Switched Control of Motor Assistance and Functional Electrical Stimulation for Biceps Curls^†

Round 1

Reviewer 1 Report

General remarks:

The Authors present a hybrid rehabilitation technique for impaired limbs. In particular, the elbow joint is taken into account in this paper as a simple test case scenario. The presented strategy merges the Functional Electrical Stimulation (FES) to the action of a DC motor to help patients with impaired upper limbs performing a biceps curl. The Authors rigorously describe the model and accurately investigate its stability. The second part of the paper focuses instead on the experimental protocol and the test results.

Although the topic is on the whole very interesting, it seems to me that such a method is largely borrowed from previous publications and, specifically, from  “Rouse, C. A., Cousin, C. A., Duenas, V. H., & Dixon, W. E. (2017, December). Switched motorized assistance during switched functional electrical stimulation of the biceps brachii to compensate for fatigue. In 2017 IEEE 56th Annual Conference on Decision and Control (CDC) (pp. 5912-5918). IEEE”. The stability analysis and the discussion about the results are therefore not very original. Moreover, it is not very clear what the innovative contribution is compared to previous works. For all of the reasons above, in my opinion, the paper shall need deep modification, in particular to the second part, in order to add novel information and give better proofs of the claimed results.

Last but not least, more than one-third of the total references are from the Authors' research group, which seems to be excessive compared to the added value they bring to the article. I suggest revising the use of such many references.

Please, consider all these notes as a sincere interest in your work with the only aim of improving it.

Specific comments:

1. Abstract: "Rehabilitation robotics IS an emerging tool..."
2. Abstract: FES acronym is used before definition. Please consider defining the acronym the first time is used.
3. Materials and Methods: right below Eq. 1, Q is defined as the "set of forearm angles"; few lines below, it is said, "N distinct electrode channels that are placed along the biceps brachii in predefined regions of Q". What do the authors mean here by "predefined regions" of a "set of angles"?
4. Materials and Methods: In Eq. 12, 13, 14, 20, 26 the parenthesis formatting is a little bit confusing.
5. Materials and Methods: right below Eq. 12, the decay constant ρ is here introduced for the first time with no explanation. I suggest moving here the few lines reported in the Discussion section where its aim is presented. 
6. Materials and Methods: right below Eq. 16, are the constants C_M and C_m the same from Property 2? If yes please refer explicitly to it.
7. Test Setup: has the electrode placement procedure any references?
8. Protocol: how much does the fastening influence the elbow motion during flexion? Is this setup itself influencing the level of FES to perform a specific movement? Is the DC motor controlled to follow the elbow motion even when the FES is not saturated or is it just about reversibility? 
9. Protocol: the two FES thresholds are very subjective: how do the authors identify them?
10. Discussion: in the very first lines it is said, "The results in Table 1 show that the average position and velocity errors of the impaired arm are similar to that of the unimpaired arm for both participants, despite each having movement disorders that significantly limit performing daily activities with their impaired arm." I am not such an expert in anatomy but, if the considered disorder interests only the neurological part, this should be quite an expectable result since the control system replaces the neurological disorder. Poorly speaking, in both cases (healthy and compromised arm), there is good "hardware" plus an artificial control system. Please consider discussing this point.
11. Discussion: right below it is reported, "Moreover, the motor only contributes as needed and the FES activates the biceps throughout flexion, ensuring that the exercises align with rehabilitation goals that may involve building strength and muscle mass. Thus, the FES and motor controllers developed in this paper are a promising development for rehabilitation robotics." However, this is not very clear from the results. Please consider deepening this point.
12. Discussion: overcoming the chattering problem highlighted in [19] seems to be one of the major contributions of this paper. Nonetheless, it is not clear from the results of how this has been achieved.
13. References: there are too many references from the Authors' research group (more than one-third of the total). Please consider limiting this number to the most important ones.

I do really hope these suggestions could be useful to the Authors to improve the quality and effectiveness of their research work.

Yours faithfully, the reviewer.

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Organization and Style: The document is well organized and easy to read.

Technical Accuracy: The paper seems technically accurate, based upon the assumptions that are made. Some detailed comments are included as suggestions for the authors

Presentation: The presentation of the work is formally correct and rigorous from a methodological point of view.

Adequacy of Citations: fairly complete

Some comments:

- In the Abstract, the first appearance of the acronym FES is made without having defined its meaning, this must be corrected

- The Introduction ends with the following paragraph:

"The performance of the controllers is demonstrated on both arms of two participants with hemiparesis, with average root mean square (RMS) position errors of 4.55 degrees and 4.36 degrees for the impaired and unimpaired arms, respectively"

Why is this data relevant? The reader should be given a brief explanation of its meaning and importance in the context of the paper.

- In lines 104-11 seven properties are indicated by their analytical expression. It should be explained what implications each one has for the proposed system and, therefore, why it is relevant in the context of the Paper

- In lines 196-198 the authors said that the purpose of the experiments were to verify the feasibility, safety, and performance of the controllers for rehabilitation purposes. ¿how is the safety aspect addressed with the experiment described?

- Lines 225-226: "A saturation limit for the muscle control input was established based on comfort."

How is comfort quantified to determine the limit? Is it subjective?

- In the results of the experiment, the position and velocity errors are compared between the affected arm and the healthy arm of each of the two subjects. However, it is indicated that the first participant has affected both arms ("The first participant had post-polio syndrome that effected their right arm noticeably more than the left,") and that the second has damaged the spine, which may have affected both sides as well. What sense then does it make to assess the validity of the strategy comparing not with a healthy limb but with one that is also partially affected?

- Some figure (block diagram) is missing that would facilitate the description of the control strategy presented.

- A compressed file is provided which contains mostly supplementary material but the purpose of which is unclear, despite the inclusion of numerous files. I would like the authors to clarify to this reviewer the purpose of these files and if there is relevant content that should be taken into account in the review of the work presented.

Formal aspects:

- check the use of commas and semicolons in the list of keywords

- from line 189 and until the end all the text is in italics

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The reviewer whish to thank the Authors for the effort made answering its comments. Nevertheless, there are still three points that have not been fully discussed and therefore need further revision.

1. Comment 3: the issue highlighted in the first revision step has not been solved. I apologize for not having been clear enough on this point. What the Authors mean with "set of angles" was already clear, what still is not clear is what the Authors mean when they say that "the N electrode channels are placed along the biceps brachii in predefined regions/ranges of Q". I understand that the electrode channels are placed along the biceps brachii, and it makes sense, but what do you mean by adding "in predefined regions/ranges of Q"? Since Q is a set of angles, it sounds weird to me claiming to have placed electrode channels on it. Please consider clarifying this point. Besides, few lines above, it is said that Q is the set of forearm angles; do the Authors mean elbow joint angles? Please also consider rephrasing it.
2. Comment 5: the reviewer appreciated the addition of a brief description of the decay constant ρ. However for the sake of clarity, in my opinion, such a first apparition of the parameter still needs a proper description of its contribution to the work. The reader is left unaware of it until a few lines from the end of the article. Since there is already a good description of it in the Discussion section, I once again suggest moving the corresponding lines from the Discussion to the addressed point. 
3. Comment 12: this point should be carefully revised. It is very hard for the reader to keep up with the results if they are not presented in the paper. I would strongly recommend reporting directly the claimed comparison in the text. Please consider adding to this very paper at least one figure from [19] where the chattering problem is clearly visible, and then use it as a term of comparison. 

Once again, please consider all these comments as a sign of sincere interest in your work with the only aim of improving it.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf