) direction was faster than while standing with MCS or BF (ES = 1.2, 2.0). Previous studies have reported increased muscle activity of tibialis anterior muscles [1] and medial gastrocnemius [15] in the standing position while wearing the MBT. In addition, Landry et al. [7] reported increased muscle activity in the standing position while wearing the MBT shoes compared with a stable
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
G Model YFOOT-1366; No. of Pages 4 4
ARTICLE IN PRESS T. Demura et al. / The Foot xxx (2015) xxx–xxx
control shoe, which persisted even after a 6-week accommodation period. Hence, wearing MBT or SW affects not only walking but also body sway in the front–back direction during static standing. Mean velocity in the left–right direction (ES = 1.6–2.2), mean velocity in the front–back direction (ES = 3.1–3.8), and sway area (ES = 1.7–2.2) were higher when wearing the MBT as compared with the SW. Nakayama [16] reported that while standing on a slope as compared with level ground, muscle activity of the rectus femoris, vastus medialis, and vastus lateralis muscles and the lateral head of the gastrocnemius muscle increases. Furthermore, the muscle activity’s tendency becomes stronger with increasing slope angle. He discussed that an increase of muscle activity depends on holding the standing position on the slope. MBT has a larger round angle and a smaller contact area than the SW (Figs. 1 and 2). Therefore, the posture assumed while wearing the MBT was more unstable and the body sway was larger than when wearing the SW. From the abovementioned discussion, when wearing shoes with curved rocker bottom soles, compared to barefeet or when wearing general shoes, static standing becomes unstable, particularly in the front–back direction, and the effect is greater in the MBT with steeper arc soles than the SW. In addition, because standing posture becomes unstable while wearing the SW or MBT, it is considered that muscle activity increases to maintain a stable posture. From the present results, a certain training effect on leg muscle groups may be expected by wearing either type of shoes. Originally, the MBT was designed based on a principle similar to that of the training device, i.e., the enhancement of leg strength and prevention of its deterioration. Although the sole of the SW is similar to that of the MBT, it has a relatively small round angle. Hence, it is highly likely that the MBT stimulates leg muscles more strongly even during static standing and has a larger training effect than the SW. However, the MBT increases the risk of falls in the elderly due to a higher level of posture instability caused by inferior leg strength. Therefore, from the safety perspective, the SW may be preferable to the MBT for the elderly. From the results of this study, the following point is clear: although static standing posture becomes unstable when wearing MBT or SW with rounded soles, the degree and content of instability differ. This may be useful knowledge for people with poor leg strength wearing these shoes. It may be noted that the results of this study broadly reflected the intention of the manufacturer that developed the SW. In the future, it will be necessary to examine electromyographs during standing and the training effect after wearing the MBT or
the SW for a certain period of time using a variety of subjects to clarify their training methods and quantify their effect. 5. Conclusion In conclusion, static standing posture becomes more unstable when wearing the MBT or SW with curved rocker bottom soles than when wearing more conventionally soled shoes with a typical toespring or when barefoot. In addition, the MBT has lower stability in the front–back direction than the SW. References [1] Nigg B, Hintzen S, Ferber R. Effect of an unstable shoe construction on lower extremity gait characteristics. Clin Biomech 2006;21(1):82–8. [2] Waddington G, Seward H, Wrigley T, Lacey N, Adams R. Comparing wobble board and jump-landing trainig effects on knee and ankle movement discrimination. J Sci Med Sport 2000;3(4):449–59. [3] Waddington G, Adams R. The effect of a 5-week Wobble-Board exercise intervention on ability to discriminate different degrees of ankle inversion, barefoot and wearing shoes: a study in healthy elderly. J Am Geriatr 2004;52(4):573–6. [4] Romkes J, Rudmann C, Brunner R. Changes in gait and EMG when walking with the Masai Barefoot Technique. Clin Biomech 2006;21(1):75–81. [5] Demura T, Demura S, Yamaji S, Yamada T, Kitabayashi T. Gait characteristics when walking with rounded soft sole shoes. Foot 2011;22:18–23. [6] Federolf PRL, Nigg B. The effect of footwear on postural control in bipedal quiet stance. Footwear Sci 2012;4:115–22. [7] Landry SC, Nigg BM, Tecante KE. Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait Posture 2010;32(2):215–9. [8] Nigg BFP, von Tscharner V, Nigg S. Unstable shoes: functional concepts and scientific evidence. Footwear Sci 2012;4:73–82. [9] Lord SR, Bashford GM, Howland A, Munroe BJ. Effects of shoe collar height and sole hardness on balance in older women. J Am Geriatr 1999;47(6):681–4. [10] Wilson ML, Rome K, Hodgson D, Bell P. Effect of textured foot orthotics on static and dynamic postural stability in middle-aged females. Gait Posture 2008;27(1):36–42. [11] Brenton-Rule A, Bassett S, Waish A, Rome K. The evaluation of walking footwear on postural stability in healthy older adults: an exploratory study. Clin Biomech 2011;26(8):885–7. [12] Yamamoto M, Yoshida T. Body tracking test (BTT)·Galvanic body sway test (GBST). Equilib Res 2011;70(3):135–44. [13] Demura S, Yamaji S, Noda M, Kitabayashi T, Nagasawa Y. Examination of parameters evaluation the center of foot pressure in static standing posture from the viewpoints of trial-to-trial reliability and interrelationships among parameters. Equilib Res 2001;60(1):44–55. [14] Ogaya S, Ikezoe T, Tsuboyama T, Ichihashi N. Postural control on a wobble board and stable surface of young and elderly people. Rigakuryoho Kagaku 2009;24(1):81–5. [15] Sousa A, Tavares J, Macedo R, Rodrigues A, Santos R. Influence of wearing an unstable shoe on thigh and leg muscle activity and venous response in upright standing. Appl Ergon 2012;43(5):933–9. [16] Nakayama Y. Influence of postural strategy on standing on a slope: electromyographic analysis. Rigakuryoho Kagaku 2009;24(6):817–20.
Please cite this article in press as: Demura T, et al. Effect of shoes with rounded soft soles in the anterior–posterior direction on the center of pressure during static standing. Foot (2015), http://dx.doi.org/10.1016/j.foot.2015.02.004
