The Influence of Sleep, Menstrual Cycles, and Training Loads on Heart Rate Variability: A Four-Year Case Study on an Elite Female Slalom Kayaker

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor,

Thank you for the opportunity to review the manuscript titled "The Influence of Sleep, Menstrual Cycle, and Training Load on Heart Rate Variability: A Four-Year Study in an Elite Female Kayaker." This study examines various factors influencing heart rate variability (HRV) in an elite female kayaker over a four-year period, analyzing the relationships between HRV, sleep quality and duration, menstrual cycle, illness, training load, and athletic performance. The main findings reveal that sleep quality and the follicular phase of the menstrual cycle have significant positive associations with HRV, while training load, sleep duration, and illness did not show statistically significant effects. Additionally, neither HRV values nor trends were significant predictors of athletic performance.

The four-year longitudinal design with daily measurements (1,394 data points) provides a robust dataset rarely seen in elite athlete research. The examination of multiple factors affecting HRV simultaneously is a notable strength, as is the focus on a female athlete, addressing an important gap in the literature, particularly female athletes. The methodology is sound, the results are clearly presented, and the conclusions are well-supported by the data. With some minor revisions, this manuscript will make a meaningful contribution to the field of sports physiology and performance monitoring.

General Comments

1. Introduction: The introduction establishes a solid foundation for the study but would benefit from a clearer description of the relationship between sympathetic/parasympathetic activity and HRV. Additionally, a brief explanation of the significance of rMSSD specifically as the chosen HRV parameter would strengthen this section.
2. Methods: The methodology is generally well-described, but some aspects require minor clarification. Specifically, more information about how the Performance Index (PI) was calculated would enhance reproducibility. A brief explanation of why the specific time frame of 60 consecutive days was chosen for the HRV trend analysis would also be beneficial.
3. Results: While the tables and figures effectively present the data, the addition of visual representations showing the longitudinal trends in HRV over the four-year period would enhance the clarity of the findings and highlight the temporal patterns that may be present.
4. Discussion: The discussion effectively contextualizes findings within existing literature but would benefit from a more thorough exploration of why training load showed no significant effect on HRV, contrary to some previous research findings. Additionally, a brief paragraph addressing the limitations of a single-subject design would strengthen this section.

Specific Comments

Abstract

• Line 9: The statement "The study aimed to examine the influence of some parameters on heart rate variability" is somewhat vague. Consider specifying the parameters explicitly in this summary statement.

Introduction

• Lines 33-34: The sentence "Technological developments in the last decade have allowed us to measure HRV using sports watches or smartphones that operate according to the photoplethysmography (PPG) principle" should include a citation for the reliability/validity of these methods compared to ECG.
• Line 33 and 52: The uncited statements "(ref?)" need to be referenced.

Methods

• Lines 114-121: The acute training load (ATL) formula is presented, but it would be helpful to provide more context about how intensity (I) was specifically quantified (e.g., RPE scale, perceived exertion, etc.). It is important to reference the equation in text and include a title of the equation.
• Lines 122-128: The Performance Index (PI) calculation could be described in more detail. Specifically, how were the "competition quality" and "event significance" quantified and weighted in the formula? Please, include the equation as you realized with ATL. It is important to reference the equation in text and include a title of the equation.
• Lines 129-132: The rationale for choosing a 60-day window for the HRV trend analysis should be briefly explained.
• Line 135: The statistical significance threshold is set at p < 0.001, which is quite stringent. Normally, the significance is stablished at p < 0.05, so a brief justification for this choice would be helpful.

Results

• Lines 145-150: The authors report average values for morning rMSSD and resting HR, but confidence intervals or ranges would provide more complete information about the distribution of these values.
• Figure 1: The statistical significance indicators (* and **) are present in the figure, but their specific p-value thresholds are not explicitly stated in the figure caption.
• Figure 2: Similarly, the significance indicator (*) should be clearly defined in the caption.
• The manuscript would benefit from a figure showing the longitudinal trends in HRV over the four-year period, potentially highlighting competition periods, to provide context for the findings.

Discussion

• Lines 224-232: The authors note varying findings regarding the relationship between training load and HRV in previous research, but the discussion of why their study found no significant effect could be expanded.
• Lines 233-250: The discussion of HRV and competitive performance is thorough but could be strengthened by addressing potential sport-specific factors that might influence the relationship between HRV and kayaking performance specifically.
• Lines 251-256: The statement about sympathetic dominance potentially benefiting performance needs more substantiation or explanation of the physiological mechanism behind this assertion.

Conclusion

• The conclusion effectively summarizes the main findings but would benefit from a more explicit statement about the practical implications for coaches and athletes monitoring HRV.
• The limitations section (Lines 286-294) appropriately acknowledges the constraints of a single-subject design but could further address the challenges of generalizing these findings to athletes in different sports or competitive levels.

References and style

• Throughout the manuscript, the format of the journal has not followed. Please, review the entire manuscript and utilize the styles of the journal. If you use the Microsoft Word template, they are included above in the first section of the toolbar.
• Several references in the text are marked with "(ref?)" indicating missing citations that need to be addressed.
• The reference formatting is consistent, but the authors should ensure all DOIs are included where available.

This manuscript addresses an important topic in elite athlete monitoring and provides valuable insights into factors affecting HRV in female athletes. The four-year longitudinal approach with a large number of data points is a significant strength. With the suggested minor revisions, particularly regarding the clarification of methodological details and enhanced visual representation of longitudinal data, this manuscript will make a valuable contribution to the field. For the reasons mentioned above, I recommend Minor Revisions for the present manuscript.

Best regards,

The Reviewer

Author Response

Thank you for take your time and revised our article. In the attachment you will find our corrections. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear author,

please make these corrections:

In the title please add- case report at the and

in line 33, 52- the reference is missing, please add it

in line 236 - reduce the space between dot and first letter

Please add exact data on HRV values in the introduction — simply state the exact ranges for low and high HRV values according to gender and age.

In discussion- add sentences and reference where it is clear that your methodology of measuring HRV with Apple device and app based on photoplethysmography is valid and precise compared to other methods such as TMG, or CMJ etc.

Can you add some data in this section of correlation of the achieved sports successes of this athlete and HRV results during period of monitoring ? 

 

Author Response

Thank you for take your time and revised our article. In the attachment you will find our corrections. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The study presents a well-structured and comprehensive investigation into the factors influencing heart rate variability (HRV) in an elite female athlete over four years. The methodology is sound, and the longitudinal design strengthens the reliability of the findings. However, there are some areas where clarity, depth, and consistency can be improved.

 

Line 23: The title is clear but consider specifying "Elite Female Kayaker" to "Elite Female Slalom Kayaker" to better reflect the study's specific population.

Line 19-21: The conclusion mentions "sympathetic dominance in the pre-competition period," but this is not explicitly tested in the study. Please clarify.

 

Line 32: "Technological developments in the last decade..."  This statement would be stronger with a reference supporting the claim that PPG-based HRV monitoring has become a widely accepted method.

Line 52: "Additionaly, , it is less sensitive..."  Typo: "Additionally," (extra comma present).

Line 5558: The discussion on HRV's fluctuations throughout the training cycle is useful. However, citing a study that specifically examines HRV patterns in water-based endurance sports could strengthen this section.

Line 73-74: "Therefore, when interpreting HRV measurements, it is important to consider the broader context..."  This is a critical point, but it would be helpful to briefly state how the study accounts for external variables beyond those measured.

 

Line 91-93: The participant's description is clear, but does her training regimen vary significantly between seasons? If so, did this impact HRV?

Line 101: Apple Watch Series 4 and 6 were used for sleep monitoring. Given research on potential inaccuracies in consumer-grade devices, did you compare these to a validated method?

Line 110-112: The self-reported questionnaire method could introduce bias. Were there any validity checks for sleep quality reports?

Line 121-127: The performance index calculation is well-described. However, a brief explanation of why ICF ranking points were used instead of direct race times would be useful.

 

Line 146-147: "During the competition week, the rMSSD trend was, on average, 7.51 ± 18.37% lower..." The large standard deviation suggests high variability. Was there a statistical test to confirm this drop was meaningful?

Line 154-157: The random effects model showed substantial day-to-day variability. Would including a measure like intraclass correlation (ICC) strengthen the reliability of these findings?

Line 163-166: The menstrual cycle findings align with previous research. However, could you clarify whether the subject was using any hormonal contraception? This could affect HRV fluctuations.

 

 Discussion:

Line 187-190: The statement on sleep quality affecting parasympathetic activity aligns with previous research, but a clearer distinction between subjective and objective sleep measures would improve the argument.

Line 224-226: The discussion on training load and HRV variability is useful. However, given the case study design, a more cautious interpretation of causality is recommended.

Line 246-247: "Our study indicates a similar pattern to previous studies..."  It would be helpful to explicitly state if HRV differences between competition and noncompetition weeks were statistically significant.

Line 254-256: The suggestion that pre-competition HRV decreases may indicate an advantage of sympathetic dominance is interesting. Could this be further validated with stress markers like cortisol?

 

 Conclusion & Limitations:

Line 272-275: The case study design limits generalizability. Have you considered conducting similar research across multiple athletes for comparison?

Line 282-284: The reliance on self-reported data is a valid limitation. If future studies introduce actigraphy or HRV measured through ECG, this could improve reliability.

Line 288-290: The paper acknowledges gender and age effects on HRV. If possible, adding a reference to studies that compare HRV responses across different demographics could be beneficial.

 

 

Line 16, 49, 54, 80: Inconsistent formatting of citations (e.g., "[ref?]" is present in multiple places).

Line 19-21: "sympathetic dominance" should be explained in simpler terms for broader audience comprehension.

Line 79-80: "Processes involved in postexercise recovery... do not affect HRV ([21–24]." The bracket format is incorrect.

Figure Labels: The caption for Figure 1 is unclear; does it refer to a subjective scale or an objective sleep measurement?

Please take these comments and observations as an earnest effort to assist in the crafting of the manuscript. 

 

Author Response

Thank you for take your time and positive value of the article. 

Author Response File: Author Response.pdf