Maximum Force Capacity of Back Extensor Muscles in Healthy Women and Men: Not Different, if Anthropometrically Normalized
, Beatrice Steiniger †, Florian Saenger ‡, Martin Marks ‡, Lena Mader §, Evgenij Dukvin and Anna SchneiderRound 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for the opportunity to review this study.
Overall, the manuscript requires substantial improvement in scientific writing before it can be considered for publication. I strongly recommend that the authors seek professional editing support and carefully revise the entire text for clarity, coherence, and academic tone.
The introduction lacks balance and sufficient background. In particular, it provides limited discussion of trunk muscle performance, methods of assessment, and their relationship to low back pain. Expanding these areas would help better contextualize the study.
The statistical reporting is currently incomplete and needs clarification. It is not clear whether normality was assessed, and if so, which methods were used. Interpretation of the effect sizes is not reported. The inclusion and interpretation of 95 percent confidence intervals are also necessary. In addition, the use of post hoc tests appears inappropriate and should be reconsidered.
The discussion section does not clearly address the clinical relevance of the findings. The authors should elaborate on how their results relate to low back pain and explain how clinicians might apply these findings in practice.
Finally, the manuscript lacks a clear conclusions section. A concise summary of the main findings and their implications should be included.
Author Response
Thank you for the opportunity to review this study.
Overall, the manuscript requires substantial improvement in scientific writing before it can be considered for publication. I strongly recommend that the authors seek professional editing support and carefully revise the entire text for clarity, coherence, and academic tone.
Answer
We have carefully checked and revised the manuscript accordingly.
The introduction lacks balance and sufficient background. In particular, it provides limited discussion of trunk muscle performance, methods of assessment, and their relationship to low back pain. Expanding these areas would help better contextualize the study.
Answer
The introduction was expanded as requested.
The statistical reporting is currently incomplete and needs clarification. It is not clear whether normality was assessed, and if so, which methods were used. Interpretation of the effect sizes is not reported. The inclusion and interpretation of 95 percent confidence intervals are also necessary. In addition, the use of post hoc tests appears inappropriate and should be reconsidered.
Answer
We have now added the Test result for normality together with the applied tests. Also. The confidence intervals have now been provided.
In relation to your comment regarding the use of post hoc tests, we would like to reiterate that the presented analysis is based on data from previously published studies, which were re-analysed with respect to the parameters presented. We agree with the reviewer that post hoc analyses should be avoided; however, we would also like to ask how such analyses should be appropriately conducted. We would be grateful for any guidance in this regard.
The discussion section does not clearly address the clinical relevance of the findings. The authors should elaborate on how their results relate to low back pain and explain how clinicians might apply these findings in practice.
Answer
We have now expanded the discussion accordingly.
Finally, the manuscript lacks a clear conclusions section. A concise summary of the main findings and their implications should be included.
Answer
A conclusion was added, as requested.
Reviewer 2 Report
Comments and Suggestions for Authorsplease see my comments attached
Comments for author File: 
Comments.pdf
Author Response
First, I would like to recognize the authors for the work they have presented.
- I am a bit confused as to why p = 0.0055 is presented as not a relevant difference between sexes. I am not sure how to revise that, but it takes a while for the reader to understand what you mean.
Answer
We understand the concern regarding the traceability of the results. Therefore, the Abstract has been revised to once again explain in detail the basis for classifying results as relevant or non-relevant. The corresponding passage in the Methods section has also been slightly revised to improve clarity.
- In the abstract, it was stated that 124 females and 115 males were included, but in Table 1, you are presenting 115 women and 124 men. This is probably a reporting error that you should revise.
Answer
According to your and comments of the other reviewers we have carefully checked the cohort data. Due to changes with respect of the included studies some necessary adoptions in the submitted manuscript got lost. We included 239 participants: 115 females and 124 males of overall five previous studies. This has now been checked and changed throughout the manuscript
- In the introduction section, I would strongly support and explain why normalization of upper body torque (trunk extensors specifically) is superior to the rest of the outcome parameters.
Answer
We have now added a respective paragraph, detailing the superiority of normalized data in comparison to not normalized outcome parameters.
- Did you include participants from seven prior investigations or five? 5. Was there an overlap of participants?
Answer
In this analysis data of five prior investigations were included. There was no overlap between studies with respect to study participants. This has now been added to the respective section.
- Did you obtain information regarding the type and duration of exercise? It is important for overall strength as well as trunk strength.
Answer
We are not sure if we get this comment right.
If you are asking whether participants classified as physically active performed specific types of physical activity, this was not explicitly documented. However, we had defined extensive physical activity as an explicit exclusion criterion; accordingly, for example, intensive and specific strength training or endurance training did not occur. We are aware that specific training programs can influence the characteristics of trunk muscle activity, and this has already been published. Consequently, we ensured that only at most moderate levels of physical activity without specific training focus were included.
We have not added this statement as a revision to the manuscript but provide it here solely in response to the comment. If you would prefer an explicit inclusion in the manuscript text, we would be happy to do so.
- I think you should explain how you got the torque. You mention that the highest force value was taken as the MVC value and that this value was analyzed both as force and as torque. This needs to be explained in detail.
Answer
To calculate torque values from the measured force data we have determined the distance between the horizontal projection of the spina scapulae on the spine and the horizontal level of the iliac crest. This has now been added to the respective paragraph in the methods section.
- Also, your protocol needs to be explained in more detail so that someone can replicate the study. The text says that MVC was performed 3 times, but also says that the first trial was a practice at 50%. Was that an extra trial + the 3 true MVCs?
Answer
The Centaur device contains a separate MVC test program. This consists of three successive MVC tests. As we know from former investigations that MVC testing is sensitive according practice and motivation, we decided to use the first test for habituation purposes. We have now added the information about the test organization in the respective paragraph.
- Did you check for normality assumptions, homogeneity of variance, etc.?
Answer
Yes, we did. we have now added this particular information at the beginning of the statistics section.
- In the results section, p = 0.0055 is statistically significant. You should state that and then say but effect size and mean difference did not meet the predefined relevance thresholds or something similar to that. Otherwise, you are confusing the reader.
Answer
We aimed to avoid repeating previously described content. To prevent confusion, we have now included the relevant information in the Results section.
- What is the effect size for UBTR? You are providing two different values (0.42 and 0.36).
Answer
Thank you for figuring that out. This was a typo error. The correct ES for UBTR value is 0.364. We have corrected that in the manuscript.
- Also, is it UBT? Or UBW? If you use one on the table, you should have the footnote for the one used. Also, I would suggest using consistent terminology throughout, even though both may be correct. 1
Answer
In the device just force values were detected. This accounts for maximum force and also for the upper body weight. The unit was always Newton. For interindividual comparisons of force and also anthropometric measures we converted all force values into torque values. The procedure is provided in our answer to your question No. 7 and is also detailed in the manuscript.
We have now checked and corrected all respective descriptions.
- You need a figure legend.
Answer
We have now provided the respective legends for the figures.
- There are so many confounding variables that I would be a little bit careful with the conclusions. Also, utilizing t-tests for the analysis is probably not the best way to go. A regression analysis might have been better to test whether sex predicts MVC after adjusting for anthropometric variables.
Answer
We have now added more information at the end of the discussion to elaborate the matter in more detail. Also the limitations section was expanded accordingly. Regarding the mentioned regression analysis, we have now added the respective analysis. We thank the reviewer for this important hint which improves the results considerably.
Reviewer 3 Report
Comments and Suggestions for AuthorsThank you for the opportunity to review your manuscript, “Maximum force capacity of back extensor muscles in healthy women and men: not different, if anthropometrically normal ized”
This is a brief report comparing trunk extensor strength in 239 healthy adults (124 women and 115 men) using maximal voluntary contraction (MVC) tests. Initial results showed that men possess significantly greater absolute strength and torso weight than women. However, when strength was normalized to upper body weight, no relevant differences were found between the sexes. The study determined that both groups maintain a physiological strength reserve of approximately 100% of their upper body weight. This suggests that the lower absolute strength in women is compensated for by a lighter torso, resulting in similar functional capacities. Therefore, anthropometrically, relative back strength is comparable between men and women.
The aim was to investigate the extent to which such differences can be observed when strength values are normalized for anthropometric characteristics. The primary outcome parameter was the maximal strength capacity of the trunk extensors normalized to upper body weight.
The study is well-designed with an appropriate methodology and presented in an easy-to-follow format. The sample size of 239 people is substantial, and the statistical analysis is conservative. Furthermore, the inherent limitations of a cross-sectional experimental study are addressed.
Minor comments:
There is a discrepancy in the participant count. The authors mention a sample of 239 people in the Methods section (Line 80), but later refer to 258 participants in the Limitations section (Line 185). Please clarify this figure and ensure consistency throughout the manuscript.
While the normalization to upper body weight is insightful, the study would benefit from a more detailed discussion of body composition (e.g., muscle mass vs. body fat percentage). If these data were not available, this should be explicitly acknowledged in the limitations section, as muscle quality and distribution can influence MVC results.
I suggest expanding the discussion regarding the clinical implications of these findings. Additionally, it would be valuable for the authors to mention alternative or comparable assessment methods for clinicians or researchers who do not have access to the Centaur CTT device.
Author Response
Thank you for the opportunity to review your manuscript, “Maximum force capacity of back extensor muscles in healthy women and men: not different, if anthropometrically normalized”
This is a brief report comparing trunk extensor strength in 239 healthy adults (124 women and 115 men) using maximal voluntary contraction (MVC) tests. Initial results showed that men possess significantly greater absolute strength and torso weight than women. However, when strength was normalized to upper body weight, no relevant differences were found between the sexes. The study determined that both groups maintain a physiological strength reserve of approximately 100% of their upper body weight. This suggests that the lower absolute strength in women is compensated for by a lighter torso, resulting in similar functional capacities. Therefore, anthropometrically, relative back strength is comparable between men and women.
The aim was to investigate the extent to which such differences can be observed when strength values are normalized for anthropometric characteristics. The primary outcome parameter was the maximal strength capacity of the trunk extensors normalized to upper body weight.
The study is well-designed with an appropriate methodology and presented in an easy-to-follow format. The sample size of 239 people is substantial, and the statistical analysis is conservative. Furthermore, the inherent limitations of a cross-sectional experimental study are addressed.
Minor comments:
There is a discrepancy in the participant count. The authors mention a sample of 239 people in the Methods section (Line 80), but later refer to 258 participants in the Limitations section (Line 185). Please clarify this figure and ensure consistency throughout the manuscript.
Answer
According to your and comments of the other reviewers we have carefully checked the cohort data. Due to changes with respect of the included studies some necessary adoptions in the submitted manuscript got lost. We included 239 participants: 115 females and 124 males of overall five previous studies. This has now been checked and changed throughout the manuscript
While the normalization to upper body weight is insightful, the study would benefit from a more detailed discussion of body composition (e.g., muscle mass vs. body fat percentage). If these data were not available, this should be explicitly acknowledged in the limitations section, as muscle quality and distribution can influence MVC results.
Answer
We completely agree with the reviewer: the same height and weight (i.e. BMI) can be composed differently between subjects. We did not perform body composition analyses, as at the time of study execution was not in focus. On the other hand, we explicitly excluded overweight and also subjects who were physically active more than moderately (i.e. more than two times per week).
As the aim of this re- evaluation of back muscle force data was to normalize force data of both sexes to upper body weight, the found similar physiological reserve of 100% UBS seems an interesting side- effect of having established sex independent normative values of relative upper body extension force. These values can further be verified by other authors, but provide a reasonable basis. We have added a respective statement in the limitations section.
I suggest expanding the discussion regarding the clinical implications of these findings.
Answer
We have a respective paragraph at the end of the Discussion.
Additionally, it would be valuable for the authors to mention alternative or comparable assessment methods for clinicians or researchers who do not have access to the Centaur CTT device.
Answer
We have also added a respective paragraph in the limitations section.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe following section remains overly vague and should be substantially improved:
“Adequate functionality of the entire trunk musculature enables the necessary balance between spinal stability and mobility [12]. In particular, deficits in coordination of the abdominal muscles [15], as well as both coordinative insufficiencies [16,17] and, especially, strength deficits [18] of the back muscles, have been associated with nonspecific low back pain.”
The authors should better explain the underlying mechanisms and provide a clearer, more scientifically grounded rationale linking trunk muscle dysfunction to nonspecific low back pain.
Overall, the manuscript would benefit from more rigorous scientific writing and improved clarity throughout.
In addition, the use of post-hoc power analysis is not appropriate and should be reconsidered. There is extensive methodological literature showing that calculating statistical power after data collection does not provide meaningful or interpretable information regarding the study findings. Some relevant references include:
Dziak JJ, Dierker LC, Abar B. The Interpretation of Statistical Power after the Data have Been Gathered. Curr Psychol. 2020 Jun;39(3):870–877. doi:10.1007/s12144-018-0018-1.
Zhang Y, Hedo R, Rivera A, et al. Post hoc power analysis: is it an informative and meaningful analysis? General Psychiatry. 2019;32:e100069. doi:10.1136/gpsych-2019-100069.
Heckman MG, Davis JM 3rd, Crowson CS. Post Hoc Power Calculations: An Inappropriate Method for Interpreting the Findings of a Research Study. J Rheumatol. 2022;49(8):867–870. doi:10.3899/jrheum.211115.
Heinsberg LW, Weeks DE. Post hoc power is not informative. Genet Epidemiol. 2022;46:390–394. doi:10.1002/gepi.22464.
Accordingly, the authors should instead report confidence intervals for all principal outcomes, as confidence intervals provide substantially more informative evidence regarding the precision and clinical relevance of the observed effects
"Values substantially below this level are therefore of diagnostic relevance." This study cannot support this statement
Comments on the Quality of English LanguageOverall, the manuscript would benefit from more rigorous scientific writing and improved clarity throughout.
Author Response
The following section remains overly vague and should be substantially improved:
“Adequate functionality of the entire trunk musculature enables the necessary balance between spinal stability and mobility [12]. In particular, deficits in coordination of the abdominal muscles [15], as well as both coordinative insufficiencies [16,17] and, especially, strength deficits [18] of the back muscles, have been associated with nonspecific low back pain.”
The authors should better explain the underlying mechanisms and provide a clearer, more scientifically grounded rationale linking trunk muscle dysfunction to nonspecific low back pain.
Answer
We have now added the respective scientific background.
Overall, the manuscript would benefit from more rigorous scientific writing and improved clarity throughout.
Answer
We have modified the manuscript accordingly.
In addition, the use of post-hoc power analysis is not appropriate and should be reconsidered. There is extensive methodological literature showing that calculating statistical power after data collection does not provide meaningful or interpretable information regarding the study findings. Some relevant references include:
Dziak JJ, Dierker LC, Abar B. The Interpretation of Statistical Power after the Data have Been Gathered. Curr Psychol. 2020 Jun;39(3):870–877. doi:10.1007/s12144-018-0018-1.
Zhang Y, Hedo R, Rivera A, et al. Post hoc power analysis: is it an informative and meaningful analysis? General Psychiatry. 2019;32:e100069. doi:10.1136/gpsych-2019-100069.
Heckman MG, Davis JM 3rd, Crowson CS. Post Hoc Power Calculations: An Inappropriate Method for Interpreting the Findings of a Research Study. J Rheumatol. 2022;49(8):867–870. doi:10.3899/jrheum.211115.
Heinsberg LW, Weeks DE. Post hoc power is not informative. Genet Epidemiol. 2022;46:390–394. doi:10.1002/gepi.22464.
Accordingly, the authors should instead report confidence intervals for all principal outcomes, as confidence intervals provide substantially more informative evidence regarding the precision and clinical relevance of the observed effects
Answer
To clarify the matter: We did not use the post hoc analysis to justify the results retrospectively. Rather, this was intended to illustrate, based on conventional assumptions (effect size = 0.5, power = 0.8, α = 0.05), that the available sample size exceeded the number of observations required under such assumptions by approximately a factor of two. Our purpose was therefore to justify the issue of a “large sample size.” In our view, it cannot be assumed that readers are necessarily familiar with the sample sizes required for independent tests under standard assumptions.
With sample sizes of this magnitude, statistically significant p-values can be detected very easily, although such findings may not necessarily be substantively relevant. To avoid any potential misunderstanding, we have now removed this section and only retained the explanatory rationale for including effect size estimates and for considering the relationship between the DMV and the MID.
"Values substantially below this level are therefore of diagnostic relevance." This study cannot support this statement
Answer
This statement was included in response to a reviewer’s comment. We have now removed this passage.
Reviewer 2 Report
Comments and Suggestions for AuthorsI am happy with the revised version, and I have no further changes to recommend.
Author Response
I am happy with the revised version, and I have no further changes to recommend.
Thank you for the positive evaluation of the revised version of the manuscript.
Round 3
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have addressed my comments
I recommend to review the decimals for the effect sizes
Comments on the Quality of English LanguageOverall, the manuscript would benefit from more rigorous scientific writing and improved clarity throughout.
