The Association Between Shear Wave Elastography-Derived Muscle Stiffness and Muscle Force/Activation in Foot and Ankle Muscles: A Systematic Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have reviewed the manuscript titled “The Association Between Shear Wave Elastography-Derived Muscle Stiffness and Muscle Force/Activation in Foot and Ankle Muscles: A Systematic Review” (Manuscript ID: diagnostics-4323550). I have confirmed that this systematic review is based on a clear objective with clinical relevance. Establishing accurate assessment methods for foot and ankle is an important issue in the fields of physical therapy and biomechanics. I highly commend the authors for their ambitious effort to systematically evaluate the validity of Shear Wave Elastography (SWE), a relatively new technology, within this context.

However, to improve the logical consistency of the manuscript and enhance its value as a reliable reference for readers, I would like to suggest several revisions regarding potential logical gaps and inconsistencies, as detailed below.

Location: Page 2, Introduction

Issue: A logical leap is made in the argument from the description of SWE's technical advantages (e.g. non-invasiveness) to the immediate conclusion of its clinical utility for foot and ankle disorders.

Recommendation: The authors should provide a detailed rationale explaining why conventional assessment methods, such as electromyography (EMG) or isokinetic dynamometry, are inadequate in this particular context. For instance, they should address the challenges associated with accessing deep intrinsic foot muscles. Furthermore, given the prevalence of research on tendon stiffness, the physiological significance of focusing on the "muscle belly" should be more clearly articulated to justify the study's scope.

Location: Page 11

Issue: The lack of correlation between passive stiffness and Maximal Voluntary Contraction (MVC) torque in several studies highlights a limitation in using this tool for between-subject comparisons.

Recommendation: While the authors attribute this to the multi-factorial nature of MVC, it should be more directly stated that "passive SWE-derived stiffness may have limited utility as a predictive marker for comparing or estimating absolute muscle strength across different individuals." This limitation should also be reflected in the Conclusion.

A detailed and nuanced evaluation of specific studies (Soldos et al. and Ando et al.)
Soldos et al. (Ref. 22) - Location: Please refer to page 11

Comment: It is imperative to acknowledge the potential for "spurious correlation" arising from the aggregation of data from two disparate groups. Conversely, as a constructive point, the authors could suggest that this study implies that a true relationship might only be detectable when a wide range of data (diverse populations) is ensured to overcome "range restriction."

Ando et al. (Ref. 25) - Location: Kindly direct your attention to page 11

Comment: The finding that RTD improved while SWV remained unchanged following longitudinal training is critical evidence against a direct causal relationship. It is imperative to address this "dissociation" in longitudinal data to ensure that readers are cognizant of the fact that cross-sectional correlations do not necessarily imply that stiffness is the primary driver of explosive force capacity.

Location: Page 11

Issue: The hypothesis that the capacity of active SWE stiffness to "track" contraction intensity can be considered a "surrogate measure for muscle force" is not supported by the evidence.

Recommendation: The present discussion should be narrowed to emphasise its validity for the real-time monitoring of muscle activity or contraction intensity within a specific individual (within-subject). This distinction ensures logical consistency with the aforementioned difficulties in between-subject comparisons.

Location: Page 12, before the Conclusion

Issue: The manuscript would benefit from the incorporation of a more concrete roadmap to bridge the current evidence gaps.

Recommendation: It is recommended that the following three points be incorporated into a "Future Directions" section:

1. Longitudinal Intervention Studies: In order to address the lack of causality that was noted in Ando et al. (Ref. 25).

2. Large-scale, Diverse Population Studies: The objective of this study is to validate the sensitivity of SWE as a between-subject marker, whilst simultaneously controlling for statistical biases.

 

Author Response

I have reviewed the manuscript titled “The Association Between Shear Wave Elastography-Derived Muscle Stiffness and Muscle Force/Activation in Foot and Ankle Muscles: A Systematic Review” (Manuscript ID: diagnostics-4323550). I have confirmed that this systematic review is based on a clear objective with clinical relevance. Establishing accurate assessment methods for foot and ankle is an important issue in the fields of physical therapy and biomechanics. I highly commend the authors for their ambitious effort to systematically evaluate the validity of Shear Wave Elastography (SWE), a relatively new technology, within this context.

However, to improve the logical consistency of the manuscript and enhance its value as a reliable reference for readers, I would like to suggest several revisions regarding potential logical gaps and inconsistencies, as detailed below.

Response: We sincerely thank the reviewer for the careful and constructive evaluation of our manuscript. We are grateful for the reviewer's recognition of the clinical relevance of our work and the importance of systematically evaluating shear wave elastography (SWE) as an emerging assessment tool for the foot and ankle. We have carefully considered each of the points raised and have revised the manuscript accordingly. Our detailed point-by-point responses are provided below, with corresponding revisions highlighted in the updated manuscript. We believe these revisions have substantially improved the clarity, internal consistency, and overall value of the manuscript as a reference for readers in physical therapy, biomechanics, and sports medicine.

Location: Page 2, Introduction

Issue: A logical leap is made in the argument from the description of SWE's technical advantages (e.g. non-invasiveness) to the immediate conclusion of its clinical utility for foot and ankle disorders.

Recommendation: The authors should provide a detailed rationale explaining why conventional assessment methods, such as electromyography (EMG) or isokinetic dynamometry, are inadequate in this particular context. For instance, they should address the challenges associated with accessing deep intrinsic foot muscles. Furthermore, given the prevalence of research on tendon stiffness, the physiological significance of focusing on the "muscle belly" should be more clearly articulated to justify the study's scope.

Response: We thank the reviewer for this important observation. We agree that the original Introduction moved too quickly from the technical advantages of SWE to its clinical utility, and  can be enhanced by adding more material that can help in establishing why conventional functional assessments are inadequate in the foot and ankle and without explicitly justifying our focus on muscle belly rather than tendon stiffness. We have revised the Introduction by expanding paragraph 2 to address the specific limitations of surface EMG and isokinetic dynamometry in this anatomical region, including the inaccessibility of deep intrinsic foot muscles and the inability of joint-level torque measurement to isolate individual muscle contributions, and by adding a new paragraph that articulates the physiological rationale for characterizing muscle belly stiffness as distinct from the more extensively studied tendon stiffness (please see Page 2, paragraphs 2 and 3 of the revised Introduction).

Location: Page 11

Issue: The lack of correlation between passive stiffness and Maximal Voluntary Contraction (MVC) torque in several studies highlights a limitation in using this tool for between-subject comparisons.

Recommendation: While the authors attribute this to the multi-factorial nature of MVC, it should be more directly stated that "passive SWE-derived stiffness may have limited utility as a predictive marker for comparing or estimating absolute muscle strength across different individuals." This limitation should also be reflected in the Conclusion.

A detailed and nuanced evaluation of specific studies (Soldos et al. and Ando et al.)
Soldos et al. (Ref. 22) - Location: Please refer to page 11

Comment: It is imperative to acknowledge the potential for "spurious correlation" arising from the aggregation of data from two disparate groups. Conversely, as a constructive point, the authors could suggest that this study implies that a true relationship might only be detectable when a wide range of data (diverse populations) is ensured to overcome "range restriction."

Ando et al. (Ref. 25) - Location: Kindly direct your attention to page 11

Comment: The finding that RTD improved while SWV remained unchanged following longitudinal training is critical evidence against a direct causal relationship. It is imperative to address this "dissociation" in longitudinal data to ensure that readers are cognizant of the fact that cross-sectional correlations do not necessarily imply that stiffness is the primary driver of explosive force capacity.

Response: Thank you for your comments. We have made our best effort to address all your comments and have made changes based on this in the manuscript.

(a) Passive stiffness and absolute muscle strength. We agree and rather think that this is an important distinction for between-subject strength comparison was previously under-stated. We have revised Discussion paragraph 1 to state explicitly that passive SWE-derived stiffness has limited utility as a predictive marker for comparing or estimating absolute muscle strength across individuals, while preserving the nuance that passive stiffness does carry meaningful information about explosive force capacity (RTD). This caveat is also now reflected in the Conclusion.

(b) Soldos et al. (Ref. 22) and the potential for spurious correlation. We thank the reviewer for raising this important point. The correlations reported by Seldos in their study were calculated within pooled subgroups: the "non-athlete" correlation (r = 0.82) combined healthy controls and cancer patients, and the "athlete" correlation (r = 0.79) combined fast- and slow-fiber dominance subgroups. Because each of these pooled subgroups spans a substantially wider range of both muscle stiffness and force-producing capacity than its constituent subgroups individually, the high correlations may partly reflect between-group variance rather than a within-individual relationship, a recognized artifact of aggregating heterogeneous subpopulations. We have added this caveat to the Discussion. We have also incorporated the reviewer's constructive observation that this same feature carries methodological value: detecting a true SWE–force relationship may require samples with sufficient inter-individual variability to overcome range restriction, which is likely obscured in the homogeneous cohorts of young, healthy participants that dominate the existing SWE literature.

(c) Ando et al. (Ref. 25) and the longitudinal dissociation. We agree with the reviewer.  We have expanded Discussion paragraph 4 to develop this point explicitly and to caution readers that cross-sectional correlations between passive stiffness and explosive force capacity should not be interpreted as evidence of a causal relationship.

 

Location: Page 11

Issue: The hypothesis that the capacity of active SWE stiffness to "track" contraction intensity can be considered a "surrogate measure for muscle force" is not supported by the evidence.

Recommendation: The present discussion should be narrowed to emphasise its validity for the real-time monitoring of muscle activity or contraction intensity within a specific individual (within-subject). This distinction ensures logical consistency with the aforementioned difficulties in between-subject comparisons.

 

Response: Thank you for your comment. We agree that the existing evidence supports active SWE as a high-fidelity index of contraction intensity within an individual but between the subject variations demand a cautious interpretation. This distinction is logically consistent with the limitations of passive SWE for between-subject strength comparison that the reviewer raised in the preceding comment. We have revised the manuscript accordingly: Discussion paragraph 2 now frames the Chernak [19] and Vigotsky [5] findings as within-subject phenomena and explicitly notes that the Soldos et al. [22] proposal of SWE as a surrogate for maximal isometric force extends beyond what the underlying cross-group correlations can support. Corresponding adjustments have been made to the Abstract, Conclusions, and Clinical Implications sections.

Location: Page 12, before the Conclusion

Issue: The manuscript would benefit from the incorporation of a more concrete roadmap to bridge the current evidence gaps.

Recommendation: It is recommended that the following three points be incorporated into a "Future Directions" section:

1. Longitudinal Intervention Studies: In order to address the lack of causality that was noted in Ando et al. (Ref. 25).
2. Large-scale, Diverse Population Studies: The objective of this study is to validate the sensitivity of SWE as a between-subject marker, whilst simultaneously controlling for statistical biases.

Response: We thank the reviewer for the recommendation to add a dedicated Future Directions section. We agree that consolidating the research priorities into a structured roadmap will strengthen the manuscript's value as a reference. A new Section 5 (Future Directions) has been added before the Conclusions, addressing longitudinal intervention designs to resolve the causality question raised by Ando et al. [25], large-scale and demographically diverse studies to validate between-subject SWE sensitivity while controlling for statistical biases such as pooled-group artifacts, and standardization of SWE acquisition protocols as a foundational prerequisite for both. To avoid duplication, the brief future-directions sentence previously embedded in Discussion paragraph 4 has been removed, and subsequent section numbering (Conclusions, Clinical Implications) has been updated accordingly.

The changes within the manuscript are highlighted in red 

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript comprehensively explores the association between shear wave elastography (SWE)-derived foot and ankle muscle stiffness and neuromuscular force/activation levels through a systematic review. By synthesizing 20 studies comprising 637 participants, the authors clearly articulate that passive and active stiffness reflect distinct physiological phenomena: resting passive stiffness cannot predict maximal voluntary contraction (MVC) torque but is associated with the rate of torque development (RTD) at elongated muscle lengths, whereas active stiffness tracks muscle contraction intensity with high fidelity. This study provides highly valuable insights into clarifying the functional mechanisms of SWE in muscle biomechanical assessments and offers significant clinical guidance for future rehabilitation monitoring and return-to-sport evaluations. Overall, it is a review article with high academic value and strong potential for clinical translation.

Despite the excellent data integration and theoretical framework construction, there remains room for improvement regarding the depth of certain discussions and methodological justifications. First, the manuscript notes that the methodological quality of the included studies is generally "fair" or "poor," and a meta-analysis could not be performed due to massive heterogeneity in measurement protocols. It is recommended that in the discussion or conclusion section, beyond merely calling for standardization, the authors directly propose a specific set of standardized recommendations or a "minimum reporting standard checklist" for future ankle SWE studies (e.g., recommending specific joint angles, probe placements, and correction methods) to tangibly address this heterogeneity issue. Second, during the quality assessment, the authors excluded the "statistical power" item from the Downs and Black scale, reasoning that its reliability is poor in observational studies. Considering that small sample sizes (with a minimum of just 6 participants) are precisely one of the most prevalent core limitations among the included studies, the authors should supplement this with relevant literature to robustly support the methodological rationale for removing this item. Finally, although the article explores moderating factors based on muscle type and joint angle, there is little mention of the systemic bias across different ultrasound equipment platforms. It is suggested to briefly add a discussion on the limitations regarding cross-device reproducibility to ensure a more comprehensive evaluation.

Author Response

Reviewer 2
This manuscript comprehensively explores the association between shear wave elastography (SWE)-derived foot and ankle muscle stiffness and neuromuscular force/activation levels through a systematic review. By synthesizing 20 studies comprising 637 participants, the authors clearly articulate that passive and active stiffness reflect distinct physiological phenomena: resting passive stiffness cannot predict maximal voluntary contraction (MVC) torque but is associated with the rate of torque development (RTD) at elongated muscle lengths, whereas active stiffness tracks muscle contraction intensity with high fidelity. This study provides highly valuable insights into clarifying the functional mechanisms of SWE in muscle biomechanical assessments and offers significant clinical guidance for future rehabilitation monitoring and return-to-sport evaluations. Overall, it is a review article with high academic value and strong potential for clinical translation.
Response: We sincerely thank Reviewer 2 for the careful reading of our manuscript and for the generous summary of its core findings, the physiologically distinct roles of passive and active SWE-derived stiffness, the specific association of passive stiffness with rate of torque development at elongated muscle lengths, and the high-fidelity tracking of contraction intensity by active stiffness. We are especially grateful for the reviewer's recognition of the manuscript's clinical translational potential for rehabilitation monitoring and return-to-sport evaluation. Concurring what reviewer mentioned, this is a timely manuscript that helps in understanding of the stiffness in the context of muscle activation. Although, these inferences are implied in multiple studies but they are not systematically evaluated. This manuscript adds this critical insight into the existing literature. We have carefully considered each of the specific points raised below and have revised the manuscript accordingly.

Despite the excellent data integration and theoretical framework construction, there remains room for improvement regarding the depth of certain discussions and methodological justifications. First, the manuscript notes that the methodological quality of the included studies is generally "fair" or "poor," and a meta-analysis could not be performed due to massive heterogeneity in measurement protocols. It is recommended that in the discussion or conclusion section, beyond merely calling for standardization, the authors directly propose a specific set of standardized recommendations or a "minimum reporting standard checklist" for future ankle SWE studies (e.g., recommending specific joint angles, probe placements, and correction methods) to tangibly address this heterogeneity issue. Second, during the quality assessment, the authors excluded the "statistical power" item from the Downs and Black scale, reasoning that its reliability is poor in observational studies. Considering that small sample sizes (with a minimum of just 6 participants) are precisely one of the most prevalent core limitations among the included studies, the authors should supplement this with relevant literature to robustly support the methodological rationale for removing this item. Finally, although the article explores moderating factors based on muscle type and joint angle, there is little mention of the systemic bias across different ultrasound equipment platforms. It is suggested to briefly add a discussion on the limitations regarding cross-device reproducibility to ensure a more comprehensive evaluation.

Response: Thank you again for your encouraging comments and constructive feedback. We agree that the manuscript can be strengthened by moving beyond a general call for standardization, by providing literature support for our modification of the Downs and Black checklist, and by explicitly acknowledging cross-platform systemic bias as a methodological limitation. We have made three revisions you suggested accordingly:

(a) In response to the request for a concrete standardization framework, we have added a new Table 4: Proposed Minimum Reporting Standard Checklist for SWE Studies of the Foot and Ankle Musculature. We believe that this table can serve as a fantastic starting point to setup the standardization for SWE which is much needed, currently. It specifies recommended reporting elements across seven domains, including ankle joint angle, probe placement and orientation, pennation angle correction, ROI size and depth, contraction condition, concurrent recordings, and operator/device characteristics. The checklist is referenced in the Future Research section and complements the broader call for consensus guidelines previously included in that section. We believe that reviewer will really like this newly created table.

(b) In Methods (2.5), we have expanded the justification for excluding the Downs and Black Power item and provided a rationale for doing so. It has been convincingly enhanced now.

(c) In the limitations passage of Discussion paragraph 4, we have added explicit sentences addressing cross-device reproducibility, noting that different SWE platforms use distinct push-pulse and tracking algorithms and that in vitro and in vivo comparison studies have documented systematic between-platform differences in measured stiffness values. The implication for cross-study comparability has been made explicit.

 The changes within the manuscript are highlighted in red. 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Revision Comments

The authors are to be commended for their diligent efforts in revising the manuscript. In the revised background section, the authors clearly describe the limitations of conventional muscle strength and functional assessment methods, thereby effectively establishing the rationale for the application of SWE as a means to overcome these limitations. This revision has substantially improved the logical flow of the manuscript, making its objectives and significance more accessible to readers. Furthermore, the authors have demonstrated a commendable level of responsiveness to the previous comments by undertaking thorough and thoughtful revisions to the Discussion section. The additional rationale regarding intrinsic foot muscles and the physiological significance of the muscle belly has further enhanced the clarity and impact of the study. Therefore, I have only two minor points to address.

1. Alignment between the Introduction and the Evidence Base for “Foot” Muscles  Location: Discussion ／ Future Directions 

Comment: 

The revised Introduction (Page 2, Paragraphs 2–3) now clearly highlights the clinical importance and assessment challenges associated with intrinsic foot muscles. However, as demonstrated in the Results section, the current body of evidence remains heavily focused on the extrinsic muscles, particularly the triceps surae. To maintain consistency with both the manuscript title (“Foot and Ankle Muscles”) and the expanded Introduction, this evidence gap should be explicitly acknowledged in the Discussion or Limitations section.

Specifically, although the clinical need for assessing intrinsic foot muscles is substantial, the existing SWE literature predominantly provides evidence related to the extrinsic calf muscles. Furthermore, the authors should explicitly state in the “Future Directions” section that studies focusing on intrinsic foot muscles remain limited and that further investigation in this area is warranted.

2. Correction of Section Numbering  Location: Pages 12–13  Comment: There is a minor formatting issue in the section numbering. Currently, both the Conclusions and Clinical Implications sections are labeled as Section 6. Please revise the numbering (e.g., Section 6 for Conclusions and Section 7 for Clinical Implications) to maintain a consistent structural hierarchy throughout the manuscript.

Author Response

Response Letter

Reviewer 1:

The authors are to be commended for their diligent efforts in revising the manuscript. In the revised background section, the authors clearly describe the limitations of conventional muscle strength and functional assessment methods, thereby effectively establishing the rationale for the application of SWE as a means to overcome these limitations. This revision has substantially improved the logical flow of the manuscript, making its objectives and significance more accessible to readers. Furthermore, the authors have demonstrated a commendable level of responsiveness to the previous comments by undertaking thorough and thoughtful revisions to the Discussion section. The additional rationale regarding intrinsic foot muscles and the physiological significance of the muscle belly has further enhanced the clarity and impact of the study. Therefore, I have only two minor points to address.

Response: Thank you for your encouraging feedback. We really appreciate you recognizing the effort we have put in improving this manuscript. We appreciate your feedback in improving this manuscript.

1. Alignment between the Introduction and the Evidence Base for “Foot” Muscles  Location: Discussion ／ Future Directions 

Comment: 

The revised Introduction (Page 2, Paragraphs 2–3) now clearly highlights the clinical importance and assessment challenges associated with intrinsic foot muscles. However, as demonstrated in the Results section, the current body of evidence remains heavily focused on the extrinsic muscles, particularly the triceps surae. To maintain consistency with both the manuscript title (“Foot and Ankle Muscles”) and the expanded Introduction, this evidence gap should be explicitly acknowledged in the Discussion or Limitations section.

Specifically, although the clinical need for assessing intrinsic foot muscles is substantial, the existing SWE literature predominantly provides evidence related to the extrinsic calf muscles. Furthermore, the authors should explicitly state in the “Future Directions” section that studies focusing on intrinsic foot muscles remain limited and that further investigation in this area is warranted.

Response: Thank you for your comment. We agree with the reviewer that the Introduction's emphasis on intrinsic foot muscles created an inconsistency with the predominantly extrinsic-muscle evidence base synthesized in the Results. We have addressed this directly by: (a) adding an explicit statement in the Limitations section of the Discussion acknowledging that no included study examined intrinsic foot muscles via SWE in relation to force or activation outcomes; and (b) adding a fourth Future Directions paragraph specifically calling for SWE investigation of intrinsic foot muscles. Importantly, we also now cite a recently published study (Arellano et al., 2025, Gait & Posture) that directly characterizes SWE-derived stiffness of the abductor hallucis and tibialis posterior during weight-bearing transitions, providing the first empirical evidence that intrinsic and extrinsic foot muscle stiffness is measurable and task-dependent,  and use this to anchor the Future Directions call for studies linking these stiffness changes to neuromuscular force and activation outcomes. We believe that this can be a critical addition and may start a new line of research investigating this question specifically in the intrinsic foot muscles.

2. Correction of Section Numbering  Location: Pages 12–13  Comment: There is a minor formatting issue in the section numbering. Currently, both the Conclusions and Clinical Implications sections are labeled as Section 6. Please revise the numbering (e.g., Section 6 for Conclusions and Section 7 for Clinical Implications) to maintain a consistent structural hierarchy throughout the manuscript.

Response: Thank you for this comment. This has been addressed.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed all the concerns.

Author Response

Thank you for your perusal of our manuscript and helping us improve that