Differences in Body Composition between Playing Positions in Men’s Professional Soccer: A Systematic Review with Meta-Analysis

Round 1

Reviewer 1 Report

Overall, a well described and thorough piece of work, which describes the current state of the art in this topic. I only have a couple of small comments for the authors to consider:

- In the methods section, it might be cleaner to have the search strategy as a supplementary material. Within the text it is hard to read, so having it available as supplementary would allow the reader to look at it if needed.

- In the discussion/conclusion there didn't seem to be much analysis beyond describing the differences in body measures between the playing positions. The paper could be improved if the authors explored the reasons behind the differences in these anthropometric characteristics between positions (i.e. goalkeepers are taller and heavier because they need to be able to have good arm reach and they don't run as far/make primarily explosive movements, etc.). Some further interpretation of the differences in measures in this way would add to the discussion. 

Author Response

Overall, a well described and thorough piece of work, which describes the current state of the art in this topic. I only have a couple of small comments for the authors to consider:

- In the methods section, it might be cleaner to have the search strategy as a supplementary material. Within the text it is hard to read, so having it available as supplementary would allow the reader to look at it if needed.

 

Response of the authors: In accordance with the reviewer's suggestion, the sentence has been modified to facilitate the reading of the study.

 

- In the discussion/conclusion there didn't seem to be much analysis beyond describing the differences in body measures between the playing positions. The paper could be improved if the authors explored the reasons behind the differences in these anthropometric characteristics between positions (i.e. goalkeepers are taller and heavier because they need to be able to have good arm reach and they don't run as far/make primarily explosive movements, etc.). Some further interpretation of the differences in measures in this way would add to the discussion.

 

Response of the authors: Following the reviewer's suggestions, additional information has been added to the discussion to justify the results obtained.

Reviewer 2 Report

Some important points need to be addressed in this first stage:

Title: Kinanthropometry-derived parameters include human sizes, shapes, proportions, body composition, maturation status, and gross functions, in order to understand growth, exercise, performance, and nutrition. Only body composition parameters are considered in this study and the relation with growth, exercise, performance, and nutrition are poorly addressed. Therefore I suggest replacing Kinanthropometry with body composition or better addressing the concept of Kinanthropometry.

Study selection and procedures. 

- You missed the following study: Bongiovanni et al . J. Funct. Morphol. Kinesiol. 2020, 5, 73; doi:10.3390/jfmk5040073. Bioimpedance Vector References Need to Be Period‐ Specific for Assessing Body Composition and Cellular Health in Elite Soccer Players: A Brief Report 

- studies with BIA were considered. However, one of the most important body composition parameters that can be estimated with BIA is body water was not considered in this study. Please include total body water (TBW) and its components in your research outputs. Also, lean soft tissue and appendicular lean soft tissue are missing too. Regarding anthropometric-derived parameters, the fat-free mass should be included. 

- The procedures used to estimate body composition parameters must be reported. For each component is required to specify the characteristics, as well as the regression model (formula), of the predictive equation used for the estimation. 

Results

- Table 2: In the protocol column anthropometric protocols, such as ISAK or ASRM are often reported for studies where BIA was applied. This makes no sense. On the contrary, BIA guidelines for athletes should have been considered: Assessment of Body Composition in Athletes: A Narrative Review of Available Methods with Special Reference to Quantitative and Qualitative Bioimpedance Analysis. Nutrients 2021, 13, 1620. https://doi.org/10.3390/nu13051620

- Figure 2: Please provide a new figure with a better resolution. 

- Figure 3 is very representative and provides a practical message. However, it needs the following revisions:

1. Regarding the skinfold thickness in mm, you should specify which skinfold sites are considered in this sum.

2. Fat mass: different materials (BIA, DXA, anthropometry) and methods (the use of different predictive equations as well as different BIA technologies, such as foot-to-hand, and standing position) result in different fat mass estimations. Therefore, you have two options: i) report the mean value in the figure specifying the provenience of the different outputs in the text ii) report different material and methods, and the relative outputs. Please see Figure 6 of this paper: Nutrients 2023, 15, 1160. https://doi.org/10.3390/nu15051160. I would prefer the second option.

3. Include a caption for each body composition component or instrument.

4. Muscle mass: same comment as above

5. Somatotype: specify that all the players are balanced mesomorphs, regardless of the role. 

6. I would change the icon of the midfielder with the icon of the forward players. 

Practical applications:

- A dedicated section explaining how the different body composition parameters are important for soccer players is missing. All the measures considered are part of a specific level of body composition. For example, muscle and adipose tissues are part of the 4 tissue levels, and the somatotype, is the body mass of the 5 levels (the whole-body level). If we talk about fat mass it is not possible to classify it at any level given that the mass is something non-specific (it is neither atomic, molecular, cellular, nor tissue), while if instead we refer to adipocytes, lipids or adipose tissue it is possible to include it in one of 5 levels (Wang et al. 1992.). In any case, it is necessary to specify how these measures are useful to practitioners and at what part of the season they should be assessed. Are all components equally informative during the same phases of the season, or are some more important in the return to play, others in the preseason, and others during the competitive periods?

- I have another important request that represents the basis for justifying the need for the study. Although athletic equations are scarce, a wide range of generalized equations (developed on general or non-athletic populations) is present in the literature. Of course, the use of athletic equations will result in a greater accuracy (less standard error of estimation) but my question is: what is the practical application of a difference of 1 or 2 kg in the estimation of the body component? what is the difference, in the sport context, to evaluate a soccer player with an FM% equal to 13 or 9 %? How does this gain in accuracy impact the work of practitioners or nutritionists? Because if in practical terms nothing changes the novelty and the significance of the body composition assessment in soccer are compromised. 

 

Author Response

Some important points need to be addressed in this first stage:

Title: Kinanthropometry-derived parameters include human sizes, shapes, proportions, body composition, maturation status, and gross functions, in order to understand growth, exercise, performance, and nutrition. Only body composition parameters are considered in this study and the relation with growth, exercise, performance, and nutrition are poorly addressed. Therefore I suggest replacing Kinanthropometry with body composition or better addressing the concept of Kinanthropometry.

 

Response of the authors: The title has been reformulated propperly.

 

Study selection and procedures.

- You missed the following study: Bongiovanni et al . J. Funct. Morphol. Kinesiol. 2020, 5, 73; doi:10.3390/jfmk5040073. Bioimpedance Vector References Need to Be Period‐ Specific for Assessing Body Composition and Cellular Health in Elite Soccer Players: A Brief Report.

 

Response of the authors: We appreciate the reviewer's comments to improve the manuscript. However, we discarded this study as it only included values for height, weight and height, without giving specific values for the use of bioimpedance on fat mass or fat-free mass (body water, bone mass or muscle mass).

 

- studies with BIA were considered. However, one of the most important body composition parameters that can be estimated with BIA is body water was not considered in this study. Please include total body water (TBW) and its components in your research outputs. Also, lean soft tissue and appendicular lean soft tissue are missing too. Regarding anthropometric-derived parameters, the fat-free mass should be included.

 

Response of the authors: Total body water, lean soft tissue and appendicular lean soft tissue were not included in the supplementary table or meta-analysis as only one study or none described these body composition values. Due to the importance of the clinical value of fat-free mass through anthropometry it is already included in the supplementary table, although only one study described it. In our previous published paper we were able to include total body water in the tables and meta-analysis. https://doi.org/10.3390/nu15051160

 

- The procedures used to estimate body composition parameters must be reported. For each component is required to specify the characteristics, as well as the regression model (formula), of the predictive equation used for the estimation.

 

Response of the authors: The anthropometric formulas used are described in the supplementary table. We decided not to exclude articles based on the anthropometric formulas used due to the limited number of studies included. However, the formulas used are frequently used in athletes.

 

Results

- Table 2: In the protocol column anthropometric protocols, such as ISAK or ASRM are often reported for studies where BIA was applied. This makes no sense. On the contrary, BIA guidelines for athletes should have been considered: Assessment of Body Composition in Athletes: A Narrative Review of Available Methods with Special Reference to Quantitative and Qualitative Bioimpedance Analysis. Nutrients 2021, 13, 1620. https://doi.org/10.3390/nu13051620

 

Response of the authors: We appreciate the reviewer's comments. However, this is a limitation that has been specified in the limitations section due to the information we have extracted from the studies performed in BIA and DXA. In the studies where an anthropometric protocol is indicated and bioimpedance is present, bioimpedance was used to measure weight (or at least this is the data provided by the authors), except for the article by Castro Jimenez et al, which combines both methods.

 

- Figure 2: Please provide a new figure with a better resolution.

 

Response of the authors: The resolution of figure 2 has been improved following the instructions for authors.

 

- Figure 3 is very representative and provides a practical message. However, it needs the following revisions: 1. Regarding the skinfold thickness in mm, you should specify which skinfold sites are considered in this sum.

 

Response of the authors: The figure and caption has been reformulated properly.

 

2. Fat mass: different materials (BIA, DXA, anthropometry) and methods (the use of different predictive equations as well as different BIA technologies, such as foot-to-hand, and standing position) result in different fat mass estimations. Therefore, you have two options: i) report the mean value in the figure specifying the provenience of the different outputs in the text ii) report different material and methods, and the relative outputs. Please see Figure 6 of this paper: Nutrients 2023, 15, 1160. https://doi.org/10.3390/nu15051160. I would prefer the second option.

 

Response of the authors: Due to the limited number of studies included, we can only provide data in the form of the mean of the 3 measurement instruments for fat mass and muscle mass values to describe the different playing positions specifically. In our previous paper (from which the figure you mention comes from) we decided to make that figure because the aim of the paper was to evaluate the differences between the different measurement methods, not by playing positions. In addition, the values shown in this figure correspond to the values obtained with the meta-analysis performed.

 

3. Include a caption for each body composition component or instrument.

 

Response of the authors: The caption has been reformulated properly.

 

4. Muscle mass: same comment as above

 

Response of the authors: Due to the limited number of studies included, we can only provide data in the form of the mean of the 3 measurement instruments for fat mass and muscle mass values to describe the different playing positions specifically. In our previous paper (from which the figure you mention comes from) we decided to make that figure because the aim of the paper was to evaluate the differences between the different measurement methods, not by playing positions. In addition, the values shown in this figure correspond to the values obtained with the meta-analysis performed.

 

5. Somatotype: specify that all the players are balanced mesomorphs, regardless of the role.

 

Response of the authors: The figure has been edited properly.

 

6. I would change the icon of the midfielder with the icon of the forward players.

 

Response of the authors: Thank you for the comment. We have done it.

 

Practical applications:

- A dedicated section explaining how the different body composition parameters are important for soccer players is missing. All the measures considered are part of a specific level of body composition. For example, muscle and adipose tissues are part of the 4 tissue levels, and the somatotype, is the body mass of the 5 levels (the whole-body level). If we talk about fat mass it is not possible to classify it at any level given that the mass is something non-specific (it is neither atomic, molecular, cellular, nor tissue), while if instead we refer to adipocytes, lipids or adipose tissue it is possible to include it in one of 5 levels (Wang et al. 1992.). In any case, it is necessary to specify how these measures are useful to practitioners and at what part of the season they should be assessed. Are all components equally informative during the same phases of the season, or are some more important in the return to play, others in the preseason, and others during the competitive periods?

 

Response of the authors: We have extended the discussion to justify our results for the different playing positions. In addition, we have added in Future research and practical application a new point.

 

- I have another important request that represents the basis for justifying the need for the study. Although athletic equations are scarce, a wide range of generalized equations (developed on general or non-athletic populations) is present in the literature. Of course, the use of athletic equations will result in a greater accuracy (less standard error of estimation) but my question is: what is the practical application of a difference of 1 or 2 kg in the estimation of the body component? what is the difference, in the sport context, to evaluate a soccer player with an FM% equal to 13 or 9 %? How does this gain in accuracy impact the work of practitioners or nutritionists? Because if in practical terms nothing changes the novelty and the significance of the body composition assessment in soccer are compromised.

 

Response of the authors: In elite soccer there are economic sanctions on soccer players in their body composition measurements, with clubs having reference values especially for total weight and fat mass, although these values are usually based either on isolated scientific articles or on previous values recorded by the club. This article has been written with the aim of providing guidance not only to adjust these reference values based on scientific evidence with the largest possible number of subjects (in this case, 3117 professional soccer players), but also to help the technical and medical staff in taking economic sanctions to be applied. In addition, it will also serve to focus not only on total weight and fat mass, but also on the rest of the body compartments such as bone mass and muscle mass.

Round 2

Reviewer 2 Report

The majority of the previous comments and suggestions were not properly addressed. 

Author Response

We are sorry for any inconvenience caused as we believe we have addressed the reviewer's suggestions. However, we have improved the manuscript, expanding the information in the discussion section and taking into account the reviewer's comments.

The authors' responses to each of the reviewer's comments are restated below.

 

Reviewer's comments

Some important points need to be addressed in this first stage:

Title: Kinanthropometry-derived parameters include human sizes, shapes, proportions, body composition, maturation status, and gross functions, in order to understand growth, exercise, performance, and nutrition. Only body composition parameters are considered in this study and the relation with growth, exercise, performance, and nutrition are poorly addressed. Therefore I suggest replacing Kinanthropometry with body composition or better addressing the concept of Kinanthropometry.

 Response of the authors: The title has been reformulated propperly.

 

Study selection and procedures.

- You missed the following study: Bongiovanni et al . J. Funct. Morphol. Kinesiol. 2020, 5, 73; doi:10.3390/jfmk5040073. Bioimpedance Vector References Need to Be Period‐ Specific for Assessing Body Composition and Cellular Health in Elite Soccer Players: A Brief Report.

Response of the authors: We appreciate the reviewer's comments to improve the manuscript. However, we discarded this study as it only included values for height, weight and height, without giving specific values for the use of bioimpedance on fat mass or fat-free mass (body water, bone mass or muscle mass). This article does not include the data we need on body composition by playing positions.

 

- studies with BIA were considered. However, one of the most important body composition parameters that can be estimated with BIA is body water was not considered in this study. Please include total body water (TBW) and its components in your research outputs. Also, lean soft tissue and appendicular lean soft tissue are missing too. Regarding anthropometric-derived parameters, the fat-free mass should be included.

Response of the authors: Total body water, lean soft tissue and appendicular lean soft tissue were not included in the supplementary table or meta-analysis as only one study or none described these body composition values. Due to the importance of the clinical value of fat-free mass through anthropometry it is already included in the supplementary table, although only one study described it. The data on total body water can be found in another article recently published by us (systematic review with meta-analysis), whose objective was "to describe the anthropometric characteristics, body composition, and somatotype of professional male soccer players, and to compare the values reported according to the methods and equations used”: https://doi.org/10.3390/nu15051160

Even so, this aspect has been indicated in the limitations section.

 

- The procedures used to estimate body composition parameters must be reported. For each component is required to specify the characteristics, as well as the regression model (formula), of the predictive equation used for the estimation.

Response of the authors: The anthropometric formulas used are described in the supplementary table because it is not part of the aim of the study. Furthermore, We decided not to exclude articles based on the anthropometric formulas used due to the limited number of studies included. However, as the formulas used are frequently used in athletes, readers have been told where to find this information (results section) and this issue has been addressed in the discussion section.

In any case, if the reviewer considers it appropriate that the supplementary tables are part of the manuscript, we can place them in the results section.

 

Results

- Table 2: In the protocol column anthropometric protocols, such as ISAK or ASRM are often reported for studies where BIA was applied. This makes no sense. On the contrary, BIA guidelines for athletes should have been considered: Assessment of Body Composition in Athletes: A Narrative Review of Available Methods with Special Reference to Quantitative and Qualitative Bioimpedance Analysis. Nutrients 2021, 13, 1620. https://doi.org/10.3390/nu13051620

Response of the authors: We appreciate the reviewer's comments. However, this is a limitation that has been specified in the limitations section due to the information we have extracted from the studies performed in BIA and DXA. In the studies where an anthropometric protocol is indicated and bioimpedance is present, bioimpedance was used to measure weight (or at least this is the data provided by the authors), except for the article by Castro Jimenez et al, which combines both methods.

 

- Figure 2: Please provide a new figure with a better resolution.

Response of the authors: The resolution of figure 2 has been improved following the instructions for authors.

 

- Figure 3 is very representative and provides a practical message. However, it needs the following revisions: 1. Regarding the skinfold thickness in mm, you should specify which skinfold sites are considered in this sum.

Response of the authors: The figure and caption has been reformulated properly.

 

2. Fat mass: different materials (BIA, DXA, anthropometry) and methods (the use of different predictive equations as well as different BIA technologies, such as foot-to-hand, and standing position) result in different fat mass estimations. Therefore, you have two options: i) report the mean value in the figure specifying the provenience of the different outputs in the text ii) report different material and methods, and the relative outputs. Please see Figure 6 of this paper: Nutrients 2023, 15, 1160. https://doi.org/10.3390/nu15051160. I would prefer the second option.

Response of the authors: Due to the limited number of studies included, we can only provide data in the form of the mean of the 3 measurement instruments for fat mass and muscle mass values to describe the different playing positions specifically. In our previous paper (from which the figure you mention comes from) we decided to make that figure because the aim of the paper was to evaluate the differences between the different measurement methods, not by playing positions. In addition, the values shown in this figure correspond to the values obtained with the meta-analysis performed.

 

3. Include a caption for each body composition component or instrument.

Response of the authors: The caption has been reformulated properly.

 

4. Muscle mass: same comment as above

Response of the authors: Due to the limited number of studies included, we can only provide data in the form of the mean of the 3 measurement instruments for fat mass and muscle mass values to describe the different playing positions specifically. In our previous paper (from which the figure you mention comes from) we decided to make that figure because the aim of the paper was to evaluate the differences between the different measurement methods, not by playing positions. In addition, the values shown in this figure correspond to the values obtained with the meta-analysis performed.

 

5. Somatotype: specify that all the players are balanced mesomorphs, regardless of the role.

Response of the authors: The figure has been edited properly.

 

6. I would change the icon of the midfielder with the icon of the forward players.

Response of the authors: Thank you for the comment. We have done it.

 

Practical applications:

- A dedicated section explaining how the different body composition parameters are important for soccer players is missing. All the measures considered are part of a specific level of body composition. For example, muscle and adipose tissues are part of the 4 tissue levels, and the somatotype, is the body mass of the 5 levels (the whole-body level). If we talk about fat mass it is not possible to classify it at any level given that the mass is something non-specific (it is neither atomic, molecular, cellular, nor tissue), while if instead we refer to adipocytes, lipids or adipose tissue it is possible to include it in one of 5 levels (Wang et al. 1992.). In any case, it is necessary to specify how these measures are useful to practitioners and at what part of the season they should be assessed. Are all components equally informative during the same phases of the season, or are some more important in the return to play, others in the preseason, and others during the competitive periods?

Response of the authors: We have extended the discussion to justify our results for the different playing positions in discussion section. In addition, we have added in Future research and practical application a new point.

 

- I have another important request that represents the basis for justifying the need for the study. Although athletic equations are scarce, a wide range of generalized equations (developed on general or non-athletic populations) is present in the literature. Of course, the use of athletic equations will result in a greater accuracy (less standard error of estimation) but my question is: what is the practical application of a difference of 1 or 2 kg in the estimation of the body component? what is the difference, in the sport context, to evaluate a soccer player with an FM% equal to 13 or 9 %? How does this gain in accuracy impact the work of practitioners or nutritionists? Because if in practical terms nothing changes the novelty and the significance of the body composition assessment in soccer are compromised.

Response of the authors: In elite soccer there are economic sanctions on soccer players in their body composition measurements, with clubs having reference values especially for total weight and fat mass, although these values are usually based either on isolated scientific articles or on previous values recorded by the club. This article has been written with the aim of providing guidance not only to adjust these reference values based on scientific evidence with the largest possible number of subjects (in this case, 3117 professional soccer players), but also to help the technical and medical staff in taking economic sanctions to be applied. In addition, it will also serve to focus not only on total weight and fat mass, but also on the rest of the body compartments such as bone mass and muscle mass.

The discussion section has also been improved following the reviewer's instructions.

 

Round 3

Reviewer 2 Report

No further comments

Author Response

Thank you for your time and review.