Back Muscle Strength Is Associated with Self-Reported Morning-Erection Frequency in Apparently Healthy Japanese Male University Students: A Cross-Sectional Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

I appreciate the opportunity to review this manuscript, the subject matter of which aligns closely with my current research lines regarding physical fitness and reproductive health. This review has been conducted with the utmost technical rigour to provide constructive value to the study, aiming to enhance its quality and ensure the work achieves the impact and recognition it deserves within the international scientific community.

The study addresses an intriguing gap in male reproductive health by focusing on healthy young men, using morning erections as a non-invasive physiological indicator independent of sexual intercourse. The use of back muscle strength as a predictor is novel, distinguishing this work from studies relying on more common measures such as BMI or grip strength. Furthermore, the statistical methodology is robust, employing Principal Component Analysis (PCA) and clustering to validate physical profiles.

Specific Recommendations: Abstract: Key Findings: The results indicate that 50% of students report infrequent erections. I suggest highlighting this specific datum in the Conclusion of the Abstract, as it represents a highly relevant public health finding for the Japanese population.

Methodology and Measurement. Validation of the primary variable: The authors acknowledge that using a single-item measure for morning erection frequency has limited psychometric validation in this cohort. It is recommended to discuss more thoroughly how this might affect data accuracy and whether future alternatives (such as Rigiscan devices) could be considered for validation studies.

Subjective nature of the EHS: The Erection Hardness Score (EHS) may be hypothetical for participants without sexual experience. The authors should explicitly state whether participants were screened for previous sexual experience or if this was addressed as a potential bias.

Data Analysis. Sample size: An a priori sample size calculation was not performed. Although this is an exploratory study, the authors should include a post-hoc power analysis to justify whether the sample of 125 participants is sufficient to support the conclusions drawn.

Dichotomisation of variables: Erection frequency was divided a priori into "low" (scores 1–3) and "high" (4–6). It would be appropriate to justify the selection of this specific cut-off point over others (e.g., how "occasionally" was categorised).

Results and Discussion. Causality: The cross-sectional design precludes establishing causality. The discussion should be strengthened by clarifying that greater back strength might serve as a marker of an overall healthy lifestyle rather than being a direct cause of improved erectile function.

Confounding factors: While adjustments were made for BMI, age, and smoking, other unmeasured factors (e.g., testosterone levels, academic stress, or specific diet) may exist. The authors should consider adding a detailed section on "unmeasured confounders" within the study limitations.

Interpretation of PCA: The cluster analysis showed that the group with higher BMI and strength had more frequent erections. The relationship between BMI and strength in this group requires clarification; while a high BMI is usually interpreted negatively, in a cohort of physiotherapy students (who are often athletes), it may indicate higher muscle mass rather than adiposity.

Concluding Remarks. The article is original and well-structured. However, it requires a deeper discussion regarding the limitations of self-reported measures and further justification of the sample to strengthen its scientific validity.

Author Response

Comment 1

Specific Recommendations: Abstract: Key Findings: The results indicate that 50% of students report infrequent erections. I suggest highlighting this specific datum in the Conclusion of the Abstract, as it represents a highly relevant public health finding for the Japanese population.

 

Response 1:

Thank you for your careful review of our manuscript.

We had originally included the statement, "Over 50% of healthy Japanese male university students reported infrequent morning erections, suggesting that low nocturnal erectile indicators may be prevalent even in early adulthood," in the Highlights section. However, we completely agree with your point that the Highlights section cannot replace the Conclusion in the Abstract.

Because the Abstract has a strict 250-word limit, we have shortened the Background and Objectives sections. This allowed us to add the following sentence to the Conclusion of the Abstract: "In this cohort, 59.2% reported infrequent morning erections, suggesting potential relevance even in early adulthood." (Lines 57-58).

 

Comment 2

Methodology and Measurement. Validation of the primary variable: The authors acknowledge that using a single-item measure for morning erection frequency has limited psychometric validation in this cohort. It is recommended to discuss more thoroughly how this might affect data accuracy and whether future alternatives (such as Rigiscan devices) could be considered for validation studies.

 

Response 2:

Thank you for pointing this out. In this study, we evaluated morning-erection frequency using a single self-reported item. We agree that a major limitation is that the psychometric validity and reliability of this item are not fully established in this age group.

Therefore, we recognize that several biases might occur, including recall bias, social desirability bias, and bias from participants avoiding the question. We understand that these biases could potentially cause an underestimation of the associations we found in our study.

To address this limitation in the future, it would be desirable to conduct validation studies for this low-burden question. Such studies could check its consistency with existing multi-item scales and, when possible, use objective NPT measurements (e.g., RigiScan) in a subset of participants.

We have added this point to the Limitations section: "Morning-erection frequency was assessed via a single self-reported item without specific psychometric validation for this age group, leaving room for potential recall or social desirability biases. Validation studies using established multi-item questionnaires or objective NPT measures (e.g., RigiScan) in a subset are warranted." (Lines 557-559).

 

 

Comment 3

Subjective nature of the EHS: The Erection Hardness Score (EHS) may be hypothetical for participants without sexual experience. The authors should explicitly state whether participants were screened for previous sexual experience or if this was addressed as a potential bias.

 

Response 3:

Thank you for your comment. In this study, to protect participant privacy and reduce the burden of answering the questionnaire, we did not collect data on previous sexual experience (specifically, the experience of penetrative sexual intercourse) as a separate item.

Because the EHS is an assessment tool that inherently assumes penetration, self-evaluations may be hypothetical and speculative in participants with little or no experience with penetrative intercourse. We have already noted in the Limitations section that this issue could be a source of bias.

To clarify that this bias might affect how the EHS is interpreted, we have also added the following sentence to the Methods section: "Participants were not systematically screened for penetrative sexual experience; therefore, EHS responses may be hypothetical or speculative for some individuals who have little or no experience of such sexual intercourse." (Lines 161-163).

 

Comment 4

Data Analysis. Sample size: An a priori sample size calculation was not performed. Although this is an exploratory study, the authors should include a post-hoc power analysis to justify whether the sample of 125 participants is sufficient to support the conclusions drawn.

 

Response 4:

Thank you for your comment. As you have pointed out, this study was planned as an exploratory study, so we did not perform an a priori sample size calculation.

For your reference, in our main analysis (multivariate logistic regression: BMI + grip strength + back muscle strength), the number of events was 51. Since there are three main explanatory variables, the events per variable (EPV) ratio is approximately 17. We would like to note that this ratio ensures a reasonable degree of stability for estimating the coefficients.

Regarding post-hoc power, we followed recent methodological recommendations to avoid such calculations, as they merely reflect the observed p-values. Instead, we emphasized the precision of our findings using 95% confidence intervals.

While this study is appropriate for generating hypotheses, we acknowledge that larger studies with longitudinal designs are needed to estimate the effects more precisely.

To address this point, and in combination with our revisions for another reviewer's comment, we have added the following text to the Limitations section: " No a priori sample size calculation was performed; some estimates were imprecise, several p-values were close to the conventional threshold, and confidence intervals were wide, suggesting limited power for modest effects. Therefore, findings should be interpreted with caution and require replication." (Lines 551-555).

 

Comment 5

Dichotomisation of variables: Erection frequency was divided a priori into "low" (scores 1–3) and "high" (4–6). It would be appropriate to justify the selection of this specific cut-off point over others (e.g., how "occasionally" was categorised).

 

Response 5:

Thank you for your comment. We pre-specified the cutoff of 4–6 versus 1–3 for our main analysis because a score of 4 ("once every 2 days," or at least every other day) provides a relatively clear frequency anchor. From a clinical and public health perspective, we considered this an intuitive boundary to distinguish between a "regularly occurring" group and an "infrequent" group.

To ensure the clarity in this reasoning, we have added the following explanation to the Methods section: "because score 4 ('every other day') represents a face-valid threshold for regular morning erections" (Lines 148-149).

Furthermore, to reduce any concern of arbitrariness in choosing this cutoff, we would like to point out that we have already emphasized in the manuscript that our results are robust. We confirmed this robustness through sensitivity analyses that sequentially changed the thresholds (Table S9), as well as through ordinal logistic regression.

 

Comment 6

Results and Discussion. Causality: The cross-sectional design precludes establishing causality. The discussion should be strengthened by clarifying that greater back strength might serve as a marker of an overall healthy lifestyle rather than being a direct cause of improved erectile function.

 

Response 6

Thank you for this insightful comment. We have already stated in the Limitations section that causality cannot be determined due to the cross-sectional design of our study.

To explicitly show in the Discussion that back muscle strength might be a marker of overall health behaviors and conditioning (such as exercise habits, sleep, stress, and diet) rather than a direct cause, we have revised relevant text. We modified the original expression as following: "Trunk strength activities require coordinated neuromuscular function of large muscle groups and may serve as a marker of overall conditioning, recuperation, and health behaviors rather than a direct causal determinant of erectile physiology" (Lines 468-471).

We made this enhancement to ensure that our findings are not misinterpreted as indicating a causal relationship.

 

Comment 7

Confounding factors: While adjustments were made for BMI, age, and smoking, other unmeasured factors (e.g., testosterone levels, academic stress, or specific diet) may exist. The authors should consider adding a detailed section on "unmeasured confounders" within the study limitations.

 

Response 7

Thank you for your comment. We have added a mention of academic stress and medication/supplements use to the Limitations section (Line 570).

 

Comment 8

Interpretation of PCA: The cluster analysis showed that the group with higher BMI and strength had more frequent erections. The relationship between BMI and strength in this group requires clarification; while a high BMI is usually interpreted negatively, in a cohort of physiotherapy students (who are often athletes), it may indicate higher muscle mass rather than adiposity.

 

Response 8

Thank you for your comment. The BMI of Cluster 2 is 23.17 ± 2.25, which does not indicate extreme obesity. Therefore, the cluster results do not imply that "a higher BMI is better." Instead, they should be interpreted as an overall characteristic that includes greater muscle strength, a larger physique, and potentially greater lean mass.

To clarify this point, we have added the following text: "In this cohort, the higher BMI observed in the stronger cluster may partly reflect greater lean mass rather than adiposity; however, since body composition was not assessed, […]" (Lines 449-501).

Additionally, we would like to reiterate that BMI alone was not significant in the regression model.

 

Comment 9

Concluding Remarks. The article is original and well-structured. However, it requires a deeper discussion regarding the limitations of self-reported measures and further justification of the sample to strengthen its scientific validity.

 

Response 9

Thank you for your feedback. To summarize our earlier discussions regarding the limitations of relying on self-reported measures, we define three main points. First, morning-erection frequency was assessed using a single self-reported item. Second, the EHS may involve hypothetical answers for participants with little to no sexual experience. Third, non-random missing data may occur due to the sensitive nature of questions regarding sexuality.

Regarding the justification of our sample, we believe its validity is supported by two factors. We have clearly explained that the participants are physical therapy students (which limits generalizability), and we have compared the included participants with those in the excluded group to alleviate concerns about selection bias.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents an interesting and well-structured exploratory analysis examining the association between back muscle strength and self-reported morning-erection frequency in apparently healthy young men. The study addresses a relatively understudied population and introduces trunk-related strength as a potentially relevant physiological correlate of nocturnal erectile indicators.

The manuscript is clearly written, methodologically transparent, and appropriately cautious in its interpretation. The authors explicitly acknowledge the cross-sectional design, modest discrimination (AUC ≈ 0.63), and exploratory nature of the findings. The sensitivity analyses are comprehensive and strengthen the internal consistency of the results.

Overall, this is a well-prepared manuscript that contributes hypothesis-generating insights to the field of sexual and reproductive health in young men.

Suggestions for Improvement

While the study is carefully conducted, several refinements may further strengthen the manuscript:

1. Clarification of Statistical Power

Although the study is framed as exploratory, a brief comment regarding statistical power or potential fragility of borderline p-values (e.g., p = 0.045; p = 0.049) would improve transparency.

2. Multiple Testing Considerations

Given the number of sensitivity analyses and alternative dichotomization thresholds, the authors may consider briefly acknowledging the potential for type I error inflation.

3. Measurement Considerations

The use of a single-item morning-erection frequency measure is pragmatically justified; however, reiterating its non-validated nature in this age group within the Discussion could further strengthen methodological balance.

4. Contextual Framing and Emerging Evidence on Muscle Mass, Testosterone, and Erectile Function

The authors are encouraged to more explicitly integrate the emerging 2025 evidence examining the relationship between skeletal muscle mass/quality, testosterone status, and erectile dysfunction. Although still limited in number, recent clinical investigations have provided important data suggesting that sarcopenia and muscle quality parameters may independently influence erectile function and even treatment responsiveness in adult male populations.

Given the conceptual proximity of these findings to the present results, incorporating this line of evidence would substantially strengthen the biological plausibility of the proposed muscle–erectile function axis. Such integration would help bridge muscular physiology, endocrine regulation, and sexual function outcomes, thereby enhancing the mechanistic coherence and translational positioning of the manuscript.

In addition, referencing recent international guideline documents (e.g., EAU and/or AUA guidelines on erectile dysfunction) may further reinforce the clinical and public health framing of the study.

5. Mechanistic Discussion

The mechanistic hypotheses involving myokines and autonomic regulation are scientifically interesting and thoughtfully presented. However, a slightly more concise presentation may help maintain proportionality relative to the modest observed effect size and discrimination metrics.

 

Strengths

Focus on a young and underrepresented population

Objective dynamometry-based strength assessment

Intercourse-independent outcome measure

Extensive sensitivity analyses

Balanced interpretation and avoidance of overclaiming

Conclusion

The manuscript is scientifically sound, clearly presented, and suitable for publication following minor refinements. The findings are appropriately framed as hypothesis-generating and may stimulate further longitudinal and mechanistic research in the field of muscle physiology, endocrine health, and sexual function.

Author Response

Comment 1: Clarification of Statistical Power

Although the study is framed as exploratory, a brief comment regarding statistical power or potential fragility of borderline p-values (e.g., p = 0.045; p = 0.049) would improve transparency.

 

Response 1

Thank you for carefully reading our manuscript and providing your expert comments.

We completely agree with your point. Because this study was conducted as an exploratory study, we did not perform an a priori sample size calculation. Therefore, we recognize the need to clearly state that borderline p-values, such as p = 0.045 and p = 0.049, may indicate instability in our estimates.

To address this, and in combination with revisions made for another reviewer, we have added the following sentence to the Limitations section: "No a priori sample size calculation was performed; some estimates were imprecise, several p-values were close to the conventional threshold, and confidence intervals were wide, suggesting limited power for modest effects. Therefore, findings should be interpreted with caution and require replication." (Lines 551-555)

 

Comment 2: Multiple Testing Considerations

Given the number of sensitivity analyses and alternative dichotomization thresholds, the authors may consider briefly acknowledging the potential for type I error inflation.

 

Response 2

Thank you for your valuable comment. In this study, we presented our results with a focus on the main analysis, and we conducted sensitivity analyses to confirm their robustness. However, because we examined multiple sensitivity analyses and alternative thresholds, we acknowledge that an increase in type I error could occur in the nominal p-values.

To address this, we added the following sentence to the Methods section: "Given the exploratory nature and multiple sensitivity/outcome-definition analyses, p-values should be considered nominal and were not adjusted for multiplicity." (Lines 276-277). This clarifies that we did not adjust for multiplicity and that the p-values should be interpreted as nominal due to the exploratory nature of our study.

We would like to clarify further that our interpretation of the sensitivity analyses is based on the consistency of the effect sizes and directions (Figure S3, Table S9), rather than on isolated statistical significance.

 

Comment 3: Measurement Considerations

The use of a single-item morning-erection frequency measure is pragmatically justified; however, reiterating its non-validated nature in this age group within the Discussion could further strengthen methodological balance.

 

Response 3

Thank you for your suggestion. We have already stated in the Methods section that morning-erection frequency was assessed using a single self-reported item and that its psychometric validation is limited for this age group.

Following your suggestion, we have attempted to make this point even clearer by reiterating it in the Discussion section. We added the following text: "Moreover, because morning-erection frequency was assessed using a single self-reported item without age-specific psychometric validation, prevalence estimates and effect sizes should be interpreted as exploratory" (Lines 541-543).

 

Comment 4: Contextual Framing and Emerging Evidence on Muscle Mass, Testosterone, and Erectile Function

The authors are encouraged to more explicitly integrate the emerging 2025 evidence examining the relationship between skeletal muscle mass/quality, testosterone status, and erectile dysfunction. Although still limited in number, recent clinical investigations have provided important data suggesting that sarcopenia and muscle quality parameters may independently influence erectile function and even treatment responsiveness in adult male populations.

Given the conceptual proximity of these findings to the present results, incorporating this line of evidence would substantially strengthen the biological plausibility of the proposed muscle–erectile function axis. Such integration would help bridge muscular physiology, endocrine regulation, and sexual function outcomes, thereby enhancing the mechanistic coherence and translational positioning of the manuscript.

In addition, referencing recent international guideline documents (e.g., EAU and/or AUA guidelines on erectile dysfunction) may further reinforce the clinical and public health framing of the study.

 

Response 4

Thank you for your valuable suggestions. The aim of this study was to explore easily accessible proxy measures. Therefore, we did not measure muscle mass, muscle quality (e.g., muscle composition indicators such as intramuscular fat infiltration or muscle density), or blood testosterone levels. These assessments require invasive procedures or access to specialized medical facilities and laboratories, which means we cannot directly verify these factors in our current dataset.

However, we agree that recent reviews and clinical studies showing that muscle mass, muscle quality, and testosterone status may be associated with ED and treatment responsiveness in adult men provide important context. They strongly support the biological plausibility of our findings. Therefore, we have briefly cited these recent studies in the Discussion. While recognizing that the clinical significance of sarcopenia is limited in young and healthy populations, we noted that future studies should concurrently evaluate body composition, muscle quality, and hormonal indicators.

Specifically, we added the following text: "Emerging evidence in adult men suggests that reduced skeletal muscle mass [31] and poorer muscle quality [10] may be associated with erectile dysfunction and treatment responsiveness. Although our study did not measure muscle mass/quality or hormones, our findings are directionally consistent with this evolving evidence and support further mechanistic and longitudinal research. Additionally, current international guidelines (such as those from the European Association of Urology [32] and the American Urological Association [33]) advocate for lifestyle modifications and physical fitness as foundational management strategies for ED, further underscoring the clinical relevance of investigating physical strength as a correlate of sexual health." (Lines 457-465). As shown in this addition, we have incorporated references to both the EAU and AUA guidelines.

Regarding your mention of the "muscle–erectile function axis," we understood this not as a strong claim of causality, but rather as a conceptual framework where muscle function, endocrine regulation, and vascular/autonomic mechanisms are interrelated. Because our study is cross-sectional, we maintain that we cannot claim causality. We hope that our overall revisions clarifies this point and that the proposed mechanistic pathways remain strictly hypothetical.

 

Comment 5: Mechanistic Discussion

The mechanistic hypotheses involving myokines and autonomic regulation are scientifically interesting and thoughtfully presented. However, a slightly more concise presentation may help maintain proportionality relative to the modest observed effect size and discrimination metrics.

 

Response 5

We sincerely thank you for your constructive feedback. We completely agree that the discussion regarding mechanisms should be concise and appropriately proportional to the relatively modest effect size and discrimination metrics observed in this study.

Following your feedback, we have toned down our argument by explicitly stating that these pathways are speculative and by using more cautious terminology. Specifically, we revised the text as follows: "While highly speculative given the modest effect size observed, two additional physiological pathways could theoretically contribute to this association. First, skeletal muscles secrete myokines that promote angiogenesis and nitric oxide (NO) production [35,36]. Since nocturnal erections are NO-dependent [37], large muscle mass might conceivably support penile endothelial integrity. Second, trunk posture and paraspinal muscle tone can influence autonomic balance [38]. Since nocturnal erections require parasympathetic dominance [39], adequate back muscle strength may reduce paraspinal tension, potentially assisting the autonomic transition required for nocturnal tumescence." (Lines 474-481).

Reviewer 3 Report

Comments and Suggestions for Authors

Sexual and reproductive health are vital components recognized as an important focus in public health and primary care. The finding that back muscle strength is independently associated with a higher frequency of morning erections in apparently healthy young men is both novel and clinically relevant. Overall, these findings provide valuable insights for both clinicians and researchers in the field.

Comments:

Table 1. Were additional potentially relevant variables assessed among participants, such as serum testosterone levels, sleep quality, psychological stress, medication use, alcohol consumption, or smoking status? Including these factors would help determine whether the observed association is independent of known hormonal, psychological, and lifestyle influences on morning erection frequency.

Table 2. Please clarify how erection frequency was quantified. Was the number of morning erections recorded per day, per week, or over another defined period? Providing detailed descriptive statistics (e.g., mean ± SD or median with range) for erection frequency would improve clarity and interpretability.

Table 3. What criteria were used to define Cluster 1 and 2? Please clarify the clustering method applied, the variables included in the model, and whether the cutoffs were predefined or data-driven. Providing this information would improve transparency and reproducibility.

Figure 1. Were correlations between testosterone levels and erection frequency examined? Similarly, was sleep quality analyzed in relation to erection frequency? Including these correlation analyses, if available, would help clarify potential physiological and behavioral contributors to the observed associations.

Figure 2. In addition to back muscle strength, BMI and grip strength also appear to differ significantly between Cluster 1 and 2. It would be helpful to determine whether back muscle strength remains independently associated with erection frequency after controlling for BMI and overall physical strength, or whether it may reflect broader differences in general fitness.

Author Response

Comment 1

Table 1. Were additional potentially relevant variables assessed among participants, such as serum testosterone levels, sleep quality, psychological stress, medication use, alcohol consumption, or smoking status? Including these factors would help determine whether the observed association is independent of known hormonal, psychological, and lifestyle influences on morning erection frequency.

 

Response 1

Thank you for your comment. We would like to clarify the potential confounding factors among the participants.

In this study, we collected data on smoking status (never/past/current), alcohol consumption frequency (days/week), and physical activity (MET-min/week derived from the IPAQ-SF). These are presented in Table 1. To evaluate the impact of confounding factors, we conducted a sensitivity analysis separately from the main analysis, which additionally adjusted for age, smoking, alcohol, and total physical activity. As shown in Figure S3(iv), the association between back muscle strength and morning-erection frequency was preserved in the same direction.

However, we did not measure serum testosterone levels, sleep quality (such as insomnia symptoms or the PSQI), general psychological stress, or medication history in this study. Regarding sleep, although we did not assess quality indicators, we did collect data on sleep habits and timing using the Munich ChronoType Questionnaire (such as bedtime, wake time, sleep duration, and social jetlag). A descriptive comparison of these habits according to the morning-erection frequency groups is provided in Table S4.

 

Comment 2

Table 2. Please clarify how erection frequency was quantified. Was the number of morning erections recorded per day, per week, or over another defined period? Providing detailed descriptive statistics (e.g., mean ± SD or median with range) for erection frequency would improve clarity and interpretability.

 

Response 2:

Thank you for your comment. Morning-erection frequency was not recorded by counting the number of occurrences in a diary. Instead, it was evaluated using a single-item 6-point ordinal scale asking how often participants notice a morning erection upon waking (1 = "never" to 6 = "always"; a score of 4 corresponds to "every other day," which is the only score that implies a specific frequency).

Therefore, the outcome of this study should not be interpreted as the actual measured number of times per day or per week. Rather, it is a subjective frequency category based on the participants' overall assessment of their own condition.

Regarding the descriptive statistics, Table 2 presents the n (%) for each category. Additionally, the mean ± SD is already included in the table.

 

Comment 3

Table 3. What criteria were used to define Cluster 1 and 2? Please clarify the clustering method applied, the variables included in the model, and whether the cutoffs were predefined or data-driven. Providing this information would improve transparency and reproducibility.

 

Response 3

Thank you for your comment. We would like to clarify the definition and methodology of the clusters. The clustering in this study was conducted as an exploratory analysis. We performed k-means clustering using three variables: BMI, grip strength, and back muscle strength.

We evaluated the number of clusters (k) using silhouette analysis for k = 2 to 6. We adopted k = 2 because it had the highest average silhouette score (Figure S2). Therefore, the assignment to Cluster 1 and Cluster 2 was not based on predefined cutoffs but entirely data-driven.

We would like to clarify that these methodological details are already described in the main text (Sections 2.4 and 3.5) and in the Supplementary materials.

 

Comment 4

Figure 1. Were correlations between testosterone levels and erection frequency examined? Similarly, was sleep quality analyzed in relation to erection frequency? Including these correlation analyses, if available, would help clarify potential physiological and behavioral contributors to the observed associations.

 

Response 4:

Thank you for your comment. Because we did not measure serum testosterone in this study, we cannot perform a correlation analysis with morning-erection frequency. We have clearly listed this point in the Limitations section as an unmeasured confounder.

Regarding sleep, we did not evaluate sleep quality using tools such as PSQI. However, we did collect data on sleep habits and timing using the Munich ChronoType Questionnaire (such as bedtime, wake time, sleep duration, and social jetlag). A comparison of these habits by morning-erection frequency group is shown in Table S4. In our exploratory analysis, we observed a difference between the groups only in weekday bedtime, while the other indicators were generally similar.

We believe that clarifying the physiological mechanisms, including sleep quality and hormones, is a topic that should be investigated in future longitudinal studies.

 

Comment 5

Figure 2. In addition to back muscle strength, BMI and grip strength also appear to differ significantly between Cluster 1 and 2. It would be helpful to determine whether back muscle strength remains independently associated with erection frequency after controlling for BMI and overall physical strength, or whether it may reflect broader differences in general fitness.

 

Response 5:

Thank you for making this valuable point. To address exactly what you have mentioned—whether back muscle strength is independent of BMI and overall muscle strength—we used grip strength as a proxy for overall muscle strength. We have presented this multivariate model in Figure 1b.

As a result, even after adjusting for both BMI and grip strength, back muscle strength remained independently associated with morning-erection frequency. Furthermore, regarding the influence of general fitness (physical activity level), we used data from the IPAQ to adjust our model, which is shown in Figure S3(iv). This additional analysis also confirmed the independent association of back muscle strength.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been substantially revised, enriched, and strengthened. I congratulate the authors

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adequately addressed the major concerns raised during the previous review. The introduction has been strengthened with additional recent literature, and the methodology has been clarified with additional sensitivity analyses and expanded descriptions of the statistical approach. The discussion now more appropriately contextualizes the findings and explicitly acknowledges the exploratory nature and limitations of the study.

Although the study has inherent limitations, including the cross-sectional design and reliance on a single self-reported outcome measure, these issues are now transparently discussed and do not invalidate the main findings. The analyses are appropriate for an exploratory observational study, and the manuscript contributes novel preliminary data regarding the relationship between trunk muscle strength and morning erection frequency in young men.

Therefore, the manuscript can be accepted for publication in its current form.