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Additional Editor Comments:

Dear Authors,

The reviewers have made interesting comments that will help enhance your manuscript. Please follow closely their recommendations.

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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?

Reviewer #1: Yes

Reviewer #2: Yes

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

Reviewer #1: N/A

Reviewer #2: 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

Reviewer #1: Yes

Reviewer #2: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1:  This original research paper describes a Delphi study to develop a method of develop a definition of ‘heading of the ball’ (i.e., heading) in soccer and a protocol to assess the skill characteristics for the purposes of injury surveillance and technique analysis. The protocol is designed to be applied primarily to video, but other data sources (e.g., accelerometery) are also included. As far as I can determine, the study has not been published elsewhere.

The participants and methods used for the Delphi study are appropriate, largely concordant with published guidelines (ACCORD and CREDES) and reported in sufficient detail. One hundred and sixty-seven 167 participants with experience in research and/or soccer who commenced the Delphi study. The Delphi study consisted of five rounds over three phases to select the characteristics that should comprise the minimum analysis, and determine definitions. The first phase determined the fields (described as descriptors) for the analysis, the second phase defined the fields while the final phase determined the method or list of field variables used to assess/describe each field. Of the five rounds of consensus, 50% of participants completed all 5 rounds. Although the attrition is of some concern, 80+ participants is still substantial and efforts are made to report on the characteristics of respondents at each step. Subsequently, 27 fields were selected for inclusion in the minimal analysis and a further 27 characteristics, deemed not as important by participants in the Delphi study, were also defined for an expanded analysis.

The described protocol will provide a standardised approach to identifying a header and investigating characteristics of heading in soccer from video, allowing the incidence of heading and the characteristics analysed to be determined and, when combined with injury surveillance data, potentially allowing for causal inference. Whilst there are some characteristics which I expect will be challenging to evaluate reliably from video, this is beyond the scope of the current study and the authors have wisely encouraged reporting of reliability statistics. It is my opinion that the manuscript meets the aims and scope of PLoSOne and would be of interest to its readers, subject to consideration of several revisions.

Firstly, I would encourage the authors to thoroughly review and edit the manuscript to improve its readability. There were many instances where the language used is unclear. I provide three of the many examples throughout the manuscript:

“Heading incidence research could be explored as one approach of assessing the potential causal inference between heading and long-term issues with brain health” - Does you mean that you wish to explore heading incidence research or that you want to investigate the incidence of heading and heading characteristics? I believe it's the latter but the expression is overly complex. Similarly, "our understanding of heading risk then THIS needs" (lines122-3). What do you mean by THIS?, please be specific so that it is unambiguous for the reader. Finally "Research methods experts, having... consensus projects" (lines 139-40) could be simplified as "Delphi and consensus study experts", or similar. Please be clear and explicit throughout. Additionally, there are many typos throughout which need correction (e.g., “insures” vs ensures (line 116) or “can” vs could (line 118), “whereby” vs thereby (line 441), the repeated phrase on line 468, etc.).

Secondly, the introduction should be expanded to better position this work with respect to previous literature. The authors could better introduce the discussion in the field regarding ball heading and its long-term health impacts and define also define the difference between heading and non-heading impacts. From the context and literature, I suspected non-heading impacts may refer to be head-to-head contact and it wasn’t clear until very late in the manuscript that this wasn’t the case. The introduction could also briefly introduce efforts from soccer and other sports to define injury events and develop qualitative analysis methods (e.g., Tenga’s work, volleyball, tackling in rugby union etc.), what was good and what could be improved. This would then allow a discussion point on how this study improved on earlier studies, thus broadening the relevance of this study beyond heading incidence.

Thirdly, relating to clarity of reporting, I note that the project was part funded through an academic prize at an Australian university although ethics approval was obtained in Switzerland. Were there other sources of funding (e.g., FIFA) which should be included?

The direction to include reliability in studies reporting incidence with the protocol is a good one. There are several instances where Cohen’s Kappa is not appropriate, such as continuous data, multiple raters (i.e., > 2) or where one variable occurs at a substantially higher frequency than others. For this reason, the authors might consider something like “agreement of >0.80 with an appropriate agreement statistic (e.g., cohen’s Kappa, Fleiss Kappa, ICC, percentage agreement, etc.)”

Finally, the reason for hijab being bracketed with protective equipment seems unusual and needs to be better explained to the reader. The other items listed on page 16 seem have an obvious relationship with skill and performance and so need little justification. Head covering on the other hand states that participants suggested that it be included but why it is important wasn’t mentioned in this article or in the associated systematic review. Did the expert panel form the view that a hijab attenuated head force and reduce the risk of injury or might obscure vision and increase the risk of injury etc?

Reviewer #2:  Firstly, I would like to congratulation the authors for undertaking a comprehensive Delphi study. A strength of this study is the inclusion of an large number of participants.

My major comment/concern is regarding the focus on ‘incidence’ – but then limited explanation of how one should accurately and robustly collect exposure. Given exposure time is the denominator for incidence, this will have a large impact. Especially in training – exposure could be continuous (start – end), drill duration (removal of ‘dead time’) or player involvement in the drills. I think it is the authors either point the reader to where this exists, and recommend the use of an existing definition, or include this.

I do wonder if this is more than incidence – for examples some of definitions are beyond heading incidence. Isn’t heading incidence only one part of the story, especially when considering the results of this study even say that magnitude is important. Therefore, isn’t this a consensus for heading studies?

I was also surprised that a head-head collision (during a header) wasn’t included. I’m not sure if this was deliberately excluded (based on the study design), or no one thought this was important, but it does feel like it warrants more discussion. E.g., if we know the head-head collision rates during different heading activities / in different cohorts – that is very helpful when considering ‘short-term’ head injury risk? This was discussed briefly in lines 476-478, and 540-543, but it seems an omission.

My minor comments are;

At first mention (in abstract and in main documents), report ‘football (i.e., soccer)’

You report short and long-term risk in abstract, but don’t define them in the introduction. I would suggest reporting the ‘short-, medium-, and longer-term risk’…. (and defining).

Key points – include clarify what is the denominator for calculating 1000 hours.

Key points (line 108). ‘Furthermore’, not ‘further’.

Introduction (line 119). I’m not sure what is encouraging? I couldn’t follow what ‘This is’ was referring to.

Introduction (line 124). A recent systematic review needs a reference

Method – I didn’t find the method very clear early on, and think a study design or overview would be helpful. For example, the ‘each objective and phase (1-3) on line 176 came out of the blue, and I didn’t know what each objective was, as the objective (singular – not pleural) on line 129-131 is set out as one objective. Figure 1 needs to do a better job of clarifying what the three phases are – which are nicely summarised on line 177-179.

Method (line 153). In the journal information, you report University of Sydney as the ethics organisation, then Swiss Association…. In the main text. Please clarify.

Results (line 315) The abbreviations (e.g., CAF, CONCACAF) on line 315 come before they are written in full (lines 330-333).

Results (lines 326-328) The percentages reported add up to 99%

Table 1. The numbers (112, 67%) in the last row of the men or women column, don’t follow what were previous reported, therefore aren’t intuitive. It would be useful to report men and women?

Limitations (line 474, 516, 529). ‘Furthermore’, not ‘further’.

Discussion (line 507) expand on how to capture exposure as the denominator.

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Reviewer #2: Yes:  Ben Jones

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Consensus on the descriptors, definitions, and reporting methods for heading in football studies:  A Delphi study

PONE-D-25-11018R1

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Mario Lopes, Ph.D

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Dear Authors,

The authors have attended to all suggestions made by the reviewers. I congratulate the effort placed to enhance this manuscript. I consider the manuscript ready for publishing.

Reviewers' comments: