Review Reports - Biological or Prosthetic Limb—Which Is More Advantageous for Running Performance? A Narrative Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Attached document has all the comments for minor revisions.

Comments on future research directions and some research questions are very helpful for readers.

Comments for author File: Comments.pdf

Author Response

We greatly appreciate this Reviewer’s thorough review and valuable feedback on our manuscript. The Reviewer made a series of inspiring comments, which undoubtedly improved the clarity of our manuscript. Below are our responses to each comment and how we implemented the Reviewer’s feedback into our updated manuscript.

Please see attached

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The goal of this narrative review is to provide an overview of the current literature regarding leg prostheses and running performance, in particular it is trying to answer the following question (the title itself): Biological or prosthetic limb, which one is more advantageous for running performance?

The authors in the introduction highlight how, in the past decade, both researchers and the general public got interest in this question, notably because of two athletes, Oscar Pistorius and Blake Leeper, challenged the IAAF/WAA rules that prevented athletes with amputation to compete with non-amputee athletes. With the continuous advancement of materials and designs (not only in running-specific prostheses, but also in running shoes, track materials etc.) the question of fairness among competitors is important and worth exploring. Throughout the introduction the authors evidence how the main goal of this review is to compare biological and prosthetic legs and “explore whether prosthetic legs provide amputee athletes with an unfair advantage over traditional athletes during sport competitions, particularly during running” (Lines 90-92).

The authors highlight three key components related to running performance:

1) propulsion forward

2) anatomical repositioning

3) physiological limitations

After an initial screening, they included 11 articles in their review. Of these: all articles include at least one athlete with transtibial amputation (bilateral or unilateral) and 4 of them include also non-amputees.

However, when analyzing the papers and summarizing their results, the authors tend to focus on how different prostheses affect running biomechanics instead of comparing them to biological legs.

In particular, for the first two key components:

- propulsion forward

The authors mention how in Grabowski et al. (2010), Taboga et al. (2020), Beck et al. (2016) running specific prostheses with a J-shape perform better than C-shaped prostheses in terms of ground reaction forces, contact times and elastic energy return. However, neither Taboga et al. (2020) nor Beck et al. (2016) compare prosthetic and biological legs. Additionally, in Grabowski et al. (2010) prosthetic legs perform worse than biological legs. Similarly, the fact that prosthetic stiffness affects running performance in athletes with amputation (Grobler et al. 2017, Beck et al. 2017) does not directly translates into better performance compared to non-amputee runners; the authors in fact acknowledge that “no literature compares these prosthetics and their corresponding stiffness to the biological legs/bones.” (Lines 190-191)

- anatomical repositioning

They report one relevant study (Weyand et al., 2009) that observed shorter swing times and longer stride lengths in a single athlete with bilateral amputation compared to non-amputee runners. Additionally, they report 3 studies (Taboga et al. 2020, Beck et al. 2016 and 2017) where an increase in prosthetic stiffness is associated with faster stride frequencies in athletes with bilateral amputation; however, these studies do not include comparisons with biological legs. They suggest that “In theory, engineers could increase leg length to increase the ability to reach farther, decrease the weight of the prosthetic to reduce the moment of inertia of the leg and minimize swing time, and increase prosthetic stiffness to increase stride frequency as well as ground reaction forces.” (Lines 239-243). This claim is based on a pilot study of Wilson et al. (2009), which does not compare prosthetic and biological legs. Speculation on future prosthetic designs does not constitute evidence for a performance advantage of current prosthetic legs over biological legs.

In the discussion section, the authors include the following statements:

1) “The current research seems to provide evidence on the notion that prosthetics may provide an advantage in running biomechanics for people with transtibial amputation relative to their non-amputee peers.” (lines 311-313)

And

2) “while evidence points to other models possessing larger forward propulsion potential due to their ability to produce higher ground reaction forces partly due to corresponding higher stiffness levels, these concepts have not been compared to their biological limb counterparts” (lines 335-338)

The second statement directly contradicts the first. The observations that some prostheses allow runners to apply higher ground reaction forces compared to other models does not inherently imply an unfair advantage compared to non-amputee runners.

This inconsistency is not addressed by the authors, their conclusion states:

“The current state of the literature suggests that amputee athletes may have a unique running performance advantage over non-amputee athletes in certain performance domains.” (Lines 285-387)

A critical look at the 11 studies included in the review reveals how this conclusion can not be supported: one study focused on long-jump, which has limited relevance to running performance, two studies (Brüggemann et al. 2008 and Weyand et al. 2009) report a running performance advantage of prosthetic legs compared to biological legs, while all remaining studies either lack direct comparisons between prosthetic and biological legs or report inferior running performance metrics for athletes with amputation (Grabowski et al. 2010).

In light of this findings, I strongly recommend that the authors perform a major revision of the paper to accurately reflect the current state of the literature, which does not provide conclusive evidence of a running performance advantage for prosthetic over biological limbs.

Additional notes:

- In line 63, the authors mention that Oscar Pistorius “placed 8th when competing in a 4x400 relay” . It would be more appropriate to report his placement in the individual event, as the relay included three other non-amputee sprinters.

- In Table 2, the authors should provide a breakdown of participants for each study, specifying the number of athletes with single-leg amputation, double-leg amputation, and non-amputees separately.

Author Response

We greatly appreciate this Reviewer’s in-depth analysis and highly constructive feedback on our manuscript. This Reviewer provides a well-written overview of our narrative review with critical points that should be addressed adequately. A key contention of this Reviewer is that the Reviewer feels that the evidence provided in the manuscript does not sufficiently support our conclusion. One of the reasons for the insufficient evidence is the lack of direct comparative studies between amputees and non-amputees present in the modern literature. While we clearly indicated that our conclusions should be taken with caution due to the limitations of the present literature (Lines 375-376), which must be solved by future research (Lines 365-373), it remains vital to emphasize and make our readers aware of the limitations associated with our manuscript.

As shown below, we have worked through the entire manuscript to address the Reviewer’s broad-based concerns by finding ways to soften the language and address areas that may initially seem inconsistent. By doing so, we hope to align the document to a single conclusion and provide a scientifically sound narrative review for the reader to understand the present state of the literature. Thus, we hope that even if the reader’s thoughts do not perfectly align with the author’s, our review provides them with genuine and transparent information that they can use to reach said conclusion and that the findings can inspire them to help better the experiences of para-athletes through their respective professions. Finally, we addressed the Reviewer’s additional notes and implemented them into our updated manuscript.

Please see attached

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Summary

The purpose of this article is to compile the current state of the literature on the performance of prosthetic limbs compared to biological limbs in the context of athletics, particularly running and long jumping. This narrative review presents the current findings based on their specific criteria for inclusion and explores the proposed question of whether prosthetic limbs have reached the point of a competitive advantage over biological limbs in three specific domains: forward propulsion, limb repositioning, and physiological limits. The authors come to the conclusion that based on the current state of the literature, transtibial amputee runners have a potential competitive advantage in some but not all domains. The researchers also describe areas for future analysis within this scope of inquiry.

General Comments & Review

This narrative review offers a unique perspective on the impact of prosthetics in the context of running performance. Their parameters for inclusion create a sufficiently narrow scope with which to analyze their initial question, of the potential competitive advantage presented by the current technology of running specific prostheses compared to biological limbs. This question is of major relevance in the current age of understanding fairness and inclusion in many athletic contexts including para-athletics. The review is well laid out and well done along the lines of three domains forward propulsion, anatomical repositioning, and physiological limits.

The introduction lays out the evolution and advancement of running prosthetics and succinctly lays out the controversies that have surrounded bilateral transtibial amputee runners and their eligibility cases and findings. One stipulation worth mentioning in the Leeper case description: Leeper was petitioning to compete in the able-bodied Olympics on limb lengths that exceeded those allowable for the para-Olympics based on the MASH specifications. This should be mentioned as it was a key fact of the arbitration. There already are leg length parameters that para-athletes must comply with to be eligible to compete and the athlete in question was already out of compliance. There is also disagreement from other researchers on the claim that the length of prostheses does not impact running speed.

The methods for this narrative review serve the purpose of answering the question per the domains of performance they have stipulated and covers the available literature on the topic. The results of the inclusion criteria lead to the review of 11 articles that capture the view of literature as it stands at the current moment.

The results of the review of each article are told through summaries of the findings in each domain and are sound based on the knowledge available about the determinants of running performance.

Forward Propulsion: The authors report that the available evidence suggests that stiffness and shape of prosthesis although difficult to compare to biological limbs, and decreased total mechanical work required to run may be factors in a potential advantage of amputee athletes.

Anatomical Repositioning: The authors report that the available evidence suggests that the modifiable aspects of length, weight, and stiffness of prostheses could in theory be engineered to lead to a potential advantage in running speed.

Physiological Limitations: The authors report that the available evidence suggests that the aerobic capacity of amputee athletes is similar to able-bodied peers.

The Discussion reviews the findings of the literature and discusses the fact that although regulatory bodies do consider anthropomorphic measures of prosthetics, some consideration should be taken for the materials they are made out of and also be normalized for that. This statement is critical, and I believe warranted. They also state that evidence suggests that prosthetics may afford amputee runners an advantage in some but not all of the domains within the review. The modifiability of lower leg prosthetics on an individual basis is most significant. The authors point out that there is a small sampling of research on this topic and provide well founded speculation as to why. They also provide suggestions for other potential lines of inquiry into the topic. The authors end by concluding that more scientific inquiry is needed based on the foundation of the work reviewed to further understand the potentially advantageous aspects of running prostheses. This should be done to protect the fairness of sport for both traditional and para-athletes.

Specific Comments

Line 73-74- Which researchers found that prosthetic height does not offer an advantage? – The only citation is for the CAS hearing after this statement. If the authors are referring to citation 9 in this regard it should be noted that the athlete in question was running on limbs that were out of compliance with paralympic standards (+15 cm). A comment paper: “Artificially long legs directly enhance long sprint running performance” by authors Peter Weyand et al from 2022 (https://doi.org/10.1098/rsos.220397) explains that the standard by which the authors claim that the amputee maintains no advantage is based on the standard of “not being faster than the intact-limb record holder”. There is also data reported in this article from the testing done on said athlete that refutes the claim of leg length not affecting speed. The comment authors conclude that artificially long legs do enhance long sprint running performance. It is recommended that the authors provide a bit of this context or at least acknowledge that there is disagreement on the matter of prosthetic limb length and running speed or advantage offered by the length of the prostheses. Currently, these lines, read like the CAS, came to their conclusion contrary to the evidence.

Line 85 – delete “of literature”

Line 166-167- The Taboga, Beck paper referenced was retracted due to multiple reasons, not just the fact they had a low sample size. The retraction was also due to their interpretation of a non-significant finding as a lack of difference. The authors of the retracted paper also offer conclusions that are not supported by the evidence provided. Their conclusion that stiffness and leg length/ height do not impact speed is contrary to the evidence provided in the article “Appendix Table 1 and Appendix Table 2.” Leg length does impact the maximal speed that can be achieved by an athlete. It is recommended to revise this statement to reflect that there were substantial issues with the article and further justification for utilizing it here besides just that low subject number being the nature of studying this population.

Author Response

We are deeply appreciative of this Reviewer’s thorough review and precious feedback on our manuscript. We are delighted to hear that the manuscript communicated the complexities of this topic to the Reviewer in a sound manner. Further, we are thankful for this Reviewer’s specific suggestions on our manuscript and believe they will strengthen our manuscript’s quality, readability, and transparency. Listed below are our responses to each of the Reviewer’s specific points of concern and how we addressed and incorporated them into our revised manuscript.

Please see attached

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I appreciate the work the authors have done in revising their manuscript and adopting a more balanced and objective approach when comparing the the performance of prosthetic vs biological legs. I commend them for the improvements in clarity and the overall structure of the paper. I also fully agree with the main message: more research is needed to better understand the advantages and disadvantages of prosthetic legs in comparison to biological legs.

Being this a delicate topic, balancing fairness in competition with inclusivity, I encourage the authors to maintain a neutral tone and to present the findings of different studies objectively, without speculation or personal interpretation. While the manuscript was overall greatly improved, I found a couple of passages that, in my opinion, require further refinement.

In the discussion section, the following statement may lead to misinterpretation:

“Runners can change their blade shape depending on personal preference or the event being run, which can influence mechanical power and optimal ground reaction forces. This may partially explain the previously noted observations of propulsive forces proven to be greater in transtibial amputees during the take-off phase, as observed in the long jump event” (Lines 356-359)

Referencing a long jump study to suggest that prosthetic legs provide an advantage in terms of ground reaction forces (GRFs) in sprinting is questionable. Multiple studies, cited also in this review (e.g., Grabowski et al., Weyand et al.), indicate that GRFs for prosthetic limbs are lower than those of biological legs in sprinting, and do not confer a performance advantage. If anything, they may present a disadvantage.

Additionally, the following statement should be reconsidered:

“Wholistically, these features are advantageous to transtibial amputee runners, which non-amputees may find difficult or even impossible to replicate through rigorous training at some competitive levels.” (Lines 362-363)

While there is limited evidence suggesting a potential advantage in stride length and stride frequency, this appears to be counterbalanced by lower GRFs. As the authors point out there is a significant gap in the literature comparing biological and prosthetic legs and at this stage it is unlikely that a definitive conclusion, either supporting or refuting a performance advantage, can be drawn. I recommend revising this sentence to ensure a more neutral and evidence-based framing.

Similarly, in the conclusions, I encourage the authors to either remove the following sentence:

“The current state of the literature seems to suggest that amputee athletes may have a unique running performance advantage over non-amputee athletes in certain performance domains” (Lines 433-436).

Or to revise it, providing a more precise and balanced discussion, specifying both the areas where prostheses may offer advantages (such as stride length, stride frequency, and the ability to alter mechanical properties) and the areas where they present disadvantages (most notably lower GRFs).

Additional notes:

Lines 64-68: I recommend reporting only the individual results of Oscar Pistorius. In addition to his placement in each round, I suggest to include his times (Round 1: 45.44 s, Semifinal: 45.54 s) and avoid evaluative language (such as: "due to a comparatively subpar performance compared to his initial debut").

“Leeper’s prosthetic blades were too long (+15 cm above paralympic standards) and would give him an unfair competitive advantage due the additional length affecting his stride frequency” (Lines 77-79)

Do the authors mean “stride length”?

Author Response

Reviewer 2: See attached for highlights

General Commentary

I appreciate the work the authors have done in revising their manuscript and adopting a more balanced and objective approach when comparing the the performance of prosthetic vs biological legs. I commend them for the improvements in clarity and the overall structure of the paper. I also fully agree with the main message: more research is needed to better understand the advantages and disadvantages of prosthetic legs in comparison to biological legs.

We greatly appreciate this positive feedback and thank the Reviewer for reviewing our revised manuscript. We are also overjoyed that this Reviewer feels the updated manuscript is noticeably improved in its objectivity, clarity, and overall narrative structure.

Being this a delicate topic, balancing fairness in competition with inclusivity, I encourage the authors to maintain a neutral tone and to present the findings of different studies objectively, without speculation or personal interpretation. While the manuscript was overall greatly improved, I found a couple of passages that, in my opinion, require further refinement.

Thank you to this Reviewer for bringing up this critically important topic. The specific comments and suggestions given by the Reviewer are highly insightful and helpful. We fully agree with the Reviewer that it is essential to maintain a neutral tone, present the findings from the literature as objectively as possible, and avoid any baseless or unrigorous statements in the manuscript. In addition, we are grateful the Reviewer, clearly dedicated to this issue, has continued to passionately find ways to improve our manuscript to ensure the final version of this narrative review properly balances concepts relating to inclusivity and competitive fairness. Below, we have worked through the entirety of Reviewer 2’s second review to address all the comments adequately. Ultimately, we are confident that the noted changes will substantially strengthen the manuscript and its potential to fully explain the complexities of the present topic to future readers.

In the discussion section, the following statement may lead to misinterpretation:

“Runners can change their blade shape depending on personal preference or the event being run, which can influence mechanical power and optimal ground reaction forces. This may partially explain the previously noted observations of propulsive forces proven to be greater in transtibial amputees during the takeoff phase, as observed in the long jump event” (Lines 356-359)

We are thankful for this thought-provoking comment. The Reviewer cites two studies within the present review, which they suggest provide evidence that ground reaction forces are lesser in those with transtibial amputations than those with biological legs, leading to an inherent logical contradiction. First, as noted in their previous review, Grabowski et al. do not compare amputees to non-amputees. We went back and double-checked to see if Grabowski had compared their sample to a biological norm; however, this does not seem to be the case. With that said, we understand the Reviewer’s concern about findings relating to ground reaction force findings in the second cited article by Weyland et al. While Weyland et al., found peak horizontal ground reaction forces to be lesser in amputees than in the biological limbed runners, vertical ground reaction forces were found to be similar. Upon reexamination of the relevant articles, we agree that this is an area of our manuscript that needs adjustments to ensure clarity to the reader. We have expanded the relevant paragraph to emphasize the inconsistent findings regarding the ground reaction force measurements comparisons between people with amputations and their biological counterparts, which we believe should validate the concerns of the Reviewer and give the reader a more comprehensive view of the topic.

Lines 356-368: The ability to alter the structure and configuration of the lower-leg prosthetic on an individualized basis may be one of the most significant advantages transtibial runners possess. Runners can change their blade shape depending on personal preference or the event being run, which can influence mechanical power and optimal ground reaction forces. This may partially explain the previously noted observations of propulsive forces proven to be greater in transtibial amputees during the takeoff phase, as observed in the long jump event or the lower amount of mechanical work needed for the running dynamics of an amputee athlete [22]. Given that these observations were only documented in a single study (ref), there is still contention on whether prosthetics can truly confer advantages to the entirety of a running competition. While the takeoff phase of the long jump event may be instructive to advantages relating to the initiation of running motions, other studies have argued that prosthetics may be disadvantageous in force production throughout the race.

Additionally, the following statement should be reconsidered:

We are highly thankful to the Reviewer for this invaluable and inspiring comment. To the author’s knowledge, no literature accurately illustrates amputee athlete stride length and frequency to be similar to or less than those with biological limbs (see lines 69-98 and 241-284). However, we agree with the Reviewer’s sediment that the quote “Wholistically, these features are advantageous to transtibial amputee runners” may not be perceived as neutral. Therefore, we altered this sentence to help ensure it does not appear overly affirmative.

Lines 370-374: Wholistically, these modifiable features may be advantageous to transtibial amputee runners, which non-amputees may find difficult or even impossible to replicate through rigorous training at some competitive levels, as the capability to simply transition to different models that produce changes in performance is an ability that the non-amputee athlete unlikely parallels.

Similarly, in the conclusions, I encourage the authors to either remove the following sentence:

“The current state of the literature seems to suggest that amputee athletes may have a unique running performance advantage over non-amputee athletes in certain performance domains” (Lines 433-436).

This is a highly insightful comment, and we fully agree with the Reviewer that our conclusive sentence might be prone to support that amputee athletes have some advantages to improve their running performance over their non-amputee peers. Considering this sentence is the theme sentence of our conclusion, we have opted to keep it in the revised manuscript. However, we extensively rewrote it to soften our tone regarding the certain benefits resulting from the amputations. To further address this concern, we added more context following the statement in question to ensure the reader understood more of the complexities associated with the manuscript’s conclusion.

Lines 444-462: The current literature seems insufficient to either entirely support or refute a performance advantage among amputee athletes compared to their non-amputee counterparts. Some studies suggested that amputee athletes may have a unique running performance advantage over non-amputee athletes in certain performance domains, particularly regarding stride length, stride frequency, and the inherent ability to make alternations/customize the mechanical properties of their prosthetics. While some segments of running performance may elicit optimal ground reaction forces (i.e., at takeoff points relating to vertical force production), the present literature points to ground reaction forces potentially being disadvantageous during the totality of competition. Therefore, there is still a significant gap in the literature comparing the running performance between biological and prosthetic legs and more work is needed to decipher if amputees may or may not enjoy advantages during certain aspects of a race compared to others. Additionally, it is crucial to point out the limited nature of the present field of literature and the relative lack of comparative studies between amputee and non-amputee athletes, which can help the reader derive their takeaways from this narrative review. Therefore, to protect the sanctity of competition and allow all athletes to receive the full benefits of fair competition, caution should be practiced, meaning traditional athletes and para-athletes should compete in separate categories in sanctioned sporting environments until further research can clarify and add to the present observations.

However, if the Reviewer insists on removing this sentence, we will be happy to do so.

Additional notes:

Lines 64-68: I recommend reporting only the individual results of Oscar Pistorius. In addition to his placement in each round, I suggest to include his times (Round 1: 45.44 s, Semifinal: 45.54 s) and avoid evaluative language (such as: “due to a comparatively subpar performance compared to his initial debut”).

We thank the Reviewer for this additional opportunity to provide greater clarity to our introduction with a more neutral tone. While we believe all of Oscar Pistorious’ Olympic performances are relevant, we highly agree with the Reviewer that including race times would benefit the reader. We retrieved his official times from the Olympic Organization’s Website. We have made adjustments to the section in question accordingly. Please see below.

Lines 59-69: As prosthetics continue to advance due to the ever-growing understanding of biomechanics, some in both amputee and non-amputee running sports communities are concerned that different models or brands of prosthetics will offer users unfair advantages over other para-athletes or even able-bodied athletes. This topic has undergone many rounds of discussion, with each reemergence corresponding to exceptional performances by amputee athletes. For example, in 2012, Oscar Pistorius, a double transtibial amputee, became the first amputee to compete in the Olympics and placed 8th in a 4×400 relay (group time of 3:03:46 minutes). Further, Pistorious also placed second in his preliminary 400-meter race (45.44 seconds) at the 2012 Olympics; however, he later put up a subpar performance (46.54 seconds) compared to his initial debut in the semifinal heat and placed last.

“Leeper’s prosthetic blades were too long (+15 cm above paralympic standards) and would give him an unfair competitive advantage due the additional length affecting his stride frequency” (Lines 77-79). Do the authors mean “stride length”?

We are deeply grateful to the Reviewer for their attention to detail and apologize for the lack of clarity in presenting the results. We have adjusted the sentence after reexamining the respective article. Furthermore, we hope that it will read in a much more fluent manner.

Lines 75-81: This discussion was then reawakened when Blake Leeper, another double transtibial amputee, attempted to follow in Pistorius’s steps and compete in the Tokyo 2020 Olympics, only to be barred from competing due to a ruling stating Leeper’s prosthetic blades were too long (+15 cm above paralympic standards) and would give him an unfair competitive advantage due the additional length shortening the swing time and lengthening the stride length, both of which are related to increased gait speed [8].

Author Response File: Author Response.pdf