Does a Prosthetic Limb for Skiing Affect the Three-Dimensional Knee-Joint Kinematics of Unilateral Transfemoral Amputee Skiers: A Pilot Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the kinematic characteristics of the knee joint in transfemoral amputees using a ski-specific prosthesis. In particular, the experimental design combining an indoor ski simulator with a three-dimensional motion analysis system represents a major strength, as alpine skiing is a high-load motor task that is inherently difficult to analyze. However, to more clearly interpret the manuscript's main conclusions—namely, the reported “left–right differences” and “asymmetries between the prosthetic and intact limbs”—several methodological aspects require further clarification and organization. Specific comments are provided below.

 

#1 Definition of the coordinate system

Line 174

Although the global coordinate system used for the three-dimensional motion analysis is described, the relationships between the X-, Y-, and Z-axes and the direction of skiing motion and turn direction are not clearly specified. Please add a description that clarifies how the coordinate system corresponds to the skier's movement, so that the relationship between the axes and the actual motion is readily understood.

 

#2 Adequacy of the sample size

Line 121

This study compares five amputees with five control participants; however, the rationale or process for determining this sample size is not described. If this study is intended as an exploratory investigation, it would be helpful to clearly state this positioning. For example, adding a subtitle such as “a pilot study” to the title may be appropriate.

 

#3 Information on leg dominance of participants

Lines 140, 156

In Tables 1 and 2, information regarding participants’ dominant leg is not presented. In this manuscript, “leg dominance” is mentioned in Line 277 as a potential factor underlying left–right differences observed in the control group, and this concept plays an important role in interpreting the results. By clearly defining how leg dominance was determined and presenting dominance information for each participant, it would be possible to more explicitly discuss whether the observed asymmetries are attributable to leg dominance or to other factors.

 

#4 Information on leg dominance in amputees

The side of amputation (left/right) in the amputee group is not systematically described in the manuscript. While it may be inferred from participant IDs, it would be preferable to clearly present this information either in the Methods section or as part of participant characteristics. By clearly organizing the side of amputation, the interpretation of results related to left versus right turns, as well as inside versus outside limbs, would become more straightforward.

 

#5 Interpretation of left-turn versus right-turn comparisons

Line 304

Tables 5 and 6 compare left and right turns. However, when both left-sided and right-sided amputees are included in the intervention group, it is unclear to what extent a simple comparison based on anatomical left and right turns has biomechanical meaning. Given the primary focus of this study, emphasizing analyses and discussions based on functional conditions—such as whether the prosthetic limb serves as the inside leg or the outside leg—would help clarify the interpretation of the results.

Author Response

Comment 1: Definition of the coordinate system
Line 174
Although the global coordinate system used for the three-dimensional motion analysis is described, the relationships between the X-, Y-, and Z-axes and the direction of skiing motion and turn direction are not clearly specified. Please add a description that clarifies how the coordinate system corresponds to the skier's movement, so that the relationship between the axes and the actual motion is readily understood.

Response:
Thank you for this very good point. We initially omitted the definition of the local coordinate systems of the studied body segments, namely the femur and tibia. These local coordinate systems have now been added to the manuscript (paragraph between lines 193–200) and are illustrated primarily in Figure 6 to clarify how the knee flexion angle at the apex of each turn was calculated. This information has also been added to Figure 5.

Comment 2: Adequacy of the sample size
Line 121
This study compares five amputees with five control participants; however, the rationale or process for determining this sample size is not described. If this study is intended as an exploratory investigation, it would be helpful to clearly state this positioning. For example, adding a subtitle such as “a pilot study” to the title may be appropriate.

Response:
Thank you for this very good and helpful suggestion. The small sample size is primarily due to the difficulty of recruiting such a specific population (unilateral transfemoral amputees able to ski on both limbs) in the Czech Republic. We therefore agree that framing the study as a pilot investigation is appropriate, and the subtitle “a pilot study” has been added to the title. In addition, a sample size estimation has been included in the first paragraph of the Materials and Methods section.

Comment 3: Information on leg dominance of participants
Lines 140, 156
In Tables 1 and 2, information regarding participants’ dominant leg is not presented. In this manuscript, “leg dominance” is mentioned in Line 277 as a potential factor underlying left–right differences observed in the control group, and this concept plays an important role in interpreting the results. By clearly defining how leg dominance was determined and presenting dominance information for each participant, it would be possible to more explicitly discuss whether the observed asymmetries are attributable to leg dominance or to other factors.

Response:
Thank you very much for this clear and reasonable comment. We agree that it is necessary to determine and report leg dominance for all participants, and this information has now been added to both Tables 1 and 2. We also acknowledge that there are differing views regarding the definition and assessment of leg dominance. Methods used to determine leg dominance generally fall into five categories: stability and/or balance, push-off (vertical or horizontal), manipulation, kicking, and strength dominance. Previous research has shown that the leg used to kick a ball demonstrates 100% agreement between self-reported and observed dominance in both men and women. Therefore, for the purposes of this study, leg dominance was determined using a manipulative kicking task.

Comment 4: Information on leg dominance in amputees
The side of amputation (left/right) in the amputee group is not systematically described in the manuscript. While it may be inferred from participant IDs, it would be preferable to clearly present this information either in the Methods section or as part of participant characteristics. By clearly organizing the side of amputation, the interpretation of results related to left versus right turns, as well as inside versus outside limbs, would become more straightforward.

Response:
Thank you for this comment. We agree that this information should be presented more clearly. The side of amputation has therefore been explicitly added to the participant characteristics in Table 1, in the column “Year of amputation (side of amputation)”.

Comment 5: Interpretation of left-turn versus right-turn comparisons
Line 304
Tables 5 and 6 compare left and right turns. However, when both left-sided and right-sided amputees are included in the intervention group, it is unclear to what extent a simple comparison based on anatomical left and right turns has biomechanical meaning. Given the primary focus of this study, emphasizing analyses and discussions based on functional conditions—such as whether the prosthetic limb serves as the inside leg or the outside leg—would help clarify the interpretation of the results.

Response:
Thank you for this excellent point. We agree that the original presentation was not sufficiently clear. Tables 5 and 6 have therefore been revised to emphasize functional conditions, explicitly indicating whether the prosthetic limb serves as the inside or outside leg during the turn. This modification should improve the biomechanical interpretability of the results.

Reviewer 2 Report

Comments and Suggestions for Authors

-The study includes only five participants per group, yet no power analysis is presented to justify whether the sample is sufficient to detect meaningful kinematic differences. Without such analysis, the reported significance levels risk being underpowered or prone to Type II errors. Include a post hoc or a priori power analysis to assess whether the sample size is adequate for the comparisons made, especially given the high inter-individual variability in prosthesis users.

-Amputee participants' skiing experience, proficiency, or training with the prosthesis is not described or controlled for, yet it likely has a strong influence on kinematics. This omission weakens the internal validity of the group comparisons.Provide details on participant skill levels or consider adding skiing experience as a covariate or matching criterion.

-The analysis acknowledges differences depending on whether the prosthetic or intact limb is inside or outside the turn, but it does not systematically explore turn direction as a factor. This could mask systematic biases in skiing mechanics. Include an explicit factor in the analysis for turn side relative to prosthetic limb, and assess its interaction with group status (amputee vs control).

-While kinematic asymmetries are demonstrated, no data are presented on ground reaction forces, joint torques, or load distribution, which are necessary to assess functional compensation or safety risks in amputee skiing. Acknowledge this as a major limitation and outline plans to integrate force data in future work.

-The conclusion recommends improving skiing-specific prostheses, but does not provide specific design insights or recommendations based on the observed biomechanical limitations. Translate the findings (limited knee flexion range) into design hypotheses for prosthetic engineers, modulation of flexion resistance, enhanced rotational freedom, or adaptive damping mechanisms.

Author Response

Comment 1:
The study includes only five participants per group, yet no power analysis is presented to justify whether the sample is sufficient to detect meaningful kinematic differences. Without such analysis, the reported significance levels risk being underpowered or prone to Type II errors. Include a post hoc or a priori power analysis to assess whether the sample size is adequate for the comparisons made, especially given the high inter-individual variability in prosthesis users.

Response:
Thank you for this important comment. Recruiting such a specific population (unilateral transfemoral amputees capable of skiing on both limbs) is challenging, which resulted in the small sample size. For this reason, the study has been explicitly framed as a pilot investigation, and the subtitle “a pilot study” has been added to the title. In addition, a sample size estimation and power analysis have been included in the first paragraph of the Materials and Methods section to support the exploratory nature of the study and to guide the interpretation of the results.

Comment 2:
Amputee participants' skiing experience, proficiency, or training with the prosthesis is not described or controlled for, yet it likely has a strong influence on kinematics. This omission weakens the internal validity of the group comparisons. Provide details on participant skill levels or consider adding skiing experience as a covariate or matching criterion.

Response:
Thank you for this valuable comment. Information on skiing experience is now provided in both Tables 1 and 2. For the amputee group, skiing experience before and after amputation is reported, while for the control group, years of skiing experience are included together with the inclusion criterion of holding at least a Level C ski instructor license. Individual participant results are also briefly described in the Results section, with reference to skiing experience where relevant.

Although incorporating skiing experience as a covariate (e.g., using ANOVA) would be desirable, the very small sample size limits the feasibility of such analyses and the strength of any resulting conclusions. As a pilot study, this represents an acknowledged limitation and highlights the need for future investigations with larger cohorts and more comprehensive statistical modeling. Moreover, even within the control group, where skiing experience was relatively homogeneous, substantial inter-individual variability was observed, suggesting that additional covariates influencing skiing kinematics may be difficult to control under such complex conditions. Therefore, in this initial study, we intentionally focused on a single primary outcome variable, with more detailed analyses planned for future work as additional data become available.

Comment 3:
The analysis acknowledges differences depending on whether the prosthetic or intact limb is inside or outside the turn, but it does not systematically explore turn direction as a factor. This could mask systematic biases in skiing mechanics. Include an explicit factor in the analysis for turn side relative to prosthetic limb, and assess its interaction with group status (amputee vs control).

Response:
Thank you for this excellent point, and we apologize for the lack of clarity in the original presentation. Tables 5 and 6 have been revised to explicitly distinguish between turns in which the prosthetic limb serves as the inside leg and those in which it serves as the outside leg. This functional framing clarifies the role of turn direction relative to the prosthetic limb and improves the interpretability of the inter-group comparisons.

Comment 4:
While kinematic asymmetries are demonstrated, no data are presented on ground reaction forces, joint torques, or load distribution, which are necessary to assess functional compensation or safety risks in amputee skiing. Acknowledge this as a major limitation and outline plans to integrate force data in future work.

Response:
Thank you for this important suggestion. The absence of kinetic and load-distribution data is now explicitly acknowledged as a major limitation of the study. This limitation is highlighted in the Discussion and further elaborated in a newly added section titled Methodological considerations. In addition, future research directions are outlined, including the ongoing development of a custom strain-gauge force measurement plate designed to be positioned between the ski and the ski binding. This system will enable direct assessment of load distribution and force patterns in skiing-specific prostheses in future studies.

Comment 5:
The conclusion recommends improving skiing-specific prostheses, but does not provide specific design insights or recommendations based on the observed biomechanical limitations. Translate the findings (limited knee flexion range) into design hypotheses for prosthetic engineers, such as modulation of flexion resistance, enhanced rotational freedom, or adaptive damping mechanisms.

Response:
Thank you for this excellent suggestion. A new paragraph has been added to the Conclusions section to translate the observed biomechanical limitations into specific design considerations for skiing-specific prostheses. The revised text now highlights that the reduced knee-flexion range observed on the prosthetic side, particularly during load-bearing phases of the turn, may be related to excessive flexion resistance in current prosthetic knees. Based on these findings, future prosthetic designs may benefit from improved modulation of flexion resistance, adaptive or phase-dependent damping, and increased rotational compliance to better support knee-flexion patterns closer to those of non-disabled skiers.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear,

Please find the review report attached.

Kind regards

Comments for author File: Comments.pdf

Author Response

Reviewer 3

Comment 1:

First and foremost, the title of the manuscript is excessively long, unnecessarily complex, and confusing. It should be significantly shortened and made more concise, clearly reflecting the core focus of the study.

Response: Thank you for your suggestion. You are definitely right about the title to be too long and too complex/difficult to understand. Therefore, the title has been changed as follows: “Does a Prosthetic Limb for Skiing Affect the Three-Dimensional Knee-Joint Kinematics of Unilateral Trans-femoral Amputee Skiers? A pilot study.”

Comment 2:

Regarding the abstract, it lacks essential scientific content. Specifically, it does not include any quantitative information related to the methodology, the analytical tools used for data processing, the obtained results, or the statisticals ignificance of those results. None of these key elements are adequately presented, which makes the abstract insufficient and uninformative. Concerning the keywords, they unnecessarily repeat terms already used in the title, which is methodologically unacceptable. The authors should propose appropriate and informative keywords that help readers better understand the research domain,without duplicating terms from the title.

Response: A very good point, thank you. Therefore, the abstract has been completely rewritten to your demands and satisfaction, with important outcomes included. Also, the keywords have been changed to avoid duplicating terms from the title.

Comment 3:

The introduction section is highly confusing. In particular, the paragraph (L94-105) appears to have been mistakenly included and discusses a completely different study involving individuals with amputations and healthy skiers. This paragraph is irrelevant to the present research and should be entirely removed from the manuscript. Furthermore, the explanation provided in the introduction namely that conventional prostheses are mainly used for walking and that this creates uncertainty regarding the extent to which skiing prostheses can reproduce the kinematic characteristics of experienced skiers is not sufficiently convincing and lacks adequate scientific justification.The study aim is also problematic. It is unusual that after stating the aim of the this significantly weakens the scientific structure of the manuscript. Additionally, the referencing style is inconsistent and lacks systematic order. The manuscript begins with reference number 4, then jumps to 11 and 16, and ends with reference number 20, which is also the total number of references listed. This approach is highly unusual and unacceptable in academic writing.

Response: Thank you for another valuable and constructive comment. The paragraph previously located at Lines 94–105 has been entirely removed from the Introduction, as it was not appropriate for this section and could lead to confusion regarding the focus of the study. In addition, the Introduction has been revised to strengthen the scientific justification distinguishing walking-oriented prosthetic design from the biomechanical demands of alpine skiing, and the study aim has been clarified and structurally repositioned to improve the logical flow of the manuscript.

Finally, we fully acknowledge the issue regarding the referencing style. All in-text citations and the reference list have been carefully corrected and renumbered to follow a strict sequential order according to their first appearance in the manuscript, starting from reference [1], in full compliance with academic and journal standards.

We believe that these changes have substantially improved the clarity, scientific structure, and readability of the Introduction.

Comment 4:

With respect to Section 2 (Materials and Methods), the most serious concern is the use of Microsoft Excel version 16 for data analysis. This software does not provide tools for conducting the Shapiro Wilk test to assess data normality, nor does it adequately support the Mann Whitney U test. Based on the information presented in this section, it is clear that Microsoft Excel 16 is an inadequate statistical tool for the analyses claimed. The authors are strongly encouraged touse appropriate statistical software and methods.

Response: Thank you for your point and I completely understand. I used the Microsoft Excel at first without data analysis tools and simply created my own equations and formulas based on statistical literature. However, you are right that it might be confusing and not proper to use it like this in a paper. Therefore, I changed it and used IBM SPSS Statistics software for the whole statistical analysis.

Comment 5:

In the Results section, there is no discussion whatsoever of the statistical significance of the results presented in the tables. It is remarkable and unacceptable that significance levels are not reported or interpreted in any way.

Response: Thank you for your point. However, for both groups, intervention and control, under the summarizing tables (Table 3 and 4), there is a short paragraph describing each participant and their significant results with some possible reason(s). There is no further discussion, because it is a Results section and not a Discussion section. I agree that it would be unacceptable not to include and interpret the significance levels and results, however, I can see that under both Tables for each individual participant. Nevertheless, we have added another short description into the paragraph under the Tables 5 and 6, describing and interpreting the significant results before the following interpretation using graphs.

Comment 6:

The Discussion section is also problematic, as it is almost entirely lacking references, which is methodologically inappropriate. Moreover, the manuscript does not include a Limitations section, which is an essential component of a well-structured scientific paper.

Response: Thank you for your suggestion. We have added one more section “Methodological considerations” after the Discussion section that included the limitations of the study. Regarding the lack of references in the discussion, due to a very unique and specific area of sports science that the study is focused on, there is very few references that might be used. Although we are aware of this fact, we have done our best to find the most similar studies available.

Comment 7:

Finally, the Conclusion section does not directly address the study aims, nor does it relate to hypotheses, which were never formulated. As a result, the manuscript as a whole appears confusing and poorly structured.In conclusion, while the research idea itself is very good and promising, the implementation of the study and presentation of the manuscript are insufficient for publication in its current form.

Response: Thank you for your point. The Conclusion section has been rewritten to try addressing the study aims and objectives in a more direct way. Hopefully, after the comments of all reviewers, and incorporating all the suggestions into the paper, the study will appear more clear, structured, and easy to follow.

Reviewer 4 Report

Comments and Suggestions for Authors

This manuscript compares knee-flexion kinematics in unilateral transfemoral amputee skiers using a skiing-specific prosthesis with those of able-bodied ski instructors on an indoor ski simulator. The topic is original and clinically relevant, and the use of 3D motion capture in a controlled environment is a clear strength. However, the study has major methodological and interpretive limitations. The sample size is extremely small, the statistical analysis suffers from pseudoreplication (treating repeated turns as independent observations), and only a single kinematic variable (knee flexion at the apex of the turn) is analyzed, despite the title’s reference to three-dimensional kinematics. A more cautious, proof-of-concept framing is warranted, and the statistical approach should be revised so that participants, not individual turns, constitute the unit of analysis.

 

Title, Highlights, Abstract

• Title mentions three-dimensional kinematic changes, but only the knee flexion angle in the sagittal plane at one instant is analyzed. Please change to “Knee flexion kinematics.”
• Tone down the abstract and highlights to preliminary and suggestive because of a small sample size.

Introduction

• The introduction includes results content referring to the present dataset and what it shows. That belongs in Results/Discussion, not in the Introduction.
• The gap and novelty are not sharply defined. Clearly distinguish what is new compared to prior skiing prosthesis studies, like older prostheses, different tasks, and no 3D capture.
• Early discussion of FIS regulations and classification is a bit premature. I would suggest saving policy-level implications for a cautious note in the Discussion/Conclusion.

Methods

• Extremely small sample with no a priori power/precision analysis. This must be highlighted as a major limitation and addressed as a pilot study or similar.
• Age distribution is more heterogeneous in amputees; only one female per group. These factors are not modeled or even briefly discussed.

• Although 3D kinematics are recorded, only one scalar outcome is used: knee flexion angle at the visually identified apex of the turn. That is a very narrow view of kinematic changes.

• Pseudoreplication is a major concern. Inter-group comparisons assume N=30 (6 turns × 5 skiers) per group, treating turns as independent. The same problem is for intra-individual analyses: turns from the same skier are treated as independent.
• Multiple comparisons are made (per limb, per condition, per group) with no correction (Bonferroni/FDR). The risk of inflated Type I error is high and needs to be acknowledged or corrected.
• Effect sizes (Cohen’s d) are given but not fully contextualized. With inflated N from pseudoreplication. Re-calculate using per-participant means and interpret cautiously.

 

Results

• Large blocks of text interpret each skier’s pattern (likely carving style, possibly leg dominance) speculatively and narratively. Consider moving such case-based commentary to a short subsection or supplement and keep the main results focused on group patterns.
• Reporting would be more transparent if you present per-subject mean ± SD at each condition (inside/outside, limb, group), then summarize across subjects with group means and 95% CIs, treating n=5 as the unit.

 

Discussion

• Amputees can reproduce the qualitative turning strategy (inside vs outside flexion pattern) but with systematically reduced flexion on the prosthetic side. That deserves central emphasis.
• Methodological weaknesses (small n, pseudoreplication, single kinematic outcome, simulator context) are not explicitly discussed as major limitations.

Conclusions

• The conclusion correctly reiterates the main observation of reduced prosthetic-side flexion and closer intact-side flexion. Still, it again pushes into regulatory implications that are not yet justified.

Author Response

Comments and Suggestions for Authors

This manuscript compares knee-flexion kinematics in unilateral transfemoral amputee skiers using a skiing-specific prosthesis with those of able-bodied ski instructors on an indoor ski simulator. The topic is original and clinically relevant, and the use of 3D motion capture in a controlled environment is a clear strength. However, the study has major methodological and interpretive limitations. The sample size is extremely small, the statistical analysis suffers from pseudoreplication (treating repeated turns as independent observations), and only a single kinematic variable (knee flexion at the apex of the turn) is analyzed, despite the title’s reference to three-dimensional kinematics. A more cautious, proof-of-concept framing is warranted, and the statistical approach should be revised so that participants, not individual turns, constitute the unit of analysis.

Title, Highlights, Abstract

• Title mentions three-dimensional kinematic changes, but only the knee flexion angle in the sagittal plane at one instant is analyzed. Please change to “Knee flexion kinematics.”

Response: Thank you for this important and well-taken comment. The title has been revised accordingly to accurately reflect the scope of the analysis and now refers specifically to knee joint kinematics, thereby ensuring consistency between the title and the presented results.

• Tone down the abstract and highlights to preliminary and suggestive because of a small sample size.

Response: Thank you for this helpful comment. The Abstract and Highlights have been revised to use more cautious and preliminary wording, explicitly reflecting the limited sample size and the exploratory nature of the study. Statements implying definitive conclusions have been toned down to suggestive interpretations, and conditional language has been applied where appropriate.

Introduction

• The introduction includes results content referring to the present dataset and what it shows. That belongs in Results/Discussion, not in the Introduction.

Response: Thank you for this important comment. The Introduction has been revised to remove all references to findings from the present dataset. The previously result-oriented statements were rephrased into neutral, hypothesis-based background text, emphasizing regulatory versus biomechanical considerations without referring to observed outcomes. All dataset-specific interpretations have been relocated to the appropriate Results and Discussion sections.

• The gap and novelty are not sharply defined. Clearly distinguish what is new compared to prior skiing prosthesis studies, like older prostheses, different tasks, and no 3D capture.

Response: Thank you for this valuable comment. The Introduction has been revised to more clearly define the research gap and novelty by explicitly contrasting prior skiing prosthesis studies with the present work. Specifically, previous studies are now described in terms of their investigated prosthetic configurations and experimental approaches, and their limitations regarding task specificity and measurement methods are clearly outlined. In contrast, the novelty of the present study is now explicitly stated as the quantitative analysis of knee flexion kinematics using three-dimensional motion capture during controlled simulated alpine skiing with a contemporary skiing-specific prosthesis, thereby clearly distinguishing it from earlier work.

• Early discussion of FIS regulations and classification is a bit premature. I would suggest saving policy-level implications for a cautious note in the Discussion/Conclusion.

Response: Thank you for this helpful suggestion. The Introduction has been revised to limit the discussion of FIS regulations to essential factual context only, without interpretive or policy-level implications. All evaluative statements regarding the relationship between regulatory classification and biomechanical function have been removed from the Introduction and deferred to the Discussion/Conclusion, where they are addressed more cautiously in light of the study results.

Methods

• Extremely small sample with no a priori power/precision analysis. This must be highlighted as a major limitation and addressed as a pilot study or similar.

Response: Thank you for this important comment. The study has now been explicitly framed as an exploratory pilot investigation throughout the manuscript. A sample size estimation has been added to the first paragraph of the Materials and Methods section to support the exploratory design and to guide the interpretation of knee flexion differences, rather than to ensure sufficient power for confirmatory hypothesis testing. In addition, the extremely small sample size is now clearly acknowledged as a major methodological limitation in the newly added Methodological considerations section. The Abstract, Discussion, and Conclusions have also been revised to emphasize the preliminary and hypothesis-generating nature of the findings and to caution against overinterpretation.

• Age distribution is more heterogeneous in amputees; only one female per group. These factors are not modeled or even briefly discussed.

Response: Thank you for raising this important point. The heterogeneity of age in the amputee group and the unbalanced sex distribution (only one female participant per group) are now explicitly acknowledged in the Methodological considerations section. Due to the extremely small sample size, the potential effects of age and sex on knee joint kinematics could not be statistically modeled or included as covariates, and their possible influence on the observed results is therefore recognized as a limitation of the present study.

• Although 3D kinematics are recorded, only one scalar outcome is used: knee flexion angle at the visually identified apex of the turn. That is a very narrow view of kinematic changes.

Response: Thank you for this comment. Although three-dimensional kinematic data were recorded, the analysis was intentionally restricted to a single scalar outcome (knee flexion angle at the apex of the turn) to maintain a focused, proof-of-concept approach consistent with the exploratory nature of this pilot study. This methodological choice and its limitations are now explicitly acknowledged and discussed in the Methodological considerations section, and future studies are outlined to incorporate a broader set of kinematic variables.

• Pseudoreplication is a major concern. Inter-group comparisons assume N=30 (6 turns × 5 skiers) per group, treating turns as independent. The same problem is for intra-individual analyses: turns from the same skier are treated as independent. – snažil jsem se omluvit v Methodological considerations

Response: Thank you for your valid suggestion. This is definitely a concern, although we treated each turn as a separate observation (six turns picked randomly over the course of experiment). Pseudoreplication is definitely a concern in our study, which is why we acknowledged this in the final part and paragraph of the section „Methodological considerations“ as follows:

Thirdly, the experimental protocol. It was conducted in a simulator-based environment, which may not fully replicate the dynamic conditions of outdoor alpine skiing. In addition, repeated turns within individual participants were treated as separate observations, introducing a potential risk of pseudoreplication.

• Multiple comparisons are made (per limb, per condition, per group) with no correction (Bonferroni/FDR). The risk of inflated Type I error is high and needs to be acknowledged or corrected.

Response: Thank you for this point. We are aware of this issue in our pilot study. Four assessing parameters were used to compare both groups of skiers. Thus, if we apply the Bonferroni correction and adjust the significance level (divided by four), resulting in a p-value <0.0125 considered as significant, the results will still be evaluated as significant.

• Effect sizes (Cohen’s d) are given but not fully contextualized. With inflated N from pseudoreplication. Re-calculate using per-participant means and interpret cautiously.

Response: Thank you for this. The „N“ value was changed according to participants, however, Cohen’s d did not changed significantly, the effect size stayed the same. However, the results were corrected accordingly. 

Results

• Large blocks of text interpret each skier’s pattern (likely carving style, possibly leg dominance) speculatively and narratively. Consider moving such case-based commentary to a short subsection or supplement and keep the main results focused on group patterns.

Response: Thank you very much. Each individual skier (an amputee skier, non-disabled skier, or a professional skier) has their own unique skiing style. Therefore, the results have both, the narrative nature and supplemented by a number, to allow readers a better and more complex insight into such a specific topic and give them a more explanatory nature of results.

• Reporting would be more transparent if you present per-subject mean ± SD at each condition (inside/outside, limb, group), then summarize across subjects with group means and 95% CIs, treating n=5 as the unit.

Response: Thank you for your idea. Therefore, the results have been modified to accomodate this request both, in the tables and figures.

 

Discussion

• Amputees can reproduce the qualitative turning strategy (inside vs outside flexion pattern) but with systematically reduced flexion on the prosthetic side. That deserves central emphasis.

Response: Thank you for this important observation. We agree that this represents the central finding of the study. The Discussion has been revised to explicitly emphasize that while transfemoral amputee skiers using a skiing-specific prosthesis reproduce the qualitative turning strategy observed in non-disabled skiers (inside versus outside limb flexion pattern), this preserved coordination strategy is accompanied by a systematic reduction in knee-flexion amplitude on the prosthetic side. This distinction between qualitative similarity and quantitative limitation is now highlighted as the primary interpretation of the results.

• Methodological weaknesses (small n, pseudoreplication, single kinematic outcome, simulator context) are not explicitly discussed as major limitations.

Response: Thank you for this important comment. The manuscript has been revised to explicitly identify and consolidate the major methodological limitations of the study in the Methodological considerations section. These now clearly include the extremely small sample size and exploratory pilot design, the restriction of the analysis to a single kinematic outcome, the simulator-based experimental context, and the potential risk of pseudoreplication arising from repeated turns within individual participants. These limitations are now clearly labelled as major and are discussed to emphasize the preliminary and hypothesis-generating nature of the findings and the need for cautious interpretation.

Conclusions

• The conclusion correctly reiterates the main observation of reduced prosthetic-side flexion and closer intact-side flexion. Still, it again pushes into regulatory implications that are not yet justified.

Response: Thank you for this important comment. We agree that regulatory implications should not be emphasized in the Conclusions given the exploratory nature and limited sample size of the present study. The Conclusions section has therefore been revised to remove explicit references to regulatory or classification changes. The revised Conclusions now focus on the central biomechanical findings—namely the preserved qualitative turning strategy and the reduced knee-flexion range on the prosthetic side—and on their implications for future biomechanical research and prosthetic design, while any potential regulatory relevance is framed only as a topic for future investigation.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made a commendable effort to address the concerns raised in the first round of review. The revised manuscript is clearer, more transparent in its methodological framing, and appropriately positioned as a pilot study. The additions regarding sample size estimation, participant experience, turn-side analysis, and prosthetic design implications have significantly strengthened the manuscript. However, I would like to raise a few remaining points that merit further clarification or discussion before the manuscript is suitable for publication.

First, the sample size estimation now included in the Methods section remains somewhat unclear in terms of its nature and role in the study design. It is not clearly stated whether this is an a priori power analysis performed prior to data collection or a post hoc justification based on observed effect sizes. Given the exploratory character of the study, this distinction is important for interpreting the strength of the reported statistical inferences. The authors should clarify this explicitly and, ideally, provide information on the statistical test, effect size assumption, and variability parameters used for the power estimation.

Second, while the authors appropriately emphasize the rarity of the studied population and justify the sample size accordingly, they should expand the discussion on generalizability. The participants in this study are highly selected: all were able to ski on both limbs using a specialized prosthesis, and testing took place in a controlled indoor simulation environment. This context does not reflect the majority of transfemoral amputee skiers, many of whom ski with outriggers or do not use bilateral skiing techniques. A more explicit acknowledgment of these constraints on ecological validity and generalizability is warranted in the Discussion section.

Third, there is a potential methodological issue concerning the treatment of repeated trials. The study analyzes six turns per participant, yet it is unclear whether these were averaged per subject before group-level comparisons or treated as independent observations. If the latter, this may introduce pseudoreplication, artificially inflating the sample size and underestimating inter-individual variability. The authors should clarify their statistical approach in this respect. If repeated measures were treated as independent, they should acknowledge this as a limitation and consider whether subject-level averaging would yield more conservative and representative comparisons.

Fourth, I appreciate the detailed reporting of individual participant data in Tables 3 and 4. However, the accompanying text in the Results section is lengthy and at times repetitive. The authors may consider moving the per-participant narrative to a supplementary file and retaining only a summary paragraph in the main manuscript. This would improve readability without sacrificing transparency.

Fifth, the study focuses exclusively on kinematic outcomes—specifically, knee flexion at the apex of the turn. This choice is well-justified for a pilot investigation. However, the Discussion could be strengthened by briefly referencing emerging approaches in gait biomechanics that complement traditional kinematic analyses, particularly in contexts with small and heterogeneous samples. For example, recent work by Trabassi et al. (Optimizing rare disease gait classification through data balancing and generative AI: insights from hereditary cerebellar ataxia. Sensors (2024)) on hereditary cerebellar ataxia has shown that combining data balancing techniques with generative AI can enhance gait pattern classification in rare disease settings. These computational tools may prove valuable in future prosthesis-related research, where both sample size and inter-individual variability are persistent challenges.

Finally, I encourage the authors to add a brief methodological note in the Limitations section about potential leg dominance effects, which are acknowledged in the interpretation of control group results but not explicitly modeled. Although this is a minor point, making it explicit would further reinforce the transparency of the study design.

In conclusion, the revised manuscript is significantly improved and offers a meaningful contribution to the biomechanics of skiing with lower-limb prostheses. With the above clarifications and minor additions, the manuscript will be well-positioned for publication.

Author Response

The authors have made a commendable effort to address the concerns raised in the first round of review. The revised manuscript is clearer, more transparent in its methodological framing, and appropriately positioned as a pilot study. The additions regarding sample size estimation, participant experience, turn-side analysis, and prosthetic design implications have significantly strengthened the manuscript. However, I would like to raise a few remaining points that merit further clarification or discussion before the manuscript is suitable for publication.

 

First, the sample size estimation now included in the Methods section remains somewhat unclear in terms of its nature and role in the study design. It is not clearly stated whether this is an a priori power analysis performed prior to data collection or a post hoc justification based on observed effect sizes. Given the exploratory character of the study, this distinction is important for interpreting the strength of the reported statistical inferences. The authors should clarify this explicitly and, ideally, provide information on the statistical test, effect size assumption, and variability parameters used for the power estimation.

Response:

We thank the reviewer for this important comment. We would like to clarify that the sample size estimation was performed a priori, prior to data collection, and was intended solely to support the exploratory and pilot nature of the study rather than to ensure adequate power for confirmatory hypothesis testing.

The estimation was based on effect size assumptions derived from previously published literature on knee flexion in recreational skiers and on reported differences between intact and prosthetic limbs. Specifically, a difference of approximately 20% in mean knee flexion between groups was considered clinically relevant, assuming a knee flexion range of approximately 30°–80°. Under these assumptions, and using a two-group comparison of mean knee flexion (independent samples comparison), a sample size of five participants per group was estimated to yield approximately 80% power at a 5% significance level.

We have now revised the Methods section to explicitly state the a priori nature of this estimation, the assumed effect size, and its role as a feasibility-oriented justification appropriate for an exploratory pilot study, rather than as a basis for strong inferential claims.

 

Second, while the authors appropriately emphasize the rarity of the studied population and justify the sample size accordingly, they should expand the discussion on generalizability. The participants in this study are highly selected: all were able to ski on both limbs using a specialized prosthesis, and testing took place in a controlled indoor simulation environment. This context does not reflect the majority of transfemoral amputee skiers, many of whom ski with outriggers or do not use bilateral skiing techniques. A more explicit acknowledgment of these constraints on ecological validity and generalizability is warranted in the Discussion section.

Response:

We thank the reviewer for highlighting this important point. We fully agree that the generalizability of the present findings is limited by the highly selected nature of the study population and the experimental context.

The participants were a specific subgroup of unilateral transfemoral amputee skiers who were capable of bilateral skiing using a skiing-specific prosthesis, and all testing was conducted under controlled laboratory conditions on an indoor ski simulator. As such, the results cannot be generalized to the broader population of transfemoral amputee skiers, many of whom ski using one ski with outriggers or do not employ bilateral skiing techniques in accordance with current para-alpine classification rules.

We have expanded the Discussion section (Methodological Considerations) to explicitly acknowledge these limitations in ecological validity and generalizability and to clarify that the findings are intended to describe biomechanical characteristics under controlled conditions in a selected subgroup, rather than to represent typical skiing behavior of transfemoral amputees in real-world alpine environments.

 

Third, there is a potential methodological issue concerning the treatment of repeated trials. The study analyzes six turns per participant, yet it is unclear whether these were averaged per subject before group-level comparisons or treated as independent observations. If the latter, this may introduce pseudoreplication, artificially inflating the sample size and underestimating inter-individual variability. The authors should clarify their statistical approach in this respect. If repeated measures were treated as independent, they should acknowledge this as a limitation and consider whether subject-level averaging would yield more conservative and representative comparisons.

Response:

We thank the reviewer for raising this important methodological point. We agree that the treatment of repeated turns per participant requires explicit clarification.

In the present study, six left and six right turns per participant were analyzed to characterize knee flexion behavior under each experimental condition. These repeated turns were not intended to represent independent subjects but rather repeated observations within individuals, allowing assessment of within-subject consistency under controlled conditions.

We acknowledge, however, that treating repeated turns as independent observations may introduce a risk of pseudoreplication and may underestimate true inter-individual variability at the group level. We have therefore expanded the Methodological considerations section to explicitly acknowledge this limitation and to clarify that subject-level averaging or hierarchical (mixed-effects) modeling would represent a more conservative and statistically robust approach in future studies with larger sample sizes.

Given the exploratory nature of this pilot study and the rarity of the studied population, the present analysis was intended primarily to identify systematic trends and effect directions rather than to support definitive inferential conclusions.

 

Fourth, I appreciate the detailed reporting of individual participant data in Tables 3 and 4. However, the accompanying text in the Results section is lengthy and at times repetitive. The authors may consider moving the per-participant narrative to a supplementary file and retaining only a summary paragraph in the main manuscript. This would improve readability without sacrificing transparency.

Response:

We thank the reviewer for this constructive suggestion and for recognizing the value of the detailed individual-level reporting. We agree that, in larger cohort studies, extensive per-participant narratives may reduce readability and are often better placed in supplementary materials.

In the present study, however, we intentionally retained the participant-specific descriptions in the main Results section due to the very small sample size and the exploratory nature of the work. Inter-individual variability is a key characteristic of unilateral transfemoral amputation and is central to the interpretation of prosthesis-related movement strategies. The accompanying narrative was therefore intended to support transparent interpretation of the quantitative results in Tables 3 and 4 and to contextualize large effect sizes and asymmetries that may otherwise appear inconsistent or overly generalized at the group level.

We believe that retaining this level of detail in the main manuscript improves interpretability and transparency for this rare and heterogeneous population. Nevertheless, we acknowledge the reviewer’s point and will consider relocating participant-level narratives to supplementary materials in future studies with larger sample sizes.

 

Fifth, the study focuses exclusively on kinematic outcomes—specifically, knee flexion at the apex of the turn. This choice is well-justified for a pilot investigation. However, the Discussion could be strengthened by briefly referencing emerging approaches in gait biomechanics that complement traditional kinematic analyses, particularly in contexts with small and heterogeneous samples. For example, recent work by Trabassi et al. (Optimizing rare disease gait classification through data balancing and generative AI: insights from hereditary cerebellar ataxia. Sensors (2024)) on hereditary cerebellar ataxia has shown that combining data balancing techniques with generative AI can enhance gait pattern classification in rare disease settings. These computational tools may prove valuable in future prosthesis-related research, where both sample size and inter-individual variability are persistent challenges.

Response:
Thank you for this valuable suggestion. The Discussion section has been expanded to briefly reference emerging data-driven methodological approaches, including recent work by Trabassi et al., highlighting the potential of data balancing and generative AI in small and heterogeneous biomechanical datasets. This perspective is presented strictly as a methodological direction for future research.

 

Finally, I encourage the authors to add a brief methodological note in the Limitations section about potential leg dominance effects, which are acknowledged in the interpretation of control group results but not explicitly modeled. Although this is a minor point, making it explicit would further reinforce the transparency of the study design.

Response:
Thank you for this helpful suggestion. A brief methodological note has now been added to the Methodological considerations section explicitly stating that potential leg dominance effects were acknowledged in the interpretation of control group results but were not modeled as an independent factor due to the limited sample size and exploratory nature of the study. This clarification further improves the transparency of the study design.

In conclusion, the revised manuscript is significantly improved and offers a meaningful contribution to the biomechanics of skiing with lower-limb prostheses. With the above clarifications and minor additions, the manuscript will be well-positioned for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear,

Please find my comments attached.

Kind regards

Comments for author File: Comments.pdf

Author Response

First, in the Abstract, the statistical approach is still not clearly defined. The analysis is described only in very general terms and an “examination of intra-individual and inter- exact statistical procedures used. Moreover, the Abstract does not report the level of statistical significance, nor does it mention the statistical software applied. This lack of detail limits the interpretability and transparency of the findings already at the abstract level.

Response:

We thank the reviewer for this comment. We agree that the description of the statistical approach in the Abstract was too general. To improve transparency and interpretability at the abstract level, we have revised the Abstract to more clearly specify the nature of the statistical comparisons, the significance threshold, and the statistical software used.

The revised Abstract now explicitly indicates that paired and independent group comparisons were performed, that statistical significance was evaluated at p < 0.05, and that the analyses were conducted using IBM SPSS Statistics. These additions improve clarity while maintaining the concise format appropriate for an abstract.

 

More critically, the Methods chapter (Section 2) remains the weakest part of the manuscript. Even in the revised version, there is still no explicit description of the concrete statistical tests used in the analysis. Throughout the manuscript, the authors repeatedly emphasize intra- versus inter-individual levels of analysis; however, these terms describe only the level of analysis, not the statistical methods -clear methodological limitation and requires precise clarification and explicit specification of the statistical procedures in the Methods chapter. Furthermore, it is unusual that the research hypotheses are found in the introductory paragraph of the Methods chapter, and not at the end of the Introduction chapter.

Response:

We thank the reviewer for this detailed and constructive comment. We agree that, while the statistical analyses were implemented as described, their presentation in the Methods section did not sufficiently emphasize the concrete statistical procedures used, as opposed to the conceptual level of analysis (intra- vs. inter-individual).

We have therefore revised Section 2 to explicitly specify the statistical tests applied for each type of comparison, including normality testing, paired and independent group comparisons, and effect size estimation. The description has been restructured to clearly distinguish the statistical methods from the analytical levels, thereby improving methodological transparency.

In addition, we agree that the research hypotheses are conventionally presented at the end of the Introduction. We have therefore relocated the hypothesis-related statements from the opening of the Methods section to the end of the Introduction chapter to align with standard reporting practices.

We believe these revisions substantially strengthen the clarity and rigor of the Methods section without altering the analytical approach or results.

 

In the Results chapter, additional issues are evident. For example, Table 4 does not include effect size indicators in its legend, unlike Table 3, and no explanation is provided for this inconsistency. Furthermore, the figures are not sufficiently self-explanatory. They do not specify which statistical analyses were used to obtain the presented results, particularly for the bar graphs shown in Figures 7and 8. Also, Figures 4, 5 and 6 are not sufficiently informative. Figure 4 does not explain in addition from which plane this coordinate system is observed. Concerning Figures 5 and 6, the axes are not defined: Which is mediolateral, which is anterior-posterior, which is longitudinal. So, a lot of technical omissions?

Response:

We thank the reviewer for carefully identifying these presentation inconsistencies and omissions. We agree that clearer and more consistent reporting of effect sizes, statistical procedures, and figure annotations improves transparency and interpretability.

We have therefore revised the Results section and figure captions accordingly. Specifically, we have harmonized the reporting of effect sizes across Tables 3 and 4 by explicitly including effect size definitions in both table legends. We have also expanded the captions of Figures 7 and 8 to clearly indicate the statistical comparisons used to generate the bar graphs.

In addition, we have revised the captions of Figures 4–6 to provide explicit definitions of the coordinate systems and anatomical axes (mediolateral, anterior–posterior, and longitudinal), as well as the viewing plane of the coordinate systems. These additions improve the technical clarity of the figures without altering the underlying data or analyses.

We believe these revisions address the reviewer’s concerns regarding technical completeness and consistency.

 

Finally, a major limitation concerns the Discussion chapter, which is largely restricted to comparisons with only three references. This is not sufficient, especially considering that the reference list includes a total of 15 sources. A broader and more critical integration of the existing literature is necessary to adequately contextualize the findings and strengthen the scientific contribution of the study.

Thank you for your constructive comments regarding the scope and depth of the Discussion section. In response, we have substantially expanded the Discussion to better integrate the present findings within the broader context of alpine skiing biomechanics and lower-limb amputee locomotion.

First, to support the interpretation of inside–outside limb function during alpine skiing turns, we complemented the previously cited work by Fasel et al. with additional biomechanical studies describing fundamental coordination and load-distribution strategies in alpine skiing. Specifically, we added references to Spörri et al. and Supej & Holmberg, which consistently report functional asymmetries between the inside and outside limbs as an inherent feature of alpine skiing technique.

Second, to better contextualize the asymmetries observed in the control group, we incorporated literature demonstrating that a certain degree of inter-limb asymmetry and leg dominance is common even in able-bodied alpine skiers. We added the study by Steidl-Müller et al., which reports systematic limb symmetry index deviations in competitive alpine ski racers, as well as the work by Promsri et al., which highlights the role of sensorimotor leg dominance in shaping lower-limb asymmetries during alpine skiing. These additions support our interpretation that the smaller asymmetries observed in the control group likely reflect functional leg dominance rather than structural limitations or methodological artefacts.

Third, to address concerns regarding inter-individual variability and the small sample size, we expanded the Discussion to include evidence from amputee locomotion studies. We added references to Hobara et al., who demonstrated limb-specific loading asymmetries and compensation strategies in unilateral transfemoral amputees during dynamic tasks, and to Schmid-Zalaudek et al., who provided large-cohort gait reference data showing pronounced inter-limb asymmetries and between-subject variability in transfemoral amputees. These studies support the interpretation that the observed variability in the present study represents inherent biomechanical adaptation rather than measurement error.

Finally, we strengthened the methodological outlook of the Discussion by referencing recent data-driven approaches for analyzing movement patterns in small and heterogeneous samples. In this context, we included the study by Trabassi et al., which demonstrated that combining data balancing with generative artificial intelligence can enhance gait pattern classification despite pronounced inter-individual variability.

Together, these additions substantially broaden the Discussion, improve its theoretical grounding, and clarify the biomechanical and methodological context of the present findings. We believe that the revised Discussion now provides a more comprehensive and balanced interpretation of the results.

Thank you again for your valuable feedback, which helped us to significantly improve the quality and clarity of the manuscript.

 

In conclusion, while the research idea itself is very good and promising, the implementation of the study and presentation of the manuscript are insufficient for publication in its current form.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Authors

I'm pleased with the changes made to this manuscript. 

Best regards

Author Response

Dear Reviewer,

thank you very much for your kind message and for taking the time to review the revised manuscript.

We sincerely appreciate your positive feedback and are glad that the changes addressed your comments satisfactorily.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Dear,

Please find my comments attached.

Kind regards

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf