Review Reports - From Adult Morphology to Developmental Hypothesis: Variation of the Adult Lateral Wrist Extensors—A Developmental Viewpoint

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

From adult morphology to developmental hypothesis: Variation of the adult lateral wrist extensors – a developmental viewpoint

This manuscript is very interesting; it documents adult anatomic variability of the lateral wrist extensors (ECRL/ECRB), describes interconnecting tendons (ITs) across 58 upper limbs, and proposes a developmental hypothesis (oblique mismatch between the muscle and tendon fission planes). Two fetal limbs (GW12, GW17) are examined qualitatively to assess developmental timing. The topic is important for hand surgery, sports medicine, and anatomical education. The dissection work is careful, and the idea of leveraging adult variants to infer development is interesting. However, several issues limit the current impact:

(i) The developmental inference rests almost entirely on adult gross anatomy with n=2 fetal cases and without histology or modern imaging (micro‑CT/diceCT, fetal MRI reconstruction), so the develop hypothesis remains under‑supported.

(ii) There is a numerical inconsistency in the statistics (counts in Table 1 sum to 50, whereas the limb number is 58). The chi‑square analysis needs clarification and minimal strengthening.

(iii) Several editorial/terminology issues (typos, small language problems) reduce readability.

Major comments

MC1 Clarify sample accounting and statistics (gross issue; must fix).

In Results 2.2/2.5 and Table 1, the observed frequencies across the “No accessory / B→L / L→B / Both” categories sum to 50, yet the manuscript states 58 limbs were analysed. Please reconcile the denominator(s), state whether any limbs were excluded from this sub‑analysis (and why), and provide a transparent flow (n assessed → n included per analysis). Also report 95% CIs for key proportions (e.g., prevalence of any ITs, bilateral symmetry when present).

MC2 Develpmental hypothesis: support and scope.

The oblique mismatch hypothesis is elegant, but with n=2 fetal limbs and macroscopic inspection only, it remains speculative. Please temper the language (e.g., “we hypothesize” / “plausible model”), and explicitly separate observations from interpretation. Consider adding a short roadmap paragraph (“Future work”) that lists objective readouts which could test your model: (i) histology of fetal/early postnatal tendons; (ii) micro‑CT/diceCT with soft‑tissue contrast; (iii) 3D fetal MRI slice‑to‑volume reconstructions; (iv) spatial transcriptomics comparable to recent tendon development studies. Cite very recent methodological advances that are directly applicable (see “Suggested recent references”).

MC3Integrate the most recent developmental and imaging literature (2024–2026).

Add a concise literature paragraph in the Discussion summarising how dynamic mechanics and late fetal maturation shape tendon architecture, and how 3D/4D imaging can document the muscle/tendon fission planes and sheaths. Recent work shows that embryo movement modulates tendon maturation; fetal MRI slice‑to‑volume pipelines and 3D reconstruction are now clinically feasible; and micro‑CT can depict fetal soft tissues with high fidelity. These updates will modernise your Discussion.

MC4 Situate adult variants within a ‘variants are valuable’ educational/clinical framework.

Since your dataset is built on cadaveric donors, please consider a brief paragraph on the educational and translational value of body donation, and how systematic variant documentation in donors strengthens both surgical planning and hypothesis‑generation about development. This is also the most natural place to cite neutral, recent work emphasizing body donor‑based anatomical variability (see “Suggested references”).

MC5 Methods transparency (fetuses and donors).

Please provide: (a) side, sex (if known), and laterality distribution; (b) whether the same donor contributed both upper limbs; (c) fixative and storage details (already present, but consider adding any post‑mortem interval information, if available, and whether limbs were fresh vs embalmed at first dissection); (d) for fetal specimens, a brief note on anatomical planes and landmarks used to document the ECRL/ECRB split.

Minor comments

Typos/wording to fix (examples):

“bot metacarpal bones”: “both metacarpal bones”;

Line 146: “the bodies of ECRL and ECRL”: did you mean “ECRL and ECRB”?;

Ensure consistent use of ECRL/ECRB, “interconnecting tendon(s)”, and B→L / L→B notation? Please clarify.

Figure captions: Expand so that each figure is stand‑alone (define all abbreviations, planes, and landmarks; indicate scale/side when possible).
Statistics: Report exact p‑values, test statistic, df, and effect sizes where relevant; add a short power/limitations sentence acknowledging that inferential claims are constrained by sample size.
Ethics/acknowledgement: The Donor Acknowledgement is appreciated; consider a brief one‑sentence note that your findings underscore the scientific value of donated bodies for cataloguing anatomical variation and scientific progression.

Suggested recent references

The paper will benefit from adding some references to contextualise adult donors, fetal imaging, histology on embalmed cadavers or fetuses, variants within an educational/clinical framework that considers variants as an added value.

Fetal tendon maturation & mechanics

- Rolfe RA, Talak Bastürkmen ET, et al. Embryo movement is required for limb tendon maturation. Front Cell Dev Biol. 2024. https://doi.org/10.3389/fcell.2024.1466872

Histological feasibility and Fetal hand/wrist tendon development

- Trucas M, Vincis M, et al. The potential translational utility of embalmed cadaveric gastrointestinal tract specimens: a proof‑of‑concept study. Translational Research in Anatomy. 2025; 100404. https://doi.org/10.1016/j.tria.2025.100404 (to indicate that, when needed, research and clinical histological verification of fine anatomy in embalmed donors is feasible).

- Li C‑A, Hayashi S, et al. Fetal development and growth of extensor tendons and their sheaths in the dorsal side of the wrist and hand: a histological study. Folia Morphologica (Warsz) 2024 August 28 https://doi.org/10.5603/fm.101833 (Directly relevant to your system; shows dynamic increases in slips and sheath changes)

-Abuhaimed AK, et al. Histologic reliability of tissues from embalmed cadavers: can they be useful in medical education? Saudi J Med Med Sci. 2020. https://doi.org/10.4103/sjmms.sjmms_383_19 (Supports the educational feasibility of histology in embalmed tissues across organs.)

Variants in body donors, educational/clinical value

- Boadum O, Lu YF. Co‑occurrence of asymmetrical bilateral extensor carpi radialis intermedius and bilateral sternalis muscles in an anatomical donor. Folia Morphologica. 2024;83(2):461–465. https://doi.org/10.5603/FM.a2023.0049

- Vincis M, Lewis C, Barry D, Trucas M, et al. Multiple rare abdominal aortic branch variations in a centenarian woman: case report and literature review. Translational Research in Anatomy. 2024; 100366. https://doi.org/10.1016/j.tria.2024.100366

(to support the broader value of documenting anatomical variants in donated bodies and its impact on clinical examination or surgery)

3D fetal imaging

-Nagaraj UD, et al. Slice‑to‑volume reconstruction of fetal brain MR imaging in clinical practice. AJNR. 2025. https://doi.org/10.3174/ajnr.A8728 (Demonstrates feasible, higher‑quality 3D reconstructions; techniques transferrable to limb.)

Docter, D., Dawood, Y., Jacobs, K. et al. Microfocus computed tomography for fetal postmortem imaging: an overview. Pediatr Radiol 53, 632–639 (2023). https://doi.org/10.1007/s00247-022-05517-1

Overall recommendation: Major Revision. The manuscript is promising but needs (a) firmer quantitative support for the developmental claim; (b) correction of numerical/statistical inconsistencies; (c) targeted updates to the literature; (d) and minor language/formatting fixes.

Comments on the Quality of English Language

Several sentences are very long and difficult to flow with, especially in the Discussion.
MDPI requires more fluid language revision.

Author Response

Dear Reviewer 1,

Thank you for the extensive review. Below you will find your comments with our answer following them in bold. Corrections regarding your comments are in blue in the revised manuscript. We would be happy if you gave a look at the rest of the changes as well, as there might be something relevant to your comments. In yellow highlight there are some important change, linked to the values or consistently of terminology.

Major comments

MC1 Clarify sample accounting and statistics (gross issue; must fix).

Answer: The total number of observations is 58 adult limbs; the values in the table were incorrect. The statistics in the table in section Results 2.5 have been corrected, and the analysis has been redone to properly represent the cohort. The outcome from the analysis remains the same. Additional details regarding the analysis have been added to clarify all steps.

MC2 Develpmental hypothesis: support and scope.

Answer: 1. Yes, we agree it is a rather speculative proposition. The language was changed in several places to better reflect that.

The recommended literature and the “Future work” section were added. Histology, 3D MRI, and micro-CT are the logical continuations of this work. Spatial transcriptomics, although interesting, would be very hard to execute. We added the relevant literature along with some additional articles.
Fetal limbs are rather rare to obtain, hence their low number. Additionally, both limbs displayed spatial relations of the forelimb muscles similar to the adult morphology. Based on those two limbs and the literature review, we figured that no additional information would be derived by sacrificing more limbs for dissection. We also cannot obtain earlier gestational stages due to clinical constraints, which would be required for the study**, and this** is another major limitation. Yes, we are planning to continue with some histology work. Micro-CT is a very good idea for the future!

MC3 Integrate the most recent developmental and imaging literature (2024–2026).

Answer: Thank you for the suggestion! We addressed this part in the Future Work section as well.

MC4 Situate adult variants within a ‘variants are valuable’ educational/clinical framework.

Answer: The main scope of the paper was not educational, but since cadaveric donor material used for education is used, we support the idea of including a paragraph. In the beginning of the introduction, we added several sentences regarding the importance of anatomical variability and the medical malpractice cases that can arise from neglecting this aspect of anatomical education.

MC5 Methods transparency (fetuses and donors).

Answer: 1. 28 left and 30 right limbs were used. The source of the study material is a collection of prosected preparations with an average storage period of 10 years (from donors who did not object to long-term usage in anatomical education). Sadly, at this point of storage and due to some omissions in curation, the sex, limb contribution, and postmortem intervals are not available. On a more positive note, this article (and another one we are currently preparing) resulted in an upgrade to our curation procedures and a much more robust way of labelling and following up on our specimens during storage. All new donors (and future ones) are being labelled using this modified system.

Some words regarding the foetal specimens were added in the M&M.
All bodies were embalmed upon arrival at the department, and dissections were performed on the embalmed limbs. Added in M&M.

Minor comments

Typos/wording to fix (examples):

“bot metacarpal bones”: “both metacarpal bones”;

Line 146: “the bodies of ECRL and ECRL”: did you mean “ECRL and ECRB”?;

Answer: Yes, it should be ECRL and ECRB. "Bot" was changed to "both."

Ensure consistent use of ECRL/ECRB, “interconnecting tendon(s)”, and B→L / L→B notation? Please clarify.

Answer: The abstract was checked for consistency. B-to-L and L-to-B notations were introduced at two places (in the Introduction and Results) for better readability.

Figure captions: Expand so that each figure is stand‑alone (define all abbreviations, planes, and landmarks; indicate scale/side when possible).

Answer: Some clarifications have been added to the figure captions. The upper limb side was added to the figure text. Adding a scale will be very difficult and does not significantly change the level of understanding of the figures; therefore, we prefer to omit it. However, we acknowledge that its absence was an oversight on our side.

Statistics: Report exact p‑values, test statistic, df, and effect sizes where relevant; add a short power/limitations sentence acknowledging that inferential claims are constrained by sample size.

Answer: All this information was available already in the first version of the manuscript. Probably you missed it due to the poor description of this part of our analysis. In the new version we improved the text. In short, the only statistical analysis that we performed was a χ² goodness-of-fit-test to compare the distribution of the observed frequencies to a hypothetical distribution given the presence of an interconnecting tendon on either muscle is an independent event. Such a test, comparing two distributions, is always with one degree of freedom. We didn’t perform further analysis of the results (multiple comparisons, odds ratios etc), because the null hypothesis is very general and such analysis would not contribute anything to our conclusions. In other words, at this point, it is meaningless whether the probability of both accessory tendons occurring is twice or three times as high as the expected and any interpretation of such results would be too speculative.

Ethics/acknowledgement: The Donor Acknowledgement is appreciated; consider a brief one‑sentence note that your findings underscore the scientific value of donated bodies for cataloguing anatomical variation and scientific progression.
Answer: We support that idea. Added in blue.

Reviewer 2 Report

Comments and Suggestions for Authors

I only have a few minor comments to make with regards to the text and which are highlighted here in bold type:

line 47: "ECRL, ECRB, or both metacarpal bones..."

line 78/79: "Albright et al. (1978)..."

line 148: Change: "that were we saw..." to: that where we saw...

line 341: "Swinehart et al (2013)..."

Author Response

Dear Reviewer 2,

Thank you for the feedback. All corrections were applied in green. In yellow highlight there are some important change, linked to the values or consistently of terminology.

Reviewer 3 Report

Comments and Suggestions for Authors

The present study is descriptive and offers intriguing anatomical observations. However, it is hindered by its small sample size, speculative developmental interpretations and restricted clinical relevance. The scientific robustness and translational relevance of the work are collectively diminished by the overinterpretation of descriptive findings, lack of mechanistic validation and very limited fetal sample.

Specific comments:

Introduction: The hypothesis is speculative and incompletely supported. Please provide an explanation for the existing literature’s inadequacy, the rationale behind your selection of this muscle group and the clinical or developmental issue that the study resolves. Please incorporate the clinical relevance of the findings to denote the impact of the study

Method: There are only 2 fetal specimens (GW 12 and GW 17). From a mere two fetal samples, it is impossible to derive developmental conclusions. There is no demographic data that could affect anatomical variations. Moreover, inclusion criteria are missing.

The statistical methodology is restricted. There was only Chi-square test included without any correction for multiple comparisons or power calculation

Results: Descriptive results were obtained rather than analytical ones.

There is inconsistency in the reporting numbers. In lines 92,93 it is reported that 20 showed some variation. However, 18 variations were used in the percentages -31% (18/58). There were only two fetal dissections, which are insufficient to infer developmental mechanisms and migration patterns.

Discussion: The hypothesis is highly speculative and there is an overinterpretation of the statistical findings. Please add the clinical implications of the findings. Please, add the limitations section.

Author Response

Dear Reviewer 3,

Thank you for the comments. Changes in the draft, directly as a result of your suggestions are marked in red. In yellow highlight there are some important change, linked to the values or consistently of terminology.

Specific comments:

Answer: Some information for the literature and clinical relevance was added. We prefer to add these as new paragraph in the discussion. In the initial version we restrained from including clinical relevance, since the article was more hypothetical.

Method: There are only 2 foetal specimens (GW 12 and GW 17). From a mere two fetal samples, it is impossible to derive developmental conclusions. There is no demographic data that could affect anatomical variations. Moreover, inclusion criteria are missing.

Answer: 1. Yes, we do agree that two samples are far too few. However, the observation shows normal gross position of the muscle, comparable to adult. This is in accordance with previous reports. So, if some changes might be observable, we need an earlier time point foetal arm, which is near impossible to do due to some clinical constraints. We do agree that is a major limitation and it is noted in the new section 3.1.6. Future work and study limitations.

Demographic data would not induce any relevant changes here.

The statistical methodology is restricted. There was only Chi-square test included without any correction for multiple comparisons or power calculation

Answer: This is a goodness of fit Chi-square test. In the new version we improved the text. In short, the only statistical analysis that we performed was a χ² goodness-of-fit-test to compare the distribution of the observed frequencies to a hypothetical distribution given the presence of an interconnecting tendon on either muscle is an independent event. Such a test, comparing two distributions, is always with one degree of freedom. We didn’t perform further analysis of the results (multiple comparisons, odds ratios etc), because the null hypothesis is very general and such analysis would not contribute anything to our conclusions. In other words, at this point, it is meaningless whether the probability of both accessory tendons occurring is twice or three times as high as the expected and any interpretation of such results would be too speculative.

Results: Descriptive results were obtained rather than analytical ones.

Answer: Yes, most of the results are observational. The total distribution of tendons does allow for analysis.

Answer: In the original text the analysis focuses on the interconnecting tendons, not on all variations. 18 is the number of specimens showing interconnecting tendons.

Nevertheless, after redaction (due to a typo), the total number of upper limbs that showed variations is 17, and 18 is the number of variations observed, since one upper limb showed two variations. 16 are the upper limbs with interconnecting tendons. The analysis focuses on the interconnecting tendons, not on all variations.

The foetal limbs are few and it is stated in the article.

Answer: The statistical findings interpret only what can be inferred from our observations. We improved the description of the method for comparing the observed frequencies to the hypothetical distribution of the frequencies, given the presence of an interconnecting tendon on either muscle is an independent event. We also added a thorough description of how we calculated the expected frequencies under this null hypothesis.

In the new version we added new sections 3.1.5 Clinical relevance and 3.1.6. Future work and study limitations.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors make a compelling case for the importance of understanding the contributions of developmental processes in understanding how anatomic variations in the muscles and their associated tendons may arise. Because of that approach, this is an important paper that does more than just catalog anatomic variations; it also provides an explanatory framework that can generate valuable research in functional morphology.

The presentation is clear and well-reasoned, and this paper will make an important contribution to the literature. The illustrations of the anatomic dissections are clear and easy to interpret. The graphical representation of the tendon primordium in Figure 6 is a powerful summary of the relevant developmental processes that could to the variants observed in the study. The authors make a strong case that the key to these tendinous variations lies in the development of variations in the muscles' bodies.

There is, however, one significant issue that should be addressed. The discussion in Section 2.5 on the hypotheses of independent-vs-contingent probabilities for the development of interconnecting tendons is a bit telescopic. In addition, is it unclear how the data in the table used in calculating the probabilities for the observed variations were derived. They do not match probabilities on the binomial distribution for 2 options (No accessory tendons vs accessory tendons), for 3 options (no accessory tendons, compared to one-muscle and 2 muscle accessories), of for 4 options (no accessories, L to B, B to L, and both). The formula on line 195 shows only 2 probabilities in the calculation, but there are 3 different probabilities included in the table calculated across the 4 observed conditions. The total count of occurrence of accessory tendons appears to be 16, while the values used in the probability estimates are 12 for each of the sets of data for cases with accessory tendons.

The logic of why these values were chosen is unclear, but it is difficult to connect with any of the values reported in the text.

The obvious question here is whether the accessory tendons only occur in the presence of muscles that have undergone fission. This is a conditional outcome—the probability of one event depends on the probability of another.

When testing conditional probabilities, it is more appropriate to use a Bayesian approach than one that depends on independent probabilities (because even when there is no fissioning in the muscle body, the outcome of interest—the presence of an accessory tendon—does not appear to be independent of that condition).

The premise here is that the testing for the condition of the appearance of fissioning in the muscle body will reliably predict the presence of accessory tendons in these muscles.

A Bayesian estimate starts with the probability from the study's observations that 16 of 50 muscles exhibit accessory tendons. This table shows the values needed to calculate these conditional probabilities.

Prob	Definition	Prob	Definition
P(A)	Probability that accessory tendon is present	P(~B\|~A)	Probability that “test” will not ID fission when accessory tend not present
P(B\|A)	Probability that “test” will ID fission when accessory tendon present	P(B)	Probability that “test” will ID fission whether or not accessory tendon present
P(~B\|A)	Probability that “test” will not ID fission when accessory tendon present	P(~B)	P=Probability that “test” will not ID fission whether or not accessory tendon present
P(B\|~A)	Probability “test” will ID fission when accessory tendon not present

In this case the “test” is the identification of fissioning in one or both of the extensores carporum muscles.

With any of the simple online calculators of Bayesian probabilities, one can simply enter the data from the study to generate conditional probabilities for alternative outcomes. The outcome inserted here is generated by the Bayesian probability calculator at Vassarstats.net, but other calculators generate similar results.

Using the values from the paper (16 cases with accessory tendons from 50 subjects), the P(A) is 16/50, or 0.32.

For P(B|A) and P(~B|~A), we entered the “standard” 0.95: we expect that the “test” will correctly identify fissioning (or lack of fissioning) in the mm. extensores carporum 95% of the time—though this value can be adjusted based on one's experience with the materials and as more cases are added to the data.

The resulting calculation here shows high values for both proportion of true positives [P(A|B)] and proportions of true negatives [P(~A|~B)]

https://www.dropbox.com/scl/fi/d6xvk08banyocy28qmi0p/Bayesian.png?rlkey=b7o86fmxfh5f1sw66bhlahkrt&st=5qaxwyzy&dl=0

This result still confirms the same outcome as posited in the paper, but it is based on the conditional probability that muscle fissioning and accessory tendons have a common cause. This calculation is therefore an explicit test of the hypothesis that the conditions associated with the muscle/tendon primordia are predictive of the development of accessory tendons.

The only other significant issue is that on line 278, the citation is both in an incorrect form and appears to be missing from the references list.

The correct reference is Wood, J. (1867) Variations in human myology observed during the winter session of 1866–67 at King's College, London. Proc. Roy Soc. Lond. B 15:518–546.

The source for this citation is Bergman, R.A., Afifi, A.K>, Miyauchi, R. (2015) Illustrated Encyclopedia of Human Anatomic Variation. Accessed 29 March 2026. https://www.anatomyatlases.org/AnatomicVariants/AnatomyHP.shtml

The pages for the mm extensores carporum also list several additional sources not cited by the authors that include variations in ECRL and ECRB which might be of interest to the authors.

One of the benefits of using these other sources—or adding cases from other sources in the literature if the appropriate observations were included—would be the ability to update the calculations in the Bayesian probabilities based on additional information.

Minor Issues

Line 29: mismatch between pronoun and antecedent: “tendons were consistent: it”; replace “it” with “they”.

L 30: missing words: “from the ventral side ECRL” should be from the ventral side of the ECRL”

L 46: They can either start from the … “either” is more appropriately placed “They can start either from the …: the comparison is made between the prepositional phrases; they both share the same main verb (start). As written, one would expect another verb in contrast to start...for example, “either start from...or end at...”

And it is not clear why the “They” in this sentence is in boldface.

L 48 there is inconsistency in the formatting of muscle names. Some instances (line 48, for example) use the full “m. extensor pollicis longus” but in other parts of the manuscript the “m.” is not used in muscle names. Should be consistent in the formatting.

L 49. It would be good to start a new paragraph when introducing a new topic (going from osteology to development) “From a developmental standpoint...”.

In a similar vein, new paragraphs would be appropriate in line 56 (The muscle progenitors...) and line 63 (On the other hand).

L 75. “they either start from...” maybe “emerge from”

L78–79. Year is missing from in-text citation.

L 91 is a split infinitive “to indirectly gauge”; alternatives are “to gauge indirectly” or “as an indirect gauge (or indicator)...

L 155: In legend of figure 4a is “BR— m. brachioradialis” but BR is not marked in the illustration.

L 172 In legend of figure 5 is “ED— m. extensor digitorum...II—second metacarpal bone, III—third metacarpal bone ” but these labels do not appear in the image.

Typologic Errors

Line 47: “ECRB, or bot metacarpal bones” should be “both”????

L 146. “where the bodies of ECRL and ECRL bifurcated” Should one of these be ECRB?

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 4,

We thank very much for their inciteful insightful and suggestions! You will find all changes referring to your comments are in grey, in the revised version. In yellow highlight there are some important change, linked to the values or consistently of terminology.

Comments on the statistical analysis are addressed below:

At an earlier stage of our work, we too considered a Bayesian approach to identify a probable mechanism linking the observed variations, but we found that it would add additional assumptions and a level of complexity that we cannot justify, not with the observations we have. While the proposed by reviewer 4 analysis starting from the fission and going toward the accessory tendons is different than our initial line of thinking, it brings us to the same caveats that forced us to let go of the Bayesian approach in the first place: 1/ With the available literature reports, due to the different descriptions leaving too much room for interpretation, it would be very hard to establish clear prior probabilities; 2/ More importantly, given the nature of the development of muscles and tendons it is impossible to establish a test that would differentiate between the two compething mechanisms that can lead to the generation of additional conjoining tendons between the two extensors carpi: as we establish in the paper the conjoining tendons could be a result of unfinished fission in proximal-to-distal direction, or it could be a formation and persistence of a misguided myotendinous junction formation (distal-to-proximal mechanism) or even some combination of both. In light of these thoughts, we decided to instead just show that the two events of the presence of a conjoining tendon on one or the other muscle are not independent events and there is really an underlying connecting mechanism without going to far trying to lucky guess and speculate which of the competing mechanisms is leading (evenmore as the results of the Bayesian tests that we could imagine would be the same for all competing theories). That is why we have chosen a simpler test with less assumptions, namely to compare the observed distributions of the four events – “no conjoint tendons”, “conjoining tendon longus-to-brevis only”, “conjoining tendon brevis-to-longus-only”, “conjoining tendons on both muscles”, to the distributions of a null hypothesis that the presence of a conjoining tendon on either muscle is an independent event. Since the usual null-hypothesis builtinto the χ2 – test assumes uniform random distributiuon, we calculated the null distribution starting from our observation (n=58) and the observed frequencies of presence of conjoining tendon on either muscle (12/58 for ECRL and 12/58 on ECRB). Under this assumption of independence the probability of both events occurring is

P(ECRB&ECRL)=P(ECRB)*P(ECRL|ECRB)=P(ECRL)*P(ECRB|ECRL)=P(ECRB)*P(ECRL)

Starting from this, the expected number of both conjoint tendons occurring in the same arm is equal to 12/58*12/58*58 ≈ 2.5≈ 3. Correspondingly the expected number of only one conjoint tendon occurring on ECRL (ECRLonly) is 12 minus 3=9, and the opposite ECRBonly is also 12 minus 3 =9. Finally, the expected number of no conjoint tendons remains to be 58- (9+9+3)=37 .

We agree that in our submitted version we failed to clarify this approach and that is why we added a specific section explaining the calculation of the expected frequencies. We also replaced the table 1 with a diagram that hopefully shows more clearly our line of thinking.

We additionally want to thank to reviewer 4, because his reasonable critique allowed us to identify an error that stems from an earlier stage of our work: the number of studied limbs is not 50 but 58. We corrected the calculations and replaced them in the edited version.

Finally, we are grateful about the suggestion for metanalytical approach and to include as a source of prior knowledge more data from previous research and described variations, which is a great idea that we intend to pursue in a future study.

The only other significant issue is that on line 278, the citation is both in an incorrect form and appears to be missing from the references list.

The correct reference is Wood, J. (1867) Variations in human myology observed during the winter session of 1866–67 at King's College, London. Proc. Roy Soc. Lond. B 15:518–546.

Answer: Redacted

The pages for the mm extensores carporum also list several additional sources not cited by the authors that include variations in ECRL and ECRB which might be of interest to the authors.

Answer: The citation was corrected. Yes, Bergman’s encyclopedia was the 1str thing we consulted, but we don’t think the reported variations will be of value to this article.

Minor Issues

Line 29: mismatch between pronoun and antecedent: “tendons were consistent: it”; replace “it” with “they”.

Answer: Redacted

L 30: missing words: “from the ventral side ECRL” should be from the ventral side of the ECRL”

Answer: Redacted

L 46: They can either start from the … “either” is more appropriately placed “They can start either from the …: the comparison is made between the prepositional phrases; they both share the same main verb (start). As written, one would expect another verb in contrast to start...for example, “either start from...or end at...”

And it is not clear why the “They” in this sentence is in boldface.

Answer: Corrected. The bold was leftover formatting form the internal draft revision – now removed.

Answer: We tyed our best to unify the formatting.

L 49. It would be good to start a new paragraph when introducing a new topic (going from osteology to development) “From a developmental standpoint...”.

Answer: New paragraph added.

In a similar vein, new paragraphs would be appropriate in line 56 (The muscle progenitors...) and line 63 (On the other hand).

Answer: New paragraphs added.

L 75. “they either start from...” maybe “emerge from”

Answer: Redacted

L78–79. Year is missing from in-text citation.

Answer: Year Added

L 91 is a split infinitive “to indirectly gauge”; alternatives are “to gauge indirectly” or “as an indirect gauge (or indicator)...

Answer: Redacted

L 155: In legend of figure 4a is “BR— m. brachioradialis” but BR is not marked in the illustration.

Answer: Redacted

L 172 In legend of figure 5 is “ED— m. extensor digitorum...II—second metacarpal bone, III—third metacarpal bone ” but these labels do not appear in the image.

Answer: Redacted

Typologic Errors

Line 47: “ECRB, or bot metacarpal bones” should be “both”????

Answer:Yes, we meant both. Redacted.

L 146. “where the bodies of ECRL and ECRL bifurcated” Should one of these be ECRB?