PONE-D-24-04333Increased force and elastic energy storage are not the mechanisms that improve jump performance with accentuated eccentric loading during a constrained vertical jumpPLOS ONE

Dear Dr. Su,

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 review the comments attentively. Reviewer 1 has highlighted concerns that call into question the validity of the experiment. It is imperative that you address these points with detailed consideration. Please submit your revised manuscript by May 03 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.

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PLOS ONE

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Additional Editor Comments:

Dear Authors,

Please review the comments attentively. Reviewer 1 has highlighted concerns that call into question the validity of the experiment. It is imperative that you address these points with detailed consideration.

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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: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: 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

Reviewer #3: No

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Reviewer #1: Yes

Reviewer #2: Yes

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: General Comments:

This study was to investigate the effect of additional eccentric loading on jump performance (jumping height and power). It was a very interesting topic and the entire study was well executed. In general, if the load in the ECC phase is higher (i.e., higher activity level), the jump height should increase because the load is lighter in the CON phase (since the additional load is removed in the CON phase in this study). Previous studies showed that optimal loading of the ECC phase existed with squat exercise (Takarada et al. 1997), drop-jump (Komi and Bosco 1978). However, in the present study, the work of hip joint and center of mass (and jumping height) decreased under high eccentric loading condition. This result may be related to the fact that the subjects were not giving their all due to the sudden load removal when switching from the eccentric phase to the concentric phase. In addition, there are similar concerns because jumping exercises using the device in Figure 1 are quite different from normal jumping exercises. If this is the case, I think it could lead to erroneous views in research on stretch-shortening cycle exercise. Throughout this manuscript, the additional loading of the eccentric phase is discussed in connection with the increase in elastic energy. However, this argument is disconcerting because the two are not necessarily the same.

Specific Comments:

Introduction

Line 42-44

In ref 9, the effects of AEL have not been investigated. This study was a cross-sectional comparison of tendon characteristics of athletes in different disciplines.

Line 62-64

As with the comment above, I disagree with this statement.

Reviewer #2: While seemingly simple, the execution of a countermovement jump is a complex sequence of generation and release of elastic energy. The current study is designed to determine if more elastic energy is released during the propulsive phase in response to enhancing only the eccentric load during the countermovement while controlling for all other variables. I commend the authors for designing an apparatus and methodology for controlling the many potentially confounding variables in the eccentric phase. However, this study may have been too constrained with too narrow a focus. For a complete understanding of the effects of AEL on jump performance, the entire force profile of the countermovement jump (i.e. both phases) should be examined. The study by McHugh et al. (Transl Sport Med, 2021) highlights the importance of 1) unweighting during the eccentric phase and 2) the timing of peak force to occur at the turning point, to jump performance and efficiency. While potential sources of variation such as trunk angle, and countermovement depth were controlled for in the current study, an explanation of the current findings may be found by examining eccentric phase metrics, instead of focusing only on the concentric phase. I recommend major revisions to this manuscript and the inclusion and comparison of eccentric phase force and joint metrics (see McHugh et al.) under the AEL conditions in order to completely characterize the effects of AEL on the whole countermovement jump profile.

Please see all individual comments below.

INTRODUCTION

The first paragraph of the introduction is well-written and succinctly summarizes the current research findings regrading AEL.

Lines 44-49: The amount of unweighting (defined by McHugh et al. as "low force") also plays an integral role in elastic energy storage and jump performance. While more force may be needed to decelerate during the eccentric braking phase under the AEL conditions, the amount of unweighting represents the "launch pad" from which to build tension and store elastic energy. With greater the unweighting (i.e. the lower the low force), a greater amount of tension can be accumulated and ultimately released. It is possible that AEL disrupts the mechanism of optimal unweighting, which may ultimately decrease performance. This is why the whole force curve must be examined, not only concentric phase metrics.

METHODS

The Methods are very detailed and precisely describe each aspect of the experiment and the equipment and set-up involved.

Line 195: Joint ranges of motion and a joint stiffness measurement may be more useful to characterize any changes occurring during the eccentric phase. Also, eccentric work should be considered as potential factor as there may be a optimal amount of energy which can be absorbed during the descent before an impairment in concentric energy generation occurs.

Lines 190-191: Why was average descent speed as a metric instead of maximal descent speed? The body must be decelerated from its maximum downward velocity by the end of the countermovement. The amount of force required to do this relative to the amount of unweighting represents the total amount of tension accumulated in the lower extremity. The average velocity throughout the whole countermovement does not accurately characterize the kinematics of the descent.

RESULTS

As stated earlier, the eccentric phase metric should be included in the analysis for complete characterization of the effects of AEL.

Figure 2C: I feel that maximum descent speed should be examined instead of average

Figure 3A: Based on the limited data presented in the graph, it does seem like there is a difference in the amount of unweighting between some of the conditions. It is difficult to determine this with certainty because the eccentric portion of the force profiles are not included. Please include the entire profile for each condition.

Also, it is clear that the peak concentric force does to coincide with the turning point in any of these conditions. According to McHugh et al., this is an indication of a biomechanically inefficient jump. This might be due to the constraining of so many of the eccentric phase variables using the sled. This point should be addressed in the discussion.

Figure 4A: Based on the profiles of the joint powers, it seems that there may be differences in the peak magnitude of eccentric power at the hip across conditions. However, this needs to be explored more rigorously. Additionally, the duration of the eccentric phase seems to increase in the AEL conditions, which may have implications on negative work done at each joint.

DISCUSSION

Line 322: Storage and return of elastic energy are dependent on the amount of unweighting and the total force generated during braking. As these variables were not included in the analysis, it is difficult to determine if changes to these variables were responsible for the negative effects on performance.

Reviewer #3: SUMMARY:

The study investigates the effect of accentuated eccentric loading or AEL, on the performance of human participants executing a countermovement jump. The authors used a sliding rail mechanism to control several aspects of the jumping movement in order to reduce extraneous factors not purely related to the added load during eccentric movements. Force plates, motion capture, and electromyography were used to measure ground reaction forces, joint and center of mass kinematics, and electrical activity of notable knee extensor muscles, respectively. Linear mixed-effects models were used to estimate the effect of eccentric load (10, 20 and 30% body weight) relative to a baseline jump with minimal eccentric load on multiple parameters including jump height, descent velocity, ground reaction forces, joint moments, power, and work. The study found a slight reduction in jumping height at higher eccentric loads, perhaps due to reduced hip extensor moments and power. The authors conclude that added eccentric load does not enhance jumping performance in a controlled jumping movement.

The study is rigorously designed and executed, and the manuscript is clearly written. In particular, I appreciate the authors’ honesty in rejecting both hypotheses which are reasonably developed based on previous research on the topic. In general, I believe this paper will add a nice contribution to the literature and will point researchers toward future questions more tightly aimed at understanding the AEL phenomenon and the factors underlying it. The following are a few minor-to-moderate comments which I hope can still improve the paper further.

REVIEWER COMMENTS:

Probably my biggest comment is related to the loading strategy used in the experiment. The authors suspend weight plates from a cable attached to the backrest where participants perform the jumping movement (either 10, 20 or 30% body weight). While this approach is mostly fine, there are some limitations to consider.

First, the static loading conditions are not actually static. That is, the tension force experienced by participants is closer to the load weight plus the inertial force of the load. If we assume that tension force is always greater than zero (see next point) and the cable is sufficiently rigid, then we can also assume that the vertical motion of the load and sliding rail are roughly equivalent during experiments. As such, any accelerations of the participants in the vertical direction also contribute to fluctuations in tension force. This could be one reason why descent speeds were slower at greater loads; that is, participants may have been avoiding greater tension fluctuations during descent by employing lower accelerations to reach lower descent speeds.

Second, if the participant ever accelerates downward at a rate higher than gravitational acceleration, then the tension in the cable can drop to zero. It is theoretically possible that individuals may have slowed their descent rate at loads greater than zero in order to prevent a zero tension eventually resulting in a large force spike once the weight caught tension again. This scenario is perhaps unlikely for multiple reasons, but since there is no measurement of tension force or acceleration during the eccentric motion, the reader cannot know this for sure.

Third, another form of tension fluctuation could have occurred due to centripetal accelerations of the weight plates swinging away from a vertical orientation. While the motion of the participants was fairly well-controlled, it is unclear whether the researchers made any attempt to control the motions of the weight plates during descent. This would perhaps have only a small effect, however.

In each of the above points, questions about the load being applied arise due to a missing measurement of the tension force during the experiment. While it would likely not be realistic to redo experiments with these measurements, a fairly accurate approximation of the load force could be calculated via the summation of inertial force and load weight: Tension=mg+my ̈ where y ̈ is vertical acceleration of the load (equal to vertical acceleration of the participant) and m is the load mass. Average tension during descent could be measured and even included in the statistical model as continuous data (rather than categorical) in order to potentially account for more of the data’s variability with loading condition. At the very least, it would be helpful to show an example of approximated tension from the calculation somewhere in the manuscript.

Line 119 – was friction in the linear rail system or pulleys assessed? Can the authors estimate how much additional load any friction or neglected inertial effects might have added? If not, it might be more transparent to mention that these effects were neglected in the analysis but are assumed to be quite low.

Line 122 – when the author refer to an “inextensible” metal wire, maybe they can report the gauge or diameter of the wire to help back up their claim of rigidity.

Lines 144, 177, 198 - in most cases, a number value is reported followed by a space and the units. However, there are a few instances where there is no space between the value and the units. Some examples are “9mm” (Line 144), “15Hz” (Line 177) and “5Hz” (Line 198). The authors should check the manuscript matches the journal formatting rules.

Line 128 – is it possible that the noise of the weights hitting the box could have affected participant performance? Perhaps this is part of a slightly larger discussion regarding participant instructions. For example, were participants told about what would happen to the weights during each jump? Also, how was “maximal effort” (Line 100) enforced or motivated?

Line 324 – it is interesting that the previous simulation study predicted some of the experimental results, and the discussion that follows in Lines 392-398 adds a new dimension to the interpretation. However, the initial mention of the simulation study in Line 324 feels out of place since no further context is given. I recommend waiting until the later discussion before referencing the simulation.

Line 395 – descent is misspelled here as “desent.”

Line 431-432 – I am not familiar with the studies referenced, but perhaps it is also possible that subjects in these studies could have pushed off the released weights in order to gain some amount of extra upward impulse.

The manuscript could be improved by adding either some discussion or background regarding the impact of the study and the topic more generally. Beyond developing a better understanding of the specific phenomenon related to human jumping, do the findings (or lack thereof) tell us something useful about training strategies or sports performance, for example?

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Reviewer #2: No

Reviewer #3: No

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PONE-D-24-04333R1Increased force and elastic energy storage are not the mechanisms that improve jump performance with accentuated eccentric loading during a constrained vertical jumpPLOS ONE

Dear Dr. Su,

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 Aug 23 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,

Žiga Kozinc

Academic Editor

PLOS ONE

Additional Editor Comments:

Dear Authors,

thank you for all the efforts in revising your manuscript. Reviewer 1 responded again, stating they are not completely satisfied with your responses. Please see their comments and try to see if you can be even more critical towards your results (eg., see the point on inter-individual variability in EMG responses) After this final revision, we will make a final decision.

Kind regards,

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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

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: Partly

Reviewer #2: Yes

Reviewer #3: Yes

**********

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

**********

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

Reviewer #3: 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: Thank you for your responses. As pointed out previously, however, I disagreed with two points.

Firstly, the reduction of the works of hip joint and center of mass with higher eccentric loading conditions may be related to the fact that the participants were not giving their all due to the sudden load removal when switching from the eccentric phase to the concentric phase. The authors rejected this possibility, because no differences in EMG data among all conditions were found. According to the individual EMG data (Figure 6), it is recognized that there were significant individual differences in the changes in EMG data in response to loading conditions. The individual differences were particularly pronounced in the GLUT (Fig. S4) and BF (Fig. S5). Because these muscle groups were closely related to hip joint work, I do not consider it possible to fully explain why the points I have identified (see above) are not involved in the results of this study (lower hip work with heavy loading).

Secondly, the additional loading of the eccentric phase is discussed in connection with the increase in elastic energy in this study. However, this argument is disconcerting because the two are not necessarily the same. Previous studies showed that optimal loading of the ECC phase existed with squat exercise (Takarada et al. 1997), drop-jump (Komi and Bosco 1978). For example, Takarada et al (1997) reported that power output during the concentric phase increased initially with the eccentric force, whereas they began to decline when the eccentric force exceeded >1.4 times the sum of load and body weight (JAP 83: 1749-1755).

Reviewer #2: The authors have thoroughly addressed my comments raised in a previous round of review. I feel that this manuscript is now acceptable for publication. Well done!

Reviewer #3: In general, I am satisfied by the author responses to the previous round of comments. I have listed a few minor editorial comments for the authors to consider, but otherwise I am happy to recommend publication.

Note, the following line numbers refer to the tracked-changes version of the manuscript.

Line 100: the way this line is written could be misunderstood by the reader to mean that the additional mass is spatially configured on top of the backrest. Perhaps this line could be updated for clarity: “…other AEL conditions included extra mass in addition to the mass of the backrest.”

Line 198: The letter “s” in OpenSim should be uppercase.

Line 483: change to “…the added weight, nor did it have…”

Line 528: the word novelty is misspelled as “novely”

Line 535-536: it is unclear what qualifies as a “significant reduction in force applied”, since I do not believe the authors are referring to statistical significance here (if they are, the statistics should be reported, e.g., p-value, etc.). Otherwise, I suggest a slight rewording, e.g., “…however the accelerations were unlikely to have caused a substantial reduction in the force applied.” Furthermore, this statement could be strengthened by quantification, e.g., “accelerations caused an average fluctuation in the force applied of X % participant body weight, and so are unlikely to have had a large influence on the motor patterns of participants.”

Line 554: Configuration should be plural. Similarly, knee press machine and knee extension sled should be plural, or else they should be preceded by the word “a”, e.g., “such as for use in a knee press machine…”

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Increased force and elastic energy storage are not the mechanisms that improve jump performance with accentuated eccentric loading during a constrained vertical jump

PONE-D-24-04333R2

Dear Dr. Su,

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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Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Thank you for further amending the manuscript and putting more emphasis on concerns raised by Reviewer 1. I will recommend the acceptance of the manuscript.

Reviewers' comments: