PONE-D-19-16440

Effect of sampling frequency on fractal fluctuations during treadmill walking

PLOS ONE

Dear Dr. Marmelat,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

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: Partly

Reviewer #2: Yes

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

Reviewer #1: No

Reviewer #2: Yes

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

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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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: The authors examine whether capturing spatiotemoral gait data on a treadmill at different sampling frequencies affects either the (1) magnitude or (2) reliability of fractal indices (DFA alpha) from various time series (step & stride length, step & stride time, step & stride speed). They report data from 14 participants who walked on a treadmill at their preferred speed during each of three 15-minute bouts. Each bout was captured at a different sampling frequency (60 Hz, 120 Hz, 240 Hz) in a randomized order. The study’s purpose was well-justified and the paper was well-written. The protocol and data processing methods were sufficiently described. Nevertheless, I have two primary concerns with the study’s methods: (1) sample size and (2) data analysis approach. The following specific comments are not intended to be critical, but rather are intended to help the authors improve the study and/or paper.

(Methods section, line 86 & line 144) The authors did not provide justification for their sample size and I believe the study was underpowered to accurately estimate measurement reliability. Per line 86, 17 individuals participated but per line 144, data from 3 participants were excluded, which further diminishes the sample size. The effective sample size was n = 14. Given the authors’ use of the following guidelines for interpreting ICCs (ICC < .50 = poor reliability, ICC of .50 to .75 = moderate reliability, ICC of .75 to .90 = good reliability, ICC > .90 = excellent reliability), I might have expected the authors to power the study in a way that the sample size would have been sufficient to detect reliability coefficients of .75 or higher against a minimally acceptable reliability coefficient of .50. Using those parameters across 3 measurement conditions, approximately 27 or more participants would have been required to adequately power the study (Walter SD, Eliasziw M, Donner A. Sample size and optimal designs for reliability studies. Statistics in Medicine. 1998;17:101-110.).

(Methods section, lines 133-135) In my opinion, the study design is more representative of an alternate forms reliability study, i.e., evaluating agreement between methods of measurement. Bland-Altman plots and limits of agreement analyses are perhaps better designed to address questions of agreement than are ICCs (Bland JM, Altman DG. A note on the use of the intraclass correlation coefficient in the evaluation of agreement between two methods of measurement. Computers in Biology and Medicine. 1990;20(5):337-340). Would the authors consider re-analyzing their data with Bland-Altman plots either instead of or as a supplemental analysis to the ICCs?

(Results section, lines 153-155) Per my previous comment regarding sample size, I would suggest the study was underpowered to detect accurate reliability coefficients and is a major reason the 95% confidence intervals reported in Table 1 are so wide. Informing the reader that measuring DFA alpha has “poor to moderate” or “poor to good” reliability may therefore be misleading. Those interpretations would be much more valid if the study was appropriately powered. Additionally, analyzing limits of agreement and/or systematic bias across measurement methods with Bland-Altman analyses may provide an alternative interpretation.

(Results section, Table 1) The step speed and stride speed DFA alpha values are approximately .50 (i.e., white noise) with very small standard deviations, suggesting little between-subject variability. ICC values are artificially deflated when between-subject variability is low, which would partially explain why those ICCs are so poor. I suspect that if the authors create Bland-Altman plots and analyze limits of agreement between the 60 Hz, 120 Hz and 240 Hz methods of measurement, the results will show strong agreement between methods with little to no evidence of systematic bias.

(Discussion section, line 184) Per my preceding comment, it is potentially misleading to conclude that DFA alpha “values were not reliable between conditions using different sampling frequencies.” If results are similar with an adequately powered sample size and/or systematic biases in the measures are apparent with Bland-Altman analyses, then the conclusion would be more valid.

(Discussion section, lines 205-209) The discussion content represented in lines 205-209 is potentially misleading for reasons listed above. The discussion content will be acceptable—in my opinion—only if the ICCs remain low with an adequately powered sample and if Bland-Altman analyses provide evidence of poor agreement (systematic bias) between methods.

(Discussion section, lines 225-249) The authors neglected the major limitation in their study: sample size. If the authors choose not to collect data from more participants and revise the manuscript accordingly, then the sample size limitation must be addressed as a major limitation to the study’s findings.

Reviewer #2: General: This paper provides an important technical contribution to the field of gait measurement. The authors examined the role of sample frequency and downsampling on prominently used gait metrics. Their results highlight the strengths and challenges of different sampling/processing techniques that people working in this area should be aware of. My comments below are minor and are aimed to help provide clarification in a few places in the text.

Comments and suggestions to the authors:

ABSTRACT

Lines 29-30: “Our results suggest that sampling frequency (between 60 and 240 Hz) does not significantly alter DFA”. Does the data support this? Lines 27-28 said “Intraclass correlation analysis revealed a poor reliability of DFA results between conditions using different sampling frequencies.” This seems to be a disconnect and could use some clarification in the text. (See my second comment in the Discussion section below for more context on this)

Line 32: The word “optimal” implies a data driven approach that provides support for such a claim. Since it appears that the relationship between detection accuracy and processing time was not quantified in this study, I recommend removing the word “optimal” from this sentence.

INTRODUCTION

Lines 39-44: You accurately identify that stride-to-stride fluctuations can become anti-persistent in various conditions. However, it would provide a more holistic view of work in this area if you were to also include text and references indicating that stride-to-stride fluctuations can also become more persistent in some conditions. This inclusion is important because it demonstrates gait fluctuations are modifiable in either direction (toward anti-persistence or persistence), which could be valuable for readers of this paper. In addition to some of the references you already cited, below are some additional references to consider including that show a shift toward persistence.

Rhea, C. K., Kiefer, A. W., Wittstein, M. W., Leonard, K. B., MacPherson, R. P., Wright, W. G., & Haran, F. J. (2014). Fractal gait patterns are retained after entrainment to a fractal stimulus. PLOS ONE, 9(9), e106755.

Rhea, C. K., Kiefer, A. W., D’Andrea, S. E., Warren, W. H., & Aaron, R. K. (2014). Entrainment to a real time fractal visual stimulus modulates fractal gait dynamics. Human Movement Science, 36, 20-34.

Wittstein, M. W., Starobin, J. M., Schmitz, R. J., Shulz, S. J., Haran, F. J., & Rhea, C. K. (2019). Cardiac and gait rhythms in healthy younger and older adults during treadmill walking tasks. Aging Clinical and Experimental Research, 31(3), 367-375.

Line 67-69: Please add a reference to support this statement.

METHODS

Lines 87-88: I assume this was verified via self-report, not medical record examination, correct? If so, please change this sentence to read “All participants self-reported no cognitive, neurological, muscular, or orthopedic impairments.”

RESULTS

Line 144: Please report the age (mean+/-SD) and gender of the 14 participants who were included in the analysis since these data reported in the Methods section pertains to the original 17 included in the study.

Line 166: Please add the degrees of freedom prior to your F-statistic.

DISCUSSION

Lines 182-184: It’s not clear how the following can co-exist for α-DFA:

“i) in general, mean, CV and α-DFA values of all spatiotemporal variables were similar between conditions, whether the data was collected at different sampling frequencies or downsampled, ii) α-DFA values were not reliable between conditions using different sampling frequencies”

How can α-DFA be similar between conditions collected at different sampling frequencies AND not reliable between conditions using different sampling frequencies? Some clarification needs to be made for these two points. You bring up this point again in lines 205-206. Your position on this appears to be accurate, but it would be helpful to readers if you can describe how data can be similar between conditions, yet unreliable. Many people many not understand how those two things can independently fluctuate.

Lines 192-193: Please describe how your data supports these two statements:

“Our results suggest that when the research question focuses on within-group or between-group comparisons, a sampling frequency as low as 60 Hz may be able to capture differences. While the reductions in α-DFA were not significant, 120 Hz may allow for more precise event detection.”

Line 233: The use of “dramatically” is relative and likely not a problem for most computers given current processing speeds. It will, however, increase processing speeds. Thus, I recommend removing “dramatically” from this sentence.

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Reviewer #1: No

Reviewer #2: Yes: Christopher K. Rhea

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

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PONE-D-19-16440R1

Effect of sampling frequency on fractal fluctuations during treadmill walking

PLOS ONE

Dear Dr. Marmelat,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

The revised version of this manuscript has been reviewed and both reviewers felt that the authors satisfactorily addressed their concerns.  There remain 3 minor details that should be addressed.

1) It seems from context that the statement on line 264-265 is specific to DS60, but this is not clear in the statement.  Please clarify.

2) Table 1: Leading zero missing for ICC 95% CI for Step Length  [0.811-969] should probably be [0.811-0.969]

3) Table 2: Leading zero missing for a-DFA Stride Time in Table 2 (.14) should be (0.14)

==============================

We would appreciate receiving your revised manuscript by Dec 07 2019 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

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. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Eric R. Anson

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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2. Is the manuscript technically sound, and do the data support the conclusions?

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?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

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: I appreciate the thoroughness of the authors' response to reviewers and appreciate their revisions to the manuscript. The paper will provide a valuable technical contribution to gait researchers.

Reviewer #2: The authors did an excellent job revising this manuscript. They are commended on their effort and the manuscript is recommended for publication in its current form.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Effect of sampling frequency on fractal fluctuations during treadmill walking

PONE-D-19-16440R2

Dear Dr. Marmelat,

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.

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Eric R. Anson

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PLOS ONE

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