Validation of a Smartphone Application for Measuring Peak Torque During the Nordic Hamstring Exercise in Recreational Athletes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper presents an interesting topic; however, certain revisions are necessary. Specifically, the introduction and the methodological section should be refined and expanded based on the detailed comments below.

Abstract:

31. 31. Correct the phrase ‘he app’ to ‘The app’.

Introduction:

62. 59-62. Authors state that „MRI and ultrasonography (US) are considered the criterion reference standards for diagnosis of hamstring injuries, however, both MRI and US are not practical alternatives for diagnosis of hamstring injury due to the high incidence of this injury and the costs associated with these diagnostic tests [19].” If the authors are of this opinion, please indicate what other methods should be used in practice to diagnose hamstring injuries.

The introduction provides extensive information on monitoring rehabilitation progress, injury prevention, and diagnosis; however, the authors do not specify how the peak torque parameter is utilized in these contexts. In particular, the manuscript lacks information regarding the analysis of peak torque values in hamstring injury prevention and in the assessment of an athlete’s readiness to return to sport.

Results:

L.106. This table title does not tell us anything. Please clarify it.

L.109-110. What three variables do you mean?

Table 2. Correct 0,999 to 0.999.

Check the entire results section for the Nm notation. In many places, the notation is incorrect (nm).

Materials and Methods:

L.374-381. When qualifying participants for the study, were they asked about their experience with Nordic hamstring exercises?

L.403. Correct the phrase ‘My Jump Lab pp’ to ‘My Jump Lab app’.

L.410. Correct ‘My Jump lab’ to ‘My Jump Lab’.

L.451-453. The description of the exercise is unclear. Please describe the exercise in detail. Was it based on isokinetic or isometric contractions? Why was only the dominant limb tested? How was it determined which limb was dominant?

References:

Correct the description of items 35 and 36 (remove the initial before the surname).

Check the references carefully, as some items are duplicated, e.g. 31 and 41; 34 and 42.

Delete lines 637-638.

Author Response

Comment 1: The paper presents an interesting topic; however, certain revisions are necessary. Specifically, the introduction and the methodological section should be refined and expanded based on the detailed comments below.

Response 1: Thank you for your appreciation of our manuscript

Comment 2: Abstract: 31. Correct the phrase ‘he app’ to ‘The app’.

Response 2: Thank you for your comment. We corrected the typographical error

Comment 3: Introduction: 59-62. Authors state that „MRI and ultrasonography (US) are considered the criterion reference standards for diagnosis of hamstring injuries, however, both MRI and US are not practical alternatives for diagnosis of hamstring injury due to the high incidence of this injury and the costs associated with these diagnostic tests [19].” If the authors are of this opinion, please indicate what other methods should be used in practice to diagnose hamstring injuries.

Response 3: Thank you for your comment. We added other methods commonly used in a field setting.

Comment 4: The introduction provides extensive information on monitoring rehabilitation progress, injury prevention, and diagnosis; however, the authors do not specify how the peak torque parameter is utilized in these contexts. In particular, the manuscript lacks information regarding the analysis of peak torque values in hamstring injury prevention and in the assessment of an athlete’s readiness to return to sport.

Response 4: Thank you for your comment. We added in the introduction section a paragraph regarding peak torque assessment and its importance for return to sports clearance, as well the fact that it’s not the only type of assessment that should be used in the decision-making process.

Comments 5: Results: L.106. This table title does not tell us anything. Please clarify it.

Response 5: Thank you for your comment. We added more information in the table title as well on the table caption.

Comment 6: L.109-110. What three variables do you mean?

Response 6: Thank you for your comment. We meant the three assessments that were evaluated. We have changed the sentence accordingly.

Comments 7: Table 2. Correct 0,999 to 0.999.

Response 7: Thank you for your comment. Ww have made the correction on table 2.

Comment 8: Check the entire results section for the Nm notation. In many places, the notation is incorrect (nm).

Response 8: Thank you for your comment. We reviewed the manuscript for notation correction.

Comment 9: Materials and Methods: L.374-381. When qualifying participants for the study, were they asked about their experience with Nordic hamstring exercises?

Response 9: Thank you for your comment. We did not ask about prior experience in performing the Nordic hamstring exercise.

Comment 10: L.403. Correct the phrase ‘My Jump Lab pp’ to ‘My Jump Lab app’; L.410. Correct ‘My Jump lab’ to ‘My Jump Lab’.

Response 10: Thank you for your comment. We made both corrections.

Comment 11: L.451-453. The description of the exercise is unclear. Please describe the exercise in detail. Was it based on isokinetic or isometric contractions? Why was only the dominant limb tested? How was it determined which limb was dominant?

Response 11: Thank you for your comment. We added a more detailed description of the exercise. The exercise is based on a controlled downward movement of the trunk up until the point it’s no longer sustainable (break point angle – BPA). This moment was used for eccentric peak torque estimation with My Jump Lab app.

We added the determination of dominant limb in the data collection and management section: “Dominant leg was assessed by asking participants to step on a small bench and the limb first used was considered”. This limb was the only one evaluated due to time constraints in the laboratory.

Comments 12: References: Correct the description of items 35 and 36 (remove the initial before the surname). Check the references carefully, as some items are duplicated, e.g. 31 and 41; 34 and 42. Delete lines 637-638.

Response 12: Thank you for your comment. We made the corrections in the references.

Reviewer 2 Report

Comments and Suggestions for Authors

Review of the manuscript “Validation of a Smartphone Application for Measuring Peak Torque During the Nordic Hamstring Exercise in Recreational Athletes”

1.  General comment

Thank you for inviting me to review this manuscript. The topic , validating a smartphone application for measuring eccentric hamstring strength in the field .is timely and of practical relevance to exercise physiology and sports rehabilitation. The authors address an important gap in the accessibility of strength measurement tools.

2.  Novelty and significance

The manuscript makes a meaningful contribution by comparing the mobile application with the gold-standard isokinetic dynamometer. However, it would strengthen the paper to emphasise that this is a preliminary validation (a small sample, recreational athletes) and clarify the limits of generalisability.

3.  Title, abstract and keywords

• The title appropriately reflects the
• In the abstract:
  • Typographical error noticed: “he app may serve…” should read “The app may serve…”.
  • The results section gives ICC = 76 and r ≈ 0.705, but it would be helpful if the abstract also included p-values and confidence intervals for clarity.
  • The abstract claims “excellent inter-rater reliability (ICC = 0.999; CV = 0.66%)” , please specify clearly whose ratings (which raters) and under what conditions.
• The keywords largely replicate the title. Consider adding more specific terms, e.g., “eccentric hamstring torque”, “field-based strength monitoring”.

4.  Introduction

• The authors clearly present the problem of hamstring injuries and the need for accessible strength assessment.
• Suggest shortening some lengthy paragraphs to sharpen focus.
• The sentence: “Non-modifiable risk factors include muscle composition, age, race…” — the use of “race” may be problematic; consider replacing with “genetic background” or “ethnicity” depending on context.
• A more structured overview of previous validation studies (e.g., mobile apps or field tests vs dynamometry) would help to position the current study.

5.  Methods

The description is overall adequate but requires more detail in some aspects:

• Clarify how the exercise order was randomised (e.g., random number generator, block randomisation).
• State whether the assessor analysing the application data was blinded to the dynamometer results.
• Provide more specifics about participant characteristics (e.g., training history, injury history).
• Ethics statement: you list the approval number, but please also include date of approval and name of the ethics board.
• Since this is a validation study, it would help to include the sample size calculation or power analysis.
• For reliability assessment: indicate how many raters, whether measurements were repeated, and specify the interval between them.

6.  Results

• The data appear promising: excellent inter-rater reliability and good validity correlations.
• However:
  • Provide confidence intervals for ICC and r values to reflect precision.
  • In Bland-Altman plots, report the bias and limits of agreement numerically in the text (e.g., mean difference ± 95% limits).
  • The statement “good comparative validity (ICC ≈ 76; r ≈ 0.705)” is accurate but could be clarified why ICC and r differ (they assess different things).
  • Include data on whether the mobile app systematically over- or under- estimated torque compared to dynamometer.

7.  Discussion

• The discussion appropriately interprets the findings and suggests that the mobile app may be a feasible field alternative.
• Some repetition of results could be trimmed.
• Limitations: The authors discuss some, but I suggest emphasising:
  • The sample size is modest and mostly male (26 male, 1 female) , limits generalisability.
  • Recreational athletes only — may not apply to elite or injured populations.
  • Only a single session measurement; no test–retest over days.
• Future research: good suggestions, but could add: validating in different populations, different device models, longitudinal monitoring of strength changes.
• Implications: the authors might elaborate on how coaches/practitioners can implement the app, and what caution is needed given measurement error (e.g., discuss minimal detectable change).

8.  Tables, Figures and Data Presentation

• Tables and figures are generally clear.
• In Table 3: ensure units are indicated (e.g., Nm for torque).
• Figures: the axes labels in the Bland-Altman plots should clearly indicate units (Nm) and the difference/mean.
• The high ICC value (0.999) might raise suspicion of over-fitting or too ideal conditions , comment on practicality of such reproducibility in field settings.

9.  Language and Style

• The manuscript is well written overall.
• Minor corrections: fix “he app” → “The app”; ensure consistent use of “eccentric torque” vs “peak torque”; check plural/singular (“applications … is” → “application… is”).
• Some sentences are long and could be split for clarity.
• Abbreviations: Define ICC, SEM, CV at first use (though common, still good practice).

10.  Recommendation

Overall, I recommend minor revision. The manuscript is of high potential, but I suggest addressing the methodological clarifications, adding confidence intervals, refining language, and emphasising limitations more clearly.

Author Response

Comment 1: General comment: Thank you for inviting me to review this manuscript. The topic, validating a smartphone application for measuring eccentric hamstring strength in the field is timely and of practical relevance to exercise physiology and sports rehabilitation. The authors address an important gap in the accessibility of strength measurement tools.

Response 1: Thank you for your kind words regarding our manuscript and our work.

Comment 2: Novelty and significance. The manuscript makes a meaningful contribution by comparing the mobile application with the gold-standard isokinetic dynamometer. However, it would strengthen the paper to emphasise that this is a preliminary validation (a small sample, recreational athletes) and clarify the limits of generalisability.

Response 2: Thank you for your comment. We added a sentence highlighting the preliminary results and limitations in the conclusions section.

Comment 3: Title, abstract and keywords.  The title appropriately reflects the content. In the abstract: Typographical error noticed: “he app may serve…” should read “The app may serve…”.

Response 3: Thank you for your comment. We corrected the typographical error.

•  
• Comment 4: The results section gives ICC = 0.76 and r ≈ 0.705, but it would be helpful if the abstract also included p-values and confidence intervals for clarity
• The abstract claims “excellent inter-rater reliability (ICC = 0.999; CV = 0.66%)” , please specify clearly whose ratings (which raters) and under what conditions.

Response 5: Thank you for your comment. We added p values in the abstract (as well as in table 3) as well as the confidence intervals.

• Comment 6: The keywords largely replicate the title. Consider adding more specific terms, e.g., “eccentric hamstring torque”, “field-based strength monitoring”.

Response 6: Thank you for your comment. We updated the keywords accordingly with your suggestion.

Comment 7: Introduction. The authors clearly present the problem of hamstring injuries and the need for accessible strength assessment. Suggest shortening some lengthy paragraphs to sharpen focus. The sentence: “Non-modifiable risk factors include muscle composition, age, race…” — the use of “race” may be problematic; consider replacing with “genetic background” or “ethnicity” depending on context.

Response 7: Thank you for your comment. We shortened some sentences as well as changed the word “race” to “ethnicity” as suggested.

Comment 8: A more structured overview of previous validation studies (e.g., mobile apps or field tests vs dynamometry) would help to position the current study.

Response 8: Thank you for your comment. To our knowledge there are no other smartphone app that assess isokinetic movements. We added to the introduction a study which used the same movement (Nordics) for analysing the validity of portable device for hamstrings strength assessment (Støve et al, 2025).

Comment 9: Methods The description is overall adequate but requires more detail in some aspects: Clarify how the exercise order was randomised (e.g., random number generator, block randomisation).

Response 9: Thank you for your comment. Participants which were assigned an odd number (S1, S3…) started with Nordics and participants with even numbers (S2, S4,..) started with the isokinetic assessment.  We added this information in the study design section.

•  
• Comment 10: State whether the assessor analysing the application data was blinded to the dynamometer results.

Response 10: Thank you for your comment. The assessors analysing the application data were blinded to the dynamometer results. This information was added to the data analysis section of the manuscript

•  
• Comment 11: Provide more specifics about participant characteristics (e.g., training history, injury history).

Response 11: Thank you for your comment. The questionnaire used for the recruitment of participants only asked if they were physically active (>4 hours/week) and if they had had any type of lower limb injury in the last six months. It did no asked training or injury history.

Comment 12: Ethics statement: you list the approval number, but please also include date of approval and name of the ethics board.

Response 12: Thank you for your comment. We included the date of approval. The name of the ethics board was on the statement (Ethics Committee of Egas Moniz School of Health and Science)

Comment 13: Since this is a validation study, it would help to include the sample size calculation or power analysis.

Response 13: Thank you for your comment. The information regarding sample size was already available on the methods section (4.5 Sample Size Calculation)

Comment 14: For reliability assessment: indicate how many raters, whether measurements were repeated, and specify the interval between them.

Response 14: Thank you for your comment. The information regarding raters is already present in the data analysis section of the manuscript. In it we stated that there were 2 independent evaluators, which performed 1 measurement each, in the same window of time.

Comment 15: Results The data appear promising: excellent inter-rater reliability and good validity correlations. However: Provide confidence intervals for ICC and r values to reflect precision.

Response 15: Thank you for your comment. Confidence intervals for ICC are mentioned in the text before table 3, where they are presented.

Comment 16: In Bland-Altman plots, report the bias and limits of agreement numerically in the text (e.g., mean difference ± 95% limits).

Response 16: Thank you for your comment. In the final part of the results section, bias and confidence interval were presented for each Bland- Altman plot.

Comment 17: The statement “good comparative validity (ICC ≈ 0.76; r ≈ 0.705)” is accurate but could be clarified why ICC and r differ (they assess different things).

• Response 17: Thank you for your comment. For clarification we changed the sentence to “good comparative validity and reliability” since both tests indeed assess different things.

• Comment 18: Include data on whether the mobile app systematically over- or under-estimated torque compared to dynamometer.

Response 18: Thank you for your comment. In the results section, when presenting the data for the Bland-Altan plots (figure 1 and 2), we stated that the bias results in indicating a systematic overestimation of the assessors.

Comment 19:  Discussion: The discussion appropriately interprets the findings and suggests that the mobile app may be a feasible field alternative. Some repetition of results could be trimmed.

• Response 18: Thank you for your comment. We updated the discussion section, in order to not repeat results.

Comment 19: Limitations: The authors discuss some, but I suggest emphasising: The sample size is modest and mostly male (26 male, 1 female), limits generalisability. Recreational athletes only — may not apply to elite or injured populations. Only a single session measurement; no test–retest over days.

Response 19: Thank you for your comment. We have updated the limitations section following your suggestions.

• Comment 20: Future research: good suggestions, but could add: validating in different populations, different device models, longitudinal monitoring of strength changes.

Response 20: Thank you for your comment. We have updated the suggestions for future research section.

•  
•  
•  
• Comment 21: Implications: the authors might elaborate on how coaches/practitioners can implement the app, and what caution is needed given measurement error (e.g., discuss minimal detectable change).
• Response 21: Thank you for your comment. We added suggestions for implementation in the practical applications section of the manuscript.

Comment 22: Tables, Figures and Data Presentation- Tables and figures are generally clear. In Table 3: ensure units are indicated (e.g., Nm for torque). Figures: the axes labels in the Bland-Altman plots should clearly indicate units (Nm) and the difference/mean.

Response 22: Thank you for your comment. We have implemented the suggested modifications.

Comment 23: The high ICC value (0.999) might raise suspicion of over-fitting or too ideal conditions, comment on practicality of such reproducibility in field settings.

Response 23: Thank you for your comment. Both assessors used a similar method and criteria for assessing break point angle with My Jump Lab app. Therefore, it’s not strange to have a very strong ICC value. Other validity / reliability studies listed below also reported ICC values very similar to the ones reported by our study (0.996 – 1.000).

• Støve, M. P., Kristensen, M. K., Nielsen, J., & Madsen, L. D. (2025). Validity and Intrarater Reliability of the FysioMeter—Measuring Eccentric Knee Flexor Force During the Nordic Hamstring Exercise. Measurement in Physical Education and Exercise Science, 29(3), 294–303.

https://doi.org/10.1080/1091367X.2025.2457154

• Fernandez MR, Athens J, Balsalobre-Fernandez C, Kubo M, Hébert-Losier K. Concurrent validity and reliability of a mobile iOS application used to assess calf raise test kinematics. Musculoskelet Sci Pract. 2023 Feb;63:102711. doi: 10.1016/j.msksp.2022.102711. Epub 2022 Dec 20. PMID: 36604270.
• Maia da Silva AL, Sampaio TV, Albano TR, Fernandes TLB, Bezerra MA, Lima POP. Validity and reliability of the JUMPSTER application to evaluate the vertical jump of recreational athletes. J Bodyw Mov Ther. 2025 Sep;43:165-169. doi: 10.1016/j.jbmt.2025.04.012. Epub 2025 Apr 22. PMID: 40483119.

Comment 24: Language and Style. The manuscript is well written overall. Minor corrections: Fix “he app” > “The app”; ensure consistent use of “eccentric torque” vs “peak torque”; check plural/singular (“applications .... is”> “application.... is”). Some sentences are long and could be split for clarity.

Response 24: Thank you for your comment. We revised the manuscript for typographical errors and shortened sentences for in order to add clarity.

Comment 25: Abbreviations: Define ICC, SEM, CV at first use (though common, still good practice).

Response 25: Thank you for your comment. We have defined the abbreviations at the first use in the manuscript

Comment 26: Recommendation. Overall, I recommend minor revision. The manuscript is of high potential, but I suggest addressing the methodological clarifications, adding confidence intervals, refining language,

Response 26: Thank you for your appreciation of our manuscript and our work.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have taken most of the earlier comments into account. Only minor corrections remain to be made.

L.62-66. The additional information does not convince me that palpation or the single-leg hamstring bridge exercise can be used to diagnose an athlete's readiness to return to sport or the severity of an injury. None of the methods mentioned can be a practical equivalent of MRI and ultrasonography in diagnosing the type and severity of hamstring injuries. Therefore, I propose to delete the following sentence: However, both MRI and US are not practical alternatives for diagnosis of hamstring injury due to the high incidence of this injury and the costs associated with these diagnostic tests [19]. Other methods are commonly used, such as palpation [20] or functional field tests such as the single leg bridge test [21] or the Nordics hamstring exercise to assess RTS or injury severity.

L.69. The abbreviation BPA does not stand for moment of muscle failure. The abbreviation BPA should appear elsewhere in the text.

L.71. Put a full stop instead of a comma after [22].

Author Response

Comment 1: The authors have taken most of the earlier comments into account. Only minor corrections remain to be made.

Response 1: Thank you for your comment


Comment 2: L.62-66. The additional information does not convince me that palpation or the single-leg hamstring bridge exercise can be used to diagnose an athlete's readiness to return to sport or the severity of an injury. None of the methods mentioned can be a practical equivalent of MRI and ultrasonography in diagnosing the type and severity of hamstring injuries. Therefore, I propose to delete the following sentence: However, both MRI and US are not practical alternatives for diagnosis of hamstring injury due to the high incidence of this injury and the costs associated with these diagnostic tests [19]. Other methods are commonly used, such as palpation [20] or functional field tests such as the single leg bridge test [21] or the Nordics hamstring exercise to assess RTS or injury severity.

Response 2: Thank you for your comment. We understand your point, but the sentences in question help to frame the need for practical forms of evaluation/assessment. MRI and US are expensive means of diagnostics and mandatory to understand the severity of hamstring injuries. However, simple / practical tests are also determinant to assess athletes in a field setting, hence the need for this type of assessment tools.

Consequently, we simplified the sentences in question: “However, both MRI and US are not practical alternatives for diagnosis of hamstring injury due to often costs associated with these diagnostic tests [19]. Other methods are commonly used in a field setting [20-21] to assess RTS or injury severity.”


Comment 3: L.69. The abbreviation BPA does not stand for moment of muscle failure. The abbreviation BPA should appear elsewhere in the text.

Response 3: Thank you for your comment. We updated the sentence and moved the abbreviation as requested.


Comment 4: L.71. Put a full stop instead of a comma after [22].

Response 4: Thank you for your comment. We changed the typographical error.