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. 2020 Jun 3;15(6):e0234140. doi: 10.1371/journal.pone.0234140

The effect of support surface and footwear condition on postural sway and lower limb muscle action of the older women

Meizhen Huang 1,2, Kit-lun Yick 3,*, Sun-pui Ng 4, Joanne Yip 3, Roy Tsz-hei Cheung 1,5
Editor: Pilar Serra-Añó6
PMCID: PMC7269262  PMID: 32492058

Abstract

Background

Diminished somatosensory function is a critical age-related change which is related to postural instability in the older population. Footwear is a cost-effective way to modulate the postural stability by altering sensorimotor inputs via mechanoreceptors on the plantar surface of the feet. Compared to insoles with indentions in the entire surface, we innovatively developed a textured insole with site-specific nodulous protrudous. This study thus aimed to investigate the immediate effect of the nodulous insole and supporting surface condition on static postural stability and lower limb muscle activation for healthy older women.

Methods

This is a single-session study with repeated measurements. Twenty-three healthy older women stood on the firm (i.e., concrete floor) and foam surfaces with their eyes open in the three footwear conditions, namely barefoot, plain shoes and shoes with an innovative textured insole, for 30 seconds. Static postural sway and muscle activation of biceps femoris (BF), vastus lateralis (VL), tibialis anterior (TA), and lateral gastrocnemius (LG) of the dominant leg were measured during each testing condition.

Results

Compared to a firm surface, standing on the foam could significantly increase the body sway and lower limb muscle activation (p<0.05). When standing on the foam, compared to barefoot, wearing footwear significantly decreased the VL and TA muscle activation and minimize the postural sway in medial-lateral and anterior-posterior direction, while the influence is larger for the shoes with nodulous insloe compared to the plain shoes. No significant differences between the footwear conditions for static stability and muscle activation were observed on firm surface condition.

Conclusions

For older women, footwear could improve the postural stability in the unstable surface, particularly the footwear with nodulous insole, with the underlying mechanism as enhancing the mechanoreceptors on the plantar surface of the feet.

Background

Balance is a major concern for older people [1–4]. The decline in postural stability in older adults increases the risk of falls that ultimately elevates the morbidity, mortality, and cost of health care services [1–4]. To maintain the normal static bipedal stance, individuals primarily depend on the proprioceptive and cutaneous input [5, 6]. However, diminished somatosensory function including lower plantar cutaneous sensitivity is a critical age-related change, which may increase the body sway for the older population and increase the fall risk [5–11]. And the fall risk and fall-related injury are higher in women than men of the similar age [1–3]. For community-dwelling women, a majority of fall happen at home [4] due the environmental factors such the uneven floor [2–4]. Indeed, unreliable sensory stimulus from the unstable surface would further deteriorate the postural control [6, 9, 12, 13]. While the unstable surfaces are commonly encountered in various daily life settings including thick carpets, it is imperative to develop practical strategy to enhance postural stability for the older adults.

Footwear is a cost-effective way to modulate the postural stability by augmenting plantar cutaneous sensations [14–17]. A simple approach is to facilitate the sensation is by applying indentations on the footwear insole to stimulate the mechanoreceptors [14–16]. For instance, rigid textured insole with dimpled and grid surfaces was reported to reduce the postural sway for the older people when compared to the barefoot condition [18–20] or shoes without textured insole [20]. Also, hard insoles can effectively enhance the postural stability when visual input was absent for the older adults [21] and insoles with soft texture was unanimously suggested to be detrimental for postural control for the older people [14]. However, as reported by previous research using the rigid insole with indentions on entire surface, “most participants anecdotally reported that the harder insoles were uncomfortable to stand on for an extended period of time” [18]. Indeed, it has been recognized that flexible and relatively soft soles are preferable to hard soled shoes [22]. Thus, a dilemma exists that hard insole with indentions could enhance postural control but provide discomfort for the users, which may limit its application.

To address the problem, we have developed an innovative textured insole by applying silicone nodules in the metatarsal, heel, and the foot arch area. The underlying mechanism including (1) mechanoreceptors mainly locate in the metatarsal-tarsal and heel region of foot sole [23]; (2) investigation regarding electrical site-specific stimulation to these areas has implied consequential improvement of balance [24]; (3) the raised nodules create a boundary of the plantar surface from the metatarsal heads to the heel that has been suggested to facilitate postural stabilizing reactions evoked by unpredictable postural perturbation [25]. We proposed that this design would largely maintain users’ comfort while facilities the plantar somatosensory input, particularly in the unstable surface which is common but risk for the older women.

This study, therefore, aimed to investigate the immediate effect of the novel nodulous textured insole and supporting surface condition on static postural stability and lower limb muscle activation for older women. We hypothesized that compared to plain shoes and barefoot, novel nodulous textured insole could significantly enhance the postural stability and minimize the muscle lower limb muscle activation, particularly in the foam surface, for older women.

Methods

This study was approved by the Human Subjects Ethics Sub-committee of The Hong Kong Polytechnic University (Reference Number: HSEARS20150806001). All the participants provided written informed consent before data collection.

Participants

Independent community-dwelling older women were recruited through convenient sampling. The inclusion criteria were: (1) aged more than 60; (2) able to follow simple verbal instructions, (3) no history of foot injuries during the past two years. The exclusion criteria included: (1) having any neurological conditions and musculoskeletal problems that might affect balance; (2) having contract or glaucoma; (3) people with impaired tactile foot sensitivity. The study was in accordance with the latest revision of the Declaration of Helsinki [26].

Study procedures

This was a single-session study with repeated measurements. Participants were recruited from February to March in 2016. We first interviewed participants to obtain their demographic information, following by foot sensation test and static balance test in different footwear conditions. The dominant limb was determined by a ball kicking task [27].

Foot sensation test

Tactile foot sensitivity was evaluated using Semmes Weinstein Monofilament test (North Coast Medical Inc, California, United States) which was widely used valid tactile sensory [28]. Monofilaments, starting with 1.65 mm, were randomly applied to test regions including first, second, third toes, first, third, and fifth metatarsal heads, and medial and lateral arches of the midfoot. Participants were asked to close eyes and indicate whether stimulations were perceived. A maximum of three stimulations per monofilament was applied on each region. The inability to detect 5.07 mm/10 g monofilament at first toes, first and third metatarsal heads was deemed as impaired tactile foot sensitivity [29].

Testing conditions

Each participant then underwent three different footwear conditions, namely barefoot, plain shoes made of soft terry textiles (13F302, Yeshunag, Zhejiang, China) (Fig 1a), and nodulous insole shoes with medial arch support and silicone protrusions at the metatarsal heads and lateral heel (KE-1300T, Shin-Etsu Chemical Co., Ltd.) (Fig 1b), in a randomized sequence. The barefoot condition was simulated by placing pressure-sensing insoles onto the bottom of the feet, and then secured by standard cotton socks that are 1.4 mm in thickness.

Fig 1. Test shoe models.

Fig 1

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(a) The plain shoes are open-toe design with secured heel counter and an adjustable dorsal forefoot strap. The sole thickness is uniform across the plantar; (b) The nodulous shoes is the same plain shoes with raised nodulous and medial arch support.

Participants were instructed to maintain quiet standing on firm (i.e., concrete floor) and foam surfaces (46.5 cm (L) × 46.5 cm (W) × 4.5 cm (H); 0.0764 g/cm3, StimUp® Balance Pad, Will Medical, Tokyo, Japan) with their eyes open in the three footwear conditions. Thus, there were six experimental conditions which were carried out in a randomized order. The standing posture was standardized according to the method used in a previous study [30]. In brief, all the participants’ feet were 17 cm apart from the heel center, with the foot progression angle at 14°. They were asked to stand still with their hands by the sides and looking at a stationary visual target (i.e., a red spot of 2 cm in diameter) placed at eye level and 3-m in front of the participants and put equal body weight on each foot. In each condition, participants were tested thrice for 30 seconds and a rest period of one minute was allowed between each condition [30]. Data during the middle 20 seconds were used for analyses.

Measurement of static postural stability

Static postural sway was measured as the displacement of center of pressure (COP) using in-shoe pressure measurement system (Pedar®, Novel GmbH, Munich, Germany) with sampling frequency at 50 Hz under the dominant foot. The Pedar® insole sensor, which was 2 mm in thickness and fitted in accordance with foot size, was placed underneath the foot. Plantar pressure by Pedar® insole sensor had been reported with good validity and reliability [31, 32] and provided a valid approximation of COP [33, 34]. Referring to previous studies [18, 30], the COP parameters included the range of anterior-posterior (AP) and medial–lateral (ML) COP displacement, path length (PL) and the 95% confidence elliptical area (C95 area).

Measurement of electromyography

The surface EMG signal was captured using an 8-channel wireless EMG system (Clinical DTS, Noraxon USA Inc., AZ, USA) with 16-bit resolution and common-mode rejection ratio > 100 dB. After proper skin preparation, four circular Ag/AgCl bipolar electrodes (electrode diameter: 10 mm, inter-electrode distance: 22 mm) were placed on the biceps femoris (BF), vastus lateralis (VL), tibialis anterior (TA), and lateral gastrocnemius (LG) of the dominant leg according to the recommendations of Surface EMG for Non-Invasive Assessment of Muscles [35]. The raw signals were pre-amplified 1,000 times and sampled at 3,000 Hz with 500 Hz low-pass filter. Prior to postural stability test, maximum voluntary contractions (MVC) for each muscle was collected for 5 s using manual resistance [36] and repeated three times with five minutes rest period in-between each MVC test (S1 Appendix).

Data extraction and processing

The AP and ML COP displacement, COP path length and the 95% confidence elliptical area (C95 area) parameters were calculated by the inbuilt pressure measurement system (Pedar®/ expert Software, Novel GmbH, Munich, Germany).

The EMG signals were processed using MyoResearch 3 (Noraxon USA Inc, AZ, USA). All EMG data were filtered using a first-order high-pass Butterworth filter at 10 Hz with 8–10% cutoff. The data were then rectified, and the root-mean-square (EMGrms) was calculated in 100 ms windows during the 20-second time-window of the static standing trials. For each muscle, the highest EMGrms portion of 100 ms duration from the three MVC trails for each muscle was extracted and averaged, which was then used for normalization of the EMGrms value in each testing condition (i.e., EMGmvc%) [37].

Statistical analysis

Statistical analyses were conducted using SPSS (version 22, IBM, Armonk, NY). We used Shapiro-Wilk test and probability-probability plot to check the normality of dependent variables. Only EMGmvc% was positively skewed and log transformation was thus applied to the data to obtain normally distributed responses [38].

Two-way repeated-measures ANOVA (within-subject factors: three footwear conditions, two supporting surface conditions) was applied to each static stability and EMG variables. Greenhouse-Geisser epsilon adjustment was used when the sphericity assumption was violated. Post-hoc analysis using paired t-test with Bonferroni adjustment was performed if any overall significant results were identified. The effect size was expressed as partial eta squared (ηp2) [39]. A significance level of p ≤0.05 was set for two-way repeated ANOVA models.

Results

Twenty-three older women (mean age: 65.1±3.3 years) completed all the tests. The demographics are summarized in Table 1. Participants exhibited tactile foot sensitivity level within the normal threshold [29].

Table 1. Demographic data of the participants.

Demographic data Range Mean ± SD
Age (years) 60.0–73.0 65.1 ± 3.3
Tactile foot sensitivity (mm) 3.61–4.56 3.95±0.36
Body height (m) 1.42–1.66 1.54 ±0.55
Body mass (kg) 38.0–65.0 51.7±6.3
BMI (kg/m2) 15.2–32.2 22.2±3.4
Foot size (Euro) 35–40 37.4±1.1
Dominant leg Right (n = 23); Left (n = 0)
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Significant footwear condition × supporting surface condition was observed for VL EMGmvc% (p<0.001, ηp2 = 0.308), TA EMGmvc% (p = 0.002, ηp2 = 0.250), AP displacement (p = 0.002, ηp2 = 0.249), COP path length (p = 0.005, ηp2 = 0.211), COP velocity (p = 0.004, ηp2 = 0.251), 95% confidence elliptical area (p = 0.013, ηp2 = 0.179), which indicates significant interactions.

Effect of supporting surface condition

Significant effects of supporting surface were observed for all the EMG variables and postural stability (p<0.001, ηp2 = 0.150–0.782) except total path length and COP velocity. Significant larger postural sway parameters and EMGmvc% was observed when standing on the foam.

Effect of footwear conditions

A significant effect of the footwear was observed in foam but not the firm surface for VL EMGmvc% (p<0.001, ηp2 = 0.723), TA EMGmvc% (p = 0.047, ηp2 = 0.101), AP displacement (p = 0.03, ηp2 = 0.267), ML displacement (p = 0.03, ηp2 = 0.271), C95 area (p = 0.002, ηp2 = 0.254). Post-hoc analysis showed that the smallest value was observed with nodulous shoes, and highest value was in barefoot (p<0.05) (Figs 2 & 3).

Fig 2. Lower limb muscle activity.

Fig 2

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Nod shoes: nodulous shoes. * Significant difference of pair-t test, p<0.05 with Bonferroni adjustment.

Fig 3. Postural sway.

Fig 3

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Nod shoes: nodulous shoes. * Significant difference of pair-t test, p<0.05 with Bonferroni adjustment.

Discussion

Our finding demonstrated the nodulous textured insole would enhance the postural in the foam condition for the older women. Specifically, (1) when compared to firm surface, standing on the foam could significantly increase body sway and lower limb muscle activation, regardless of the footwear conditions; (2) when standing on the foam, comparing to barefoot, wearing footwear could immediately decrease the lower limb muscle activation and minimize the postural sway in ML and AP direction, while the influence was larger for the nodulous shoes compared to the plain shoes; (3) No significant differences between the footwear conditions for static stability and muscle activation was observed on firm surface condition.

In agreement with the previous studies, standing on foam surface increases the postural sway [18, 19]. Meanwhile, the effect of footwear condition on the postural sway was only observed on the foam surface, not the firm surface [18, 19]. When standing on a foam surface, the accuracy of the proprioception input was affected [6, 13], thus more demanding for sensory input from the mechanoreceptors.

As expected, the nodulous insole significantly enhanced the postural stability when standing on the foam surface, demonstrating by the reduction in AP and ML sway, and reduced C95 area; while plain shoes only show reduced AP sway on the foam surface (Fig 3). These findings were also comparable with previous studies applying hard insole with dimples covering the entire insole [18–20]. ML sway is an important clinical parameter because ML sway has been shown to increase with age more than AP [40] and be greater in fallers than age-matched non-fallers [9]. Several possible mechanisms regarding the static posttrial control by insole have been suggested. First, the nodulous surface may produce higher plantar pressure at the elevated parts of the textured sole, providing stronger sensory stimulation to the mechanoreceptors [18–20, 41]. We also specifically placed the nodulous in the metatarsal-tarsal and heel region, where is the mechanoreceptors predominately locate [23]. Second, footwear is also suggested as a filter for the pressure input to the sole [42], which may remove the disturbance information during standing on the foam. Therefore, we observed a trend that wearing plain insole shoes may also improve the postural stability in our study. Third, footwear may optimize the biomechanical alignment of lower limbs, thus promoting the efficiency of postural control [16, 42]. We created arch support for the nodulous insole (Fig 1). However, no difference in muscle activation was shown in the between the nodulous and plain shoes, though better postural control was observed in the condition of the nodulous shoes. This may imply the influence due to biomechanical alignment change would be trivial. Collectively, nodulous insole shoes could promote the static postural control possibly due to enhancement of mechanoreceptors in the sole.

One interesting finding of the present study is that footwear could significantly decrease muscle activity for the vastus lateralis and tibial anterior muscle but not other tested muscles. Standing on an unstable supporting surface largely challenge the stability of knee joint [43] and subtalar joint [44]. Thus, knee extensors (e.g., VL) would be greatly activated to main the knee stability. Tibial anterior, the major foot inverter muscles, activate with other extrinsic and intrinsic foot muscles to maintain proper foot alignment [44]. Thus, in consistent with previous findings, standing on the foam significantly increase the TA muscle activation [43, 45].

This study acknowledges a few limitations. First, only older women were involved in this study. The previous study suggested that the gender difference might exist in balance control and somatosensory function for the healthy elderly [46], thus recruiting the same gender in this study could minimize the potential confounding factors. However, future studies should examine both male and female participants, and also other individuals with neurological conditions [41]. Second, for the barefoot condition, the participant stood on the pressure sensor. However, the insole sensor is flexible with 2-mm thickness, which could largely preserve the barefoot condition. Third, only muscle activation and COP changes in the dominant foot were measured. In this study, we standardize the feet position (i.e., feet were 17 cm apart from the heel center, with the foot progression angle at 14°), and the participants were instructed to maintain equal body weight between two feet. Thus, the measurement in the dominant side could represent the performance of the lower limb. More, the measures of the insole pressure sensor are reported comparable to the traditional force plate [46]. Fourth, the foam surface was used in this study. Although standing on foam does not have much external validity as it is not representative of real life situation, e.g. standing on soil or other unstable surfaces, this condition provides more information on the effect of footwear to balance control as the somatosensory information is challenged.

Conclusion

For older women, footwear could improve the postural stability in the unstable surface, particularly the footwear with nodulous insole, with the underlying mechanism as enhancing the mechanoreceptors on the plantar surface of the feet.

Supporting information

S1 Appendix. Testing maneuver of maximum voluntary contractions.

(DOCX)

Click here for additional data file. (15.1KB, docx)
S2 Appendix. List of abbreviations.

(DOCX)

Click here for additional data file. (19KB, docx)
S1 Data

(XLSX)

Click here for additional data file. (12.9KB, xlsx)
S2 Data

(XLSX)

Click here for additional data file. (20.4KB, xlsx)
S3 Data

(XLSX)

Click here for additional data file. (123.6KB, xlsx)
S1 File

(DOCX)

Click here for additional data file. (28.7KB, docx)
S2 File

(CSV)

Click here for additional data file. (598B, csv)

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

The work is supported by funding from the Innovation and Technology Fund (ITF) (ITS/359/14) and the Innovation and Technology Commission (AiDesign Lab Project).

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  • 42.Nigg BM. The role of impact forces and foot pronation: a new paradigm. Clinical journal of sport medicine. 2001;11(1):2–9. 10.1097/00042752-200101000-00002 [DOI] [PubMed] [Google Scholar]
  • 43.Fransson P-A, Gomez S, Patel M, Johansson L. Changes in multi-segmented body movements and EMG activity while standing on firm and foam support surfaces. European journal of applied physiology. 2007;101(1):81–9. 10.1007/s00421-007-0476-x [DOI] [PubMed] [Google Scholar]
  • 44.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. [DOI] [PubMed] [Google Scholar]
  • 45.Loram ID, Maganaris CN, Lakie M. Paradoxical muscle movement during postural control. Medicine & science in sports & exercise. 2009;41(1):198–204. [DOI] [PubMed] [Google Scholar]
  • 46.Wolfson L, Whipple R, Derby CA, Amerman P, Nashner L. Gender differences in the balance of healthy elderly as demonstrated by dynamic posturography. Journal of Gerontology. 1994;49(4):M160–M7. 10.1093/geronj/49.4.m160 [DOI] [PubMed] [Google Scholar]

Decision Letter 0

Pilar Serra-Añó

20 Mar 2020

PONE-D-20-00586

The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people

PLOS ONE

Dear Dr. Yick,

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ABSTRACT

In the abstract, the results of the condition without foam must be provided.

Why do the results of correlations appear in the abstract and are not explained as objectives?

This is also the case in the introduction. Correlations appear as hypotheses but are omitted as objectives. Nor is there any rationale in the introduction that explains the need to evaluate these relationships between variables. Please add it or remove the correlations analyses.

The conclusions should also be specific as to whether you are comparing with firm ground or with foam.

Add keywords.

INTRODUCTION

After conducting a general review of the article it is observed that the type of sole has no effect on the firm ground. It only has an effect when tested on an unstable surface.

In this respect, the only thing the authors contribute is "As the healthy participants could achieve their best performance on a firm surface [7, 9], it may not be sufficiently discriminating to detect the change of postural sway across footwear conditions. Indeed, textured insole was reported to reduce the postural sway when standing on foam, when compared to the barefoot condition [9, 10].

The objective of the study should not be to force the support condition to observe differences (fishing), in postural sway, as stated in the paragraph, but rather to determine what is the need to evaluate on unstable surfaces (are they common in the population?) and from there see if differences exist or not according to the type of sole. The approach of the study should change so that the objectives of the study make sense and serve to contribute to the field of footwear design.

Methods

Please review methodological guidelines, such as CONSORT for example, to add the missing methodological aspects that allow replicability of the data. For example, you should add recruitment time, reliability of measurement tools (StimUp® Balance 5 99 Pad, Will Medical, Tokyo, Japan; Pedar®, Novel GmbH, Munich, Germany), etc.

Statistics

Only EMGmvc% was positively skewed? Indicate, if it is true, that the others met the assumption of normality. The statistics used for the main study are very well designed. However, here in this section, there is also no reference to correlations of muscle activation with postural sway.

Results

Change the + symbols to parentheses.

The results do not show the correlations that are in the abstract either.

Change "indicates positive interaction effects" by "significant interaction"

Please, add the legends to the figures.

Discussion

More information on the mechanisms by which alterations in postural control and muscle activation occur is lacking in the discussion. As well as discussing  the differences between what happens on firm ground and on unstable surfaces.

It should be added, in limitations, that the use of foam, as a form of unstable soil does not have much external validity as it is not representative of real life. It would be advisable to slightly modify what the authors have put forward.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper examined the footwear insole texture and supporting surface conditions on static posrtural stability and lower limb muscle activation for health older people. While this is a potential very valuable study there is some work needed to get it to the standard for this journal. There were some typographical errors I have included some in specific comments. Please refer to the specific comments below:

1. The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people

It is suggested that the topic should be change to "The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old women' because only women participants involved in this study.

2. Start with capital letter for 'compared' in the Results abstract section.

3. Start with capital letter for 'for' in the Conclusions abstract section.

4. Change 'old people' to 'old women' in the Background section.

5. What correlation methods/analysis used for your study? There were correlation results in the Abstract but the information was not available in the Results section. There were also no Correlation Analysis information found in the Data Analysis section. The correlation results also were not discussed in the Discussions section. Please revise this issues accordingly.

6. 'pedar' should start with capital letter..i.e. line 130...Please check and revise accordingly.

7. It is good to state the minimum age of your participants, i.e. how many participants are age from 60-65 years old. In the discussion section, you mentioned 'elderly' and by definition, elderly refers to person who are age 65 years old and above...but in Table 1, the age minimum age range of your participants are 60 years old..please revise according regarding the use of 'elderly' and older population/older women.

8. To our knowledge, this is the first study that investigates the footwear condition and supporting surface conditions on static postural stability and lower limb muscle activation for healthy old women.

As to our knowledge, no studies have investigated the effect of footwear insole condition on both static postural stability and lower limb muscle activity for older people.

Please standardized the term used for helthy old momen and older people. What is the main participants involved in this study?

9. As to our knowledge, no studies have investigated the effect of footwear insole condition on both static postural stability and lower limb muscle activity for older people.

The authors should provide a detailed argument before coming to this statement. The information in the same paragraph were only on textured insoles, spike insoles that was inserted to the participants shoes during the testing session. There should be an intensive information on how does the footwear used in this were different from the previous study. Is it a readymade footwear that is available in the market?

10. How do you make sure that all participants have the minimum foot sensitivity level before they were accepted in your study? Do you conducted any test on foot sensitivity?

Reviewer #2: I am grateful for the possibility to revise this research study.

The effect of support surface and footwear condition on postural sway and lower limb

muscle action of the old people is a trend topic in the current research literature and may be a main focus of interest for readers.

Results of the abstract need to reflect the findings with respect to both groups and the lack of significant differences of balance, and also you need reflect the meaning of AP and ML because these abbreviates are not reflected clearly

Introduction may be improved adding new information in order to provide an adequate state-of-the-art including some references. I suggest to include this references include in the atteched to complet this requeriment

Lines 55-68

Rodríguez-Sanz D, Tovaruela-Carrión N, López-López D, Palomo-López P, Romero-Morales C, Navarro-Flores E, et al. Foot disorders in the elderly: A mini-review. Disease-a-Month. 2018 Mar;64(3):64–91.

Calvo-Lobo C, García AR, Iglesias MEL, López-López D, Rodríguez-Sanz D, Romero-Morales C, et al. The relationship between shoe fitting and foot health of persons with down syndrome: A case control study. Int J Environ Res Public Health. 2018 May 14;15(5).

López-López D, Marañon-Medina J, Losa-Iglesias ME, Calvo-Lobo C, Rodríguez-Sanz D, Palomo-López P, et al. The influence of heel height related on quality of life on the foot in a sample of women. Rev Assoc Med Bras. 2018 Apr;64(4):324–9.

Roca-Dols A, Elena Losa-Iglesias M, Sánchez-Gómez R, Becerro-de-Bengoa-Vallejo R, López-López D, Palomo-López P, et al. Electromyography activity of triceps surae and tibialis anterior muscles related to various sports shoes. J Mech Behav Biomed Mater. 2018/06/05. 2018 Oct;86:158–71.

Roca-Dols A, Losa-Iglesias ME, Sánchez-Gómez R, López-López D, Becerro-de-Bengoa-Vallejo R, Calvo-Lobo C. Electromyography comparison of the effects of various footwear in the activity patterns of the peroneus longus and brevis muscles. J Mech Behav Biomed Mater. 2018/03/13. 2018 Jun;82:126–32.

López-López D, Expósito-Casabella Y, Losa-Iglesias M, Bengoa-Vallejo RB de, Saleta-Canosa JL, Alonso-Tajes F. Impact of shoe size in a sample of elderly individuals. Rev Assoc Med Bras. 2016 Nov;62(8):789–94.

López López D, Losa Iglesias ME, Becerro de Bengoa Vallejo R, Palomo López P, Morales Ponce Á, Soriano Medrano A, et al. Optimal choice of footwear in the elderly population. Geriatr Nurs. 2015/08/12. 2015;36(6):458–61.

Roca-Dols A, Losa-Iglesias ME, Sánchez-Gómez R, Becerro-de-Bengoa-Vallejo R, López-López D, Rodríguez-Sanz D, et al. Effect of the cushioning running shoes in ground contact time of phases of gait. J Mech Behav Biomed Mater. 2018/08/25. 2018 Dec;88:196–200.

Methods are well-designed with relevant and complete information. Correct sample size calculations, good description of the properties of the outcome measurements as well as detailed statistical analyses were included. Tables, figures and redaction of the results are presented in a correct way providing a good presentation of the main finding of the study.

In line 76 to 82 in methods section I suggest authors must include a reference to Ethics requirements Helsinki declaration

Holt GR. Declaration of Helsinki—The World’s Document of Conscience and Responsibility. South Med J. 2014 Jul;107(7):407–407.

Discussion section may include future research studies secondary to the current findings of this study. Clinical considerations, limitations and overall discussion are well-presented, but future research may be useful in order to propose future research regarding this field.

Finally, I suggest to authors in discuss section to include the possbilty of influence of aging or even another pathologies like Parkinson diseease in your study finding suggest to include this references include in the attached to complete this requirement.

In line 198-214

Brognara, Navarro-Flores, Iachemet, Serra-Catalá, Cauli. Beneficial Effect of Foot Plantar Stimulation in Gait Parameters in Individuals with Parkinson’s Disease. Brain Sci. 2020 Jan 27;10(2):69.

**********

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Reviewer #1: No

Reviewer #2: No

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Attachment

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PLoS One. 2020 Jun 3;15(6):e0234140. doi: 10.1371/journal.pone.0234140.r002

Author response to Decision Letter 0

20 Apr 2020

First of all, we would like to thank the academic editor and reviewers for their insightful and constructive comments, which are very helpful in improving the quality of the manuscript. We have addressed all the comments raised in the revised manuscript. The point-by-point responses to the comments are provided below.

Academic editor:

ABSTRACT

1. In the abstract, the results of the condition without foam must be provided.

Response: Thank you for the comments. We have added the related results accordingly in the abstract (Line 43-44).

2. Why do the results of correlations appear in the abstract and are not explained as objectives? This is also the case in the introduction. Correlations appear as hypotheses but are omitted as objectives. Nor is there any rationale in the introduction that explains the need to evaluate these relationships between variables. Please add it or remove the correlations analyses.

Response: Thank you for the comments. We have removed the correlation analyses in the abstract.

3. The conclusions should also be specific as to whether you are comparing with the firm ground or with foam.

Response: It is in the foam(unstable) surface. We have specified the comparison in the results accordingly (Line 46).

4. Add keywords.

Response: We have added key words as suggested (Line 48).

INTRODUCTION

5. After conducting a general review of the article, it is observed that the type of sole has no effect on the firm ground. It only has an effect when tested on an unstable surface. In this respect, the only thing the authors contribute is "As the healthy participants could achieve their best performance on a firm surface [7, 9], it may not be sufficiently discriminating to detect the change of postural sway across footwear conditions. Indeed, textured insole was reported to reduce the postural sway when standing on foam, when compared to the barefoot condition [9, 10].” The objective of the study should not be to force the support condition to observe differences (fishing), in postural sway, as stated in the paragraph, but rather to determine what is the need to evaluate on unstable surfaces (are they common in the population?) and from there see if differences exist or not according to the type of sole. The approach of the study should change so that the objectives of the study make sense and serve to contribute to the field of footwear design.

Response: Thank you very much for your insightful comments. We agree with your suggestion. Now we have made a substantial revision in the introduction part to address the uniqueness of our footwear insole design, and presented a strong justification to conduct the study (Line XX).

Methods

6. Please review methodological guidelines, such as CONSORT for example, to add the missing methodological aspects that allow replicability of the data. For example, you should add recruitment time, reliability of measurement tools (StimUp® Balance 5 99 Pad, Will Medical, Tokyo, Japan; Pedar®, Novel GmbH, Munich, Germany), etc.

Response: Thank you for your comments. We have added the information about participant recruitment (Line 100-103) and the reliability of measurement (Line 139-140).

Statistics

7. Only EMGmvc% was positively skewed? Indicate, if it is true, that the others met the assumption of normality. The statistics used for the main study are very well designed. However, here in this section, there is also no reference to correlations of muscle activation with postural sway.

Response: We have tested the data normality and we confirmed that only EMGmvc% was positively skewed. Additional information about the statistics analysis have been provided (Line 167-168). Per your suggestion, we have removed the correlation analysis in the present study.

Results

8. Change the + symbols to parentheses.

Response: We have revised accordingly (Line 180).

9. The results do not show the correlations that are in the abstract either.

Response: Sorry for the mistake but we have removed the correlation analysis in the study.

10. Change "indicates positive interaction effects" by "significant interaction"

Response: We have revised accordingly (Line 185).

11. Please, add the legends to the figures.

Response: The legends of Figure 1 is in Line 120-122, Figure 2 in Line 197-198, Figure 3 in Line 200-201.

Discussion

12. More information on the mechanisms by which alterations in postural control and muscle activation occur is lacking in the discussion. As well as discussing the differences between what happens on firm ground and on unstable surfaces.

Response: Thank you for your suggestion. We have provided an elaborated discussion about the mechanism of the insole design and the firm/foam ground in the discussion section (Line 212-237).

13. It should be added, in limitations, that the use of foam, as a form of unstable soil does not have much external validity as it is not representative of real life. It would be advisable to slightly modify what the authors have put forward.

Response: We have highlighted the limitation in the discussion accordingly (Line 258-259).

Reviewer #1:

This paper examined the footwear insole texture and supporting surface conditions on static postural stability and lower limb muscle activation for health older people. While this is a potential very valuable study there is some work needed to get it to the standard for this journal. There were some typographical errors I have included some in specific comments. Please refer to the specific comments below:

1. The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people

It is suggested that the topic should be change to "The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old women' because only women participants involved in this study.

Response: Thank you for the insightful comments. We have changed the title as suggested.

2. Start with capital letter for 'compared' in the Results abstract section.

Response: Rectification has been made accordingly (Line 37).

3. Start with capital letter for 'for' in the Conclusions abstract section.

Response: Rectification has been made accordingly (Line 44).

4. Change 'old people' to 'old women' in the Background section.

Response: Rectifications has been made accordingly.

5. What correlation methods/analysis used for your study? There were correlation results in the Abstract but the information was not available in the Results section. There were also no Correlation Analysis information found in the Data Analysis section. The correlation results also were not discussed in the Discussions section. Please revise this issues accordingly.

Response: Thank you for your comments. We have removed the correlation analysis in the present study.

6. 'pedar' should start with capital letter i.e. line 130...Please check and revise accordingly.

Response: Rectification has been made accordingly (Line 137).

7. It is good to state the minimum age of your participants, i.e. how many participants are age from 60-65 years old. In the discussion section, you mentioned 'elderly' and by definition, elderly refers to person who are age 65 years old and above...but in Table 1, the age minimum age range of your participants are 60 years old..please revise according regarding the use of 'elderly' and older population/older women.

Response: Thank you for the comments. Now we used “old women” instead of “elderly” in the manuscript.

8. “To our knowledge, this is the first study that investigates the footwear condition and supporting surface conditions on static postural stability and lower limb muscle activation for healthy old women.”

“As to our knowledge, no studies have investigated the effect of footwear insole condition on both static postural stability and lower limb muscle activity for older people.”

Please standardized the term used for helthy old momen and older people. What is the main participants involved in this study?

Response: Thank you for the comments. We have removed that sentence.

9. “As to our knowledge, no studies have investigated the effect of footwear insole condition on both static postural stability and lower limb muscle activity for older people.” The authors should provide a detailed argument before coming to this statement. The information in the same paragraph were only on textured insoles, spike insoles that was inserted to the participants shoes during the testing session. There should be an intensive information on how does the footwear used in this were different from the previous study. Is it readymade footwear that is available in the market?

Response: Thank you for your comments. We have added more information about our insole design in the introduction section. This is a footwear prototype that is yet available in the market (Line 75-83).

10. How do you make sure that all participants have the minimum foot sensitivity level before they were accepted in your study? Do you conducted any test on foot sensitivity?

Response: Thank you for your expert comment. Yes, we did use Semmes Weinstein Monofilament test to evaluate the foot sensitivity for each participant. The information has been added to the method (Line 104-112) and result (Line 178-180) sections.

Reviewer #2:

1. I am grateful for the possibility to revise this research study. The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people is a trend topic in the current research literature and may be a main focus of interest for readers.

Response: Thank you for the positive comments about our study.

2. Results of the abstract need to reflect the findings with respect to both groups and the lack of significant differences of balance, and also you need reflect the meaning of AP and ML because these abbreviates are not reflected clearly.

Response: We have modified the abstract accordingly (Line 40).

3. Introduction may be improved adding new information in order to provide an adequate state-of-the-art including some references. I suggest to include this references include in the atteched to complet this requeriment

Lines 55-68

• Rodríguez-Sanz D, Tovaruela-Carrión N, López-López D, Palomo-López P, Romero-Morales C, Navarro-Flores E, et al. Foot disorders in the elderly: A mini-review. Disease-a-Month. 2018 Mar;64(3):64–91.

• Calvo-Lobo C, García AR, Iglesias MEL, López-López D, Rodríguez-Sanz D, Romero-Morales C, et al. The relationship between shoe fitting and foot health of persons with down syndrome: A case control study. Int J Environ Res Public Health. 2018 May 14;15(5).

• López-López D, Marañon-Medina J, Losa-Iglesias ME, Calvo-Lobo C, Rodríguez-Sanz D, Palomo-López P, et al. The influence of heel height related on quality of life on the foot in a sample of women. Rev Assoc Med Bras. 2018 Apr;64(4):324–9.

• Roca-Dols A, Elena Losa-Iglesias M, Sánchez-Gómez R, Becerro-de-Bengoa-Vallejo R, López-López D, Palomo-López P, et al. Electromyography activity of triceps surae and tibialis anterior muscles related to various sports shoes. J Mech Behav Biomed Mater. 2018/06/05. 2018 Oct;86:158–71.

• Roca-Dols A, Losa-Iglesias ME, Sánchez-Gómez R, López-López D, Becerro-de-Bengoa-Vallejo R, Calvo-Lobo C. Electromyography comparison of the effects of various footwear in the activity patterns of the peroneus longus and brevis muscles. J Mech Behav Biomed Mater. 2018/03/13. 2018 Jun;82:126–32.

• López-López D, Expósito-Casabella Y, Losa-Iglesias M, Bengoa-Vallejo RB de, Saleta-Canosa JL, Alonso-Tajes F. Impact of shoe size in a sample of elderly individuals. Rev Assoc Med Bras. 2016 Nov;62(8):789–94.

• López López D, Losa Iglesias ME, Becerro de Bengoa Vallejo R, Palomo López P, Morales Ponce Á, Soriano Medrano A, et al. Optimal choice of footwear in the elderly population. Geriatr Nurs. 2015/08/12. 2015;36(6):458–61.

• Roca-Dols A, Losa-Iglesias ME, Sánchez-Gómez R, Becerro-de-Bengoa-Vallejo R, López-López D, Rodríguez-Sanz D, et al. Effect of the cushioning running shoes in ground contact time of phases of gait. J Mech Behav Biomed Mater. 2018/08/25. 2018 Dec;88:196–200.

Response: Thank you for your suggested articles. We have cited the following references.

o Rodríguez-Sanz D, Tovaruela-Carrión N, López-López D, Palomo-López P, Romero-Morales C, Navarro-Flores E, et al. Foot disorders in the elderly: A mini-review. Disease-a-Month. 2018 Mar;64(3):64–91.

o López López D, Losa Iglesias ME, Becerro de Bengoa Vallejo R, Palomo López P, Morales Ponce Á, Soriano Medrano A, et al. Optimal choice of footwear in the elderly population. Geriatr Nurs. 2015/08/12. 2015;36(6):458–61.

4. Methods are well-designed with relevant and complete information. Correct sample size calculations, good description of the properties of the outcome measurements as well as detailed statistical analyses were included. Tables, figures and redaction of the results are presented in a correct way providing a good presentation of the main finding of the study.

Response: Thanks for the positive comments.

5. In line 76 to 82 in methods section I suggest authors must include a reference to Ethics requirements Helsinki declaration

Holt GR. Declaration of Helsinki—The World’s Document of Conscience and Responsibility. South Med J. 2014 Jul;107(7):407–407.

Response: We have included this reference accordingly (Line 97).

6. Discussion section may include future research studies secondary to the current findings of this study. Clinical considerations, limitations and overall discussion are well-presented, but future research may be useful in order to propose future research regarding this field.

Response: We have addressed potential research directions, e.g. involving different populations, in the discussion section (Line 249-250).

7. Finally, I suggest to authors in discuss section to include the possibility of influence of aging or even another pathologies like Parkinson diseease in your study finding suggest to include this references include in the attached to complete this requirement.

In line 198-214

Brognara, Navarro-Flores, Iachemet, Serra-Catalá, Cauli. Beneficial Effect of Foot Plantar Stimulation in Gait Parameters in Individuals with Parkinson’s Disease. Brain Sci. 2020 Jan 27;10(2):69.

Response: Thank you for sharing this recent publication. We have cited this paper accordingly.

Attachment

Submitted filename: Response to Reviewers_v3.docx

Click here for additional data file. (34.2KB, docx)

Decision Letter 1

Pilar Serra-Añó

12 May 2020

PONE-D-20-00586R1

The effect of support surface and footwear condition on postural sway and lower limb muscle action of the older women

PLOS ONE

Dear Dr. Yick,

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We look forward to receiving your revised manuscript.

Kind regards,

Pilar Serra-Añó

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

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The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thanks for the edited version and all authors did a good job in revising the manuscript. However there are some recommendations for this reviewed version.

1. There is repeated information on ethics approval on line 90 and 99. Please revise accordingly.

2. 23 participants involved in this study (line 180) but this number is not tally with the reported number of the dominant leg (line 184). Please revise accordingly.

Reviewer #2: This study supports novel information about The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people

This is an interesting aim with the quality of life scope.

Authors have adressed all the required modifications in a correct way.

The redaction is clear and concise with appropriated scientific terms.

The sample size calculation, structured tables and methodology are adequate and provide important contents.

Therefore, this study may support considerations about The effect of support surface and footwear condition on postural sway and lower limb muscle action of the old people

**********

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Reviewer #1: No

Reviewer #2: No

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PLoS One. 2020 Jun 3;15(6):e0234140. doi: 10.1371/journal.pone.0234140.r004

Author response to Decision Letter 1

14 May 2020

First of all, we would like to thank the academic editor and reviewers for their constructive comments, which are very helpful in improving the quality of the manuscript. We have addressed all the comments raised in the revised manuscript. The point-by-point responses to the comments are provided below.

Academic editor:

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future.

Response: Thank you for your recommendation. We have added laboratory protocols following the guidelines.

Reviewer #1:

1. There is repeated information on ethics approval on line 90 and 99. Please revise accordingly.

Response: Thank you for the error. We have removed the repeated information as suggested.

2. 23 participants involved in this study (line 180) but this number is not tally with the reported number of the dominant leg (line 184). Please revise accordingly.

Response: Rectification has been made accordingly (Line 182).

Attachment

Submitted filename: Response to Reviewers.docx

Click here for additional data file. (30.3KB, docx)

Decision Letter 2

Pilar Serra-Añó

20 May 2020

The effect of support surface and footwear condition on postural sway and lower limb muscle action of the older women

PONE-D-20-00586R2

Dear Dr. Yick,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

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With kind regards,

Pilar Serra-Añó

Academic Editor

PLOS ONE

Acceptance letter

Pilar Serra-Añó

22 May 2020

PONE-D-20-00586R2

The effect of support surface and footwear condition on postural sway and lower limb muscle action of the older women

Dear Dr. Yick:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Pilar Serra-Añó

Academic Editor

PLOS ONE

Associated Data

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    Supplementary Materials

    S1 Appendix. Testing maneuver of maximum voluntary contractions.

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    S2 Appendix. List of abbreviations.

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