PONE-D-23-27913Coordinated human-exoskeleton locomotion emerges from regulating virtual energyPLOS ONE

Dear Dr. Arami,

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Academic Editor

PLOS ONE

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This work is supported by NSERC Discovery, John R. Evans Leaders Fund (JELF)-Canada

Foundation for Innovation, Ontario Research Fund (ORF), and the New Frontiers in Research

Fund under Grant NFRFE-2018-01698.

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- AA received NSERC Discovery under Grants RGPIN-2018-04850

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NFRFE2022-620 

- AA received John R. Evans Leaders Fund Canadian Foundation for Innovation

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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 #3: Yes

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

Reviewer #1: Yes

Reviewer #3: N/A

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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 #3: 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: No

Reviewer #3: 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: This paper presents further work on the VER method. The VER allows control design in the phase space, where the control input is guided towards the limit cycle. This introduces several advantages, like time independence, coordinated motion, phase synchronization, etc.

Overall, I think the control method is very interesting and should be explored further. I also looked at the papers [38] and [39]. I believe the main flaw of the current work is the missing comparison to a “classical” controller with recorded trajectories. The controller is shown to improve the user effort compared to the passive movement and a lot of advantages are mentioned. But this does not really show that such “coordinated” controller is better than a standard, recorded trajectory one. Why is there no such comparison?

Or, alternatively, a better explanation on how one can measure the coordination between the exoskeleton and the user would help strengthen the claims.

Other major comments:

1 – Introduction: “...still far from optimal due to the unresolved human-exoskeleton coordination problem at the controller level.” -> I am not sure how the authors reached this conclusion, since there are a lot of works that currently reach very high “optimal” levels of assistance (metabolic cost reductions) using very straightforward control designs. Although, to my knowledge, these achievements were not yet generalized to different tasks. Or is this statement intended towards the current rehabilitation exoskeletons? In that case, the statement should be written differently.

2 – Introduction: “It has been shown that passive trajectory tracking controllers worsen human-exoskeleton coordination.” – there should be a citation here. And I think this is an important statement, since this paper tries to improve on this coordination problem.

3 – Figure 2 – a-d are identical from [39] and should be noted as such in the figure caption.

4 – Section 2.4 “However, to maximize the similarity of the resultant gait to natural walking, the knee and hip limit cycles are designed similar to natural walking; see Fig.2 a-d.” I understood only after looking at the [39] that the desired trajectories/limit cycles are modified to be stable. Is there any indication how this modification affects the gait?

5 - How is the “Passive” mode realized. Is the exoskeleton in a zero torque mode or is it completely turned off? Please add a sentence to clarify this. This is important, assuming that the controller is completely turned off, even having friction compensation, would undoubtedly reduce the muscle activations in the active mode.

6 – In the experimental results, the protocol is explained again. I think this is not needed, since the experimental protocol is written in section 3.1

7 – Section 4.2, Kinematics, second paragraph. Only the mass of the exoskeleton is considered to affect the gait. How about kinematic restriction imposed by the exoskeleton, like the simplified passive ankle, and DoF knee and hip?

8 – The idea about the central generator is interesting. But I would argue that more literature review in this area would be required to support this statement.

9 – Section 5, second paragraph. What is meant with the absence of conflicts?

10 – Section 5, last paragraph. “To sum up,...” sentence is strangely written, please consider revising it.

Minor comments:

- Section 2.2, line 8, there is a typo “...is almost zero, hence”

Reviewer #3: The authors should add the main conclusions of the study in their Abstract.

I recommend adding Figure S11 (Supporting material) to the content of the main text of the paper, as it illustrates the exoskeleton used, helping the understanding of Plos One readers.

Figure captions are too long. I suggest keeping only a brief presentation of the content, leaving more detailed descriptions for the main text of the paper.

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Reviewer #1: No

Reviewer #2: Yes: Dr. Vineet Vashista, Associate Professor, IIT Gandhinagar

Reviewer #3: No

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PONE-D-23-27913R1Coordinated human-exoskeleton locomotion emerges from regulating virtual energyPLOS ONE

Dear Dr. Arami,

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.

Please submit your revised manuscript by Apr 15 2024 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're 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.

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). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Imre Cikajlo, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Additional Editor Comments:

Please carefully examine the revision of the Reviewer 2 and provide point-to-point answers to resolve the remaining issues. Take in consideration also the typos addressed by the Reviewer 1.

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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: (No Response)

Reviewer #3: 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

Reviewer #3: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: 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

Reviewer #3: Yes

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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: No

Reviewer #2: Yes

Reviewer #3: 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: All my major comments were addressed.

I noticed that there are many typos in the paper. I would suggest the authors read the paper again and correct them.

Here are some that I found:

- “by” and “has” in line “Given gait variability, which can be intensified by an impairment, intention decoding based on gait kinematics has not been successful so far [33-35] despite the recent achievements in real-time gait phase estimation [36-39].”

- “Straighten” in line “Accordingly, we removed the flexion behavior of the knee joint during the stance phase; hence, the controller encourages the participants to straighten their leg during the stance.”

- “Experiment” in “The testing sessions were different for each experiment.”

- “were” in “Nine participants (7 male and 2 female, age: 23.9 ± 3.2 years, body mass: 72.3 ± 7.0 kg, height: 177 ± 6.3 cm; mean±standard deviation) participated in the Experiment 1.”

Reviewer #2: The paper discusses an interesting case of using VER. The authors have earlier worked on the topic and have extended it here with human experimentation. The manuscript is written well and present a thought out study. However, there are a few points that need more work.

- The authors provide a reference to their earlier works in the introduction to establish the contribution of the current paper. It appears that the concept has been developed earlier and a feasibility one human study has been published. Thus, the main contribution of the current work is implementation of the same over multiple subjects (n=9). This makes the current contribution of the paper weaker as only 9 subjects were tested, and no disabled walking studies were conducted. Also, it is not clear how the experiments 1 and 2 design strengthen the point that such approach can be useful with disabled walking.

- The second major comment is that the discussion section is very weak. Considering that the introduction section introduces the readers to the various strategies being tested in the community working on human in the loop, adaptive controllers, etc. the discussion section does not establish the proposed uniqueness and advantages as claimed for this work. This needs to be addressed.

- A claim "that passive trajectory tracking controllers worsen human-exoskeleton coordination" is made using reference [15] - this is an old reference. Are there newer references supporting this claim?

- Section 2.4 puts "Lack of actuation at the ankle joint in our exoskeleton results in walking pattern slightly different than natural walking. ... natural walking, the knee and hip limit cycles are designed similar to natural walking ..." This needs to be discussed further as it is not clear how this impacts the performance and how this will be taken care of disabled walking.

Reviewer #3: The authors responded to all comments satisfactorily. The work presents investigations relevant to the literature in the area of human gait analysis.

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If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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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 PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

Coordinated human-exoskeleton locomotion emerges from regulating virtual energy

PONE-D-23-27913R2

Dear Dr. Arami,

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

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Kind regards,

Imre Cikajlo, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

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