Test–Retest Reliability and Concurrent Validity of FysioMeter C-Station Assessing Lower-Limb Muscle Strength via Isometric Mid-Thigh Pulls

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study explores the reliability and concurrent validity of the FysioMeter C-station as a portable alternative to the gold standard AMTI force plates for assessing unilateral isometric mid-thigh pull (IMTP). The research addresses an important gap by evaluating a more accessible tool for muscle strength assessment in both clinical and sports settings.

Strengths:
- Clear Objective: The study's aim to test both the reliability and validity of the C-station is well-defined and addresses a relevant need for portable, practical solutions in strength assessment.
- Solid Methodology: The use of both intraclass correlation coefficient (ICC) and Pearson correlation coefficient (PCC) to assess reliability and validity, respectively, provides a robust statistical framework. The inclusion of bilateral and unilateral tests with two different modalities strengthens the study's design.
- Strong Results: The C-station demonstrated high reliability (ICC of 0.84–0.85) and strong concurrent validity (PCC ≥ 0.82) compared to the AMTI force plates, suggesting that the device is a viable alternative for practitioners.
- Practical Application: The findings have direct implications for practitioners in both clinical rehabilitation and athletic training, providing them with a portable, cost-effective option for assessing muscle strength.

Weaknesses:
- Limited Population Sample: The study focused solely on recreationally active males, which limits the generalizability of the findings. A more diverse sample, including female participants and different activity levels, would enhance the study's applicability.
- Unilateral Focus: While the study’s focus on unilateral IMTP is important, a comparison between bilateral and unilateral tests across different devices could provide additional insights into the consistency of the C-station for a wider range of strength assessments.
- Lack of Longitudinal Data: The study assessed reliability across two sessions one week apart. Future research should investigate long-term reliability to ensure the device’s consistency over extended periods, particularly in a clinical or rehabilitative setting.

Overall, the study provides strong evidence supporting the reliability and concurrent validity of the FysioMeter C-station as a practical alternative to the AMTI force plates for unilateral IMTP testing in recreationally active males. However, further research with more diverse populations and extended timelines would solidify its utility across broader applications. Despite these limitations, the findings offer promising potential for clinical and athletic use, especially in settings where portability and ease of access are essential.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for the opportunity to review this paper. The research is very interesting. Low cost systems offer a great opportunity for researching and training. The validation of these devices is very useful, and they must be done for the new technology developed.

Although the idea is interesting, there are several issues in the paper that should be reviewed.

First of all, in the introduction, the first paragraph is not well related to the aim of the study.

In line 60 you talk about the advantages in cost of the C-station vs force plate but you do not include other low cost alternative for measuring the IMTP as the strain gauges.

The main problem with the method is that you are comparing two systems (force plates and C-station) in different repetitions and moreover, the order is always the same, one after the other, so the effect of the fatigue could influence the measurement obtained. And you can affirm if each athlete did the repetition to the maximum intensity because there is not a criteria for assessing it.

As you have checked with the value obtained the AMTI plates offer values not reliable for AMTI right so if this is the gold standard, the problem is not the system, but the protocol of the measurement.

 

The data of concurrent validity of C-station and AMTI show significant differences in both legs and in the two days analyzed. This means that the system studied is not reliable. A difference of almost 10% appears in the force measurements. Looking at these data, the reliability could not be assured. 

 

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

I have carefully read the paper, which aims to study the reliability of unilateral IMTP in both the right and left legs of a commercial instrument and the concurrent validity compared to traditional force plates.

The rationale of the research question seems too forced, what they measure is isometric strength (it is not sufficiently highlighted in the text, only force which can lead to some misleading), what is the interest of measuring isometric (unilateral) strength at a certain angle, and how can this data be transferred to the programming of the training load -which is not usually isometric-?

It is not very clear how the content of the first paragraph really relates to the measurement of isometric strength (at a given angle of knee extension and hip flexion). It seems quite clear that in sport the measurement of maximal strength or power, or speed of load displacement, is well established in order to programme training loads and to objectify modifications produced by training. Much of the argument is based on the fact that ‘the isometric mid-thigh pull (IMTP) is one key outcome measure’, a statement that cannot be confirmed in the majority of studies that evaluate the effect of strength training. And the fact that this type of strength correlates with other manifestations of strength is not a good argument (it also correlates with age -in large samples- or with the weight of the person, or even with height), I think that this is not a good argument. Furthermore, the fact that it correlates in a cross-sectional study does not mean that it correlates with equal strength in a longitudinal study; it is one thing if they correlate and another if the correlation is sensitive to a change in one of the variables. There is no justification for measuring strength unilaterally (when it seems to be more a limitation of the instrument in terms of maximum tolerable strength).

There is no justification for the sample size (20 subjects) for the two purposes of the study, a sample that seems too small to have a certain statistical power.

For the objectives of the study, the design does not understand the relevance of the bilateral measurement of the strength platforms, I think they should remove it for greater clarity of the study.

In a repeatability study, there is a lack of more precise procedures to ensure the same angles in successive measurements (use of goniometers ....). How can one place the foot at ‘shoulder width’ in a unilateral measurement? In figure 2, they do not appear to present the same knee extension angles.

In table 2, the average of the individual differences (and standard deviation) between the first and second measurement should be shown, not the absolute difference between the group mean of the first and second day, which is uninformative as presented.

The interpretation of concurrent and test-retest validity they make is at least dubious, a PPR of 0.80 on the same individuals with the same instrument is doubtful that we can accept as strong, (coefficient of determination of 0.64). In the Bland-Altman graphs, you should point out what the limits represent (1.96 SD?). Concurrent validity is good when 95% of the population is at a difference between the result of two different instruments is about 800 neutons? Almost 50% of the measured value in each of the instruments?

These values should be discussed more critically.

I think they should express the limitations of the study (sample size, isometric strength, unilateral strength .....).

They should change the way of expressing the differences in table 2.

I think they should improve the introduction by being more coherent and clearer with the research questions (one-sidedness, isometric strength, test-retest, concurrent validity ....).

What are the contributions to the article by the authors?

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

Dear Authors,

 

I extend my appreciation to you for your dedication to advancing knowledge in the intriguing field of sports testing. Your efforts are commendable. However, I must express that, in my humble opinion, the manuscript is currently not ready for publication. I found some issues, especially concerning testing concepts and procedures that are unclear, posing a potential risk of misguiding the reader. Allow me to highlight those areas of concern:

-              lines 15-6: IMTP is a method to assess isometric strength not overall strength

-              line 16: I do not know at all if the second part of the phrase adds anything interesting (“…but […] approach.”)

-              lines 35-45: I know that sports injuries are a trendy topic nowadays, but these lines, are far from the goals of the papers and add nothing of interest to the reader; any person interested in isometric testing or strength testing, regardless of the ultimate goals (prevention, performance, profiling, talent selection) can benefit from it

-              lines 59-60: the main reason behind the comparison of the C-station to “traditional” force plates seems to be the ability to transport them, I would like to ask to the authors, why should anyone have to transport this testing material? Can provide the presented test relevant information to compete?

-              line 79: it looks that population is not highly specific, can be the sample a little bit scarce?

-              line 83: any reason to limit the age to the selected range 18-35 y. ? Since it is an inclusion criterion it has been stated a priori and not a definition of the current sample …

I really liked the randomization of the testing procedure but …

-              the learning effect has not been controlled, although there is a comment in the discussion, I would like to delve a little bit more into it, why appears in just one of the tests?

-              the authors did a really good job recording the preferred kicking leg, in my opinion the preferred/non-preferred would be a nicer comparison than left/right? Why did you decide to discard this information? If it is because of lacking statistical significance …

-              The testing setting has been also carefully treated, but have been any procedures to ensure the setting from day one to two for the same subject is reliable? Or they just perform the same procedure twice? (in the later, increasing the error in my opinion); this is not invalidating any procedure at all, but in the mind of the practitioner can be that more expert subjects or more reliable fittings can reduce variability from test to test

-              Lien 141: What of the three attempts was recorded for the further analysis? Best? Median? Averaged? All of them? And why?

-              Line 143: how an effort is evaluated as ‘non-maximum’?

-              Line 175: the p-value threshold is an exact value, p<0.05 is not, i.e. “p<0.05 were considered statistically significant”

-              Since different values have been compared, I strongly recommend to use a p-value correction to decrease type I errors (https://pmc.ncbi.nlm.nih.gov/articles/PMC6099145/)

-              Authors cited Hopkins and Schober et al. interpretation for effect sizes and PCC, how were ICC interpreted?

-              How was MDC calculated?

-              Do MDC and SEM magnitude interpretation?

-              I liked the discussion chapter but I would like to know how, despite the causes, can affect to testing or interpretation of results the magnitude of the differences of the AMTI compared to the C-station; is the error increasing with the magnitude in absolute terms? In relative terms? Is it stable?

I hope all the present comments can help the authors to improve the quality of their work.

Author Response

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Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The authors scientifically examine the between-session reliability of a C-station for unilateral isometric mid-thigh pull (IMTP) test and compared the validity of these measures between C-station and force plates. Peak force was measured when the participants performed unilateral IMTP on C-Station, and unilateral and bilateral IMTP on force plates. Intraclass Correlation Coefficient (ICC) and Pearson Correlation Coefficient (PCC) were used to evaluate the reliability and validity. The authors claimed C-station as reliable for measuring unilateral IMTP.

The current research outcomes have significant importance and endeavor. However, I realize that the current version of the manuscript needs some revisions. I have mentioned some general and specific comments which the authors may consider for further modifications. The authors are requested to consider the comments as a positive criticism to improvise the manuscript.

General Comments:

1.     The importance of examining peak force data for AMTI_BI is not clear in the current study. Especially when authors reported that it is a well-established testing procedure. Does this outcome contributing to support the current research objectives? If not, the authors are recommended to exclude reporting in the current study.

2.     Please consider uniform word to address the study participants, for example, line no. 87, “Twenty subjects….”.

Specific Comments:

3.     Figure 3: At a glance, it looks like the authors considered to study CS_Right, AMTI_Left and AMTI_BI only. It is recommended to either add a “representative image”  shown for  CS_Right, AMTI_Left and AMTI_BI  or include all types of considered IMTP protocol, like CS_Right, CS_Left, AMTI_Right, AMTI_Left, and AMTI_BI.

Figure 2B: Bar holding position seems incorrect in the representative image, may arise question about the credibility of the protocol. The authors are requested to replace Figure 2B with potential image.

4. In the Table 1 and Table 2 , along with reliability and Concurrent validity data, the authors have also reported Mean ± SD (N) of Peak Force data for different IMTP assessments conditions and days of assessment. However, that does not reflect in table legends or in footnotes. The authors are recommended to include this in both the table legends.

 

Author Response

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Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for the modifications made to the paper. 

I have still a few comments. The first one, you have added in the introduction the gauges as a cheap system . You said that they do not have software and this is the advantage of the  C-station but this afirmation is not correct. I send you a force sensor with a very good software with many statistical proccess applied in the analysis of the force (https://chronojump.org/product/force-sensor-kit-able-to-add-accessories/) and for sure that there are more examples of systems. 

I still consider that the changes found in the results in AMTI force plate measurement show that the repetitions have not been done with the same amount of force so the reliability could not be calculated in that way. 

Moreover the differences between both systems represents that both systems measured in a different way and the validity could not be assured.

Analysing the results, you can not see if the improvement of a training could not be detected due to the reliability of the system.

 

 

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

Creo que el artículo presenta inconsistencias importantes, que en el mejor de los casos deberían expresarse más claramente en las limitaciones del estudio, de lo contrario pueden llevar a errores en la interpretación del alcance del estudio por parte de muchos lectores inexpertos

Author Response

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Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

After several rounds of revisions, in which they have addressed most of the comments, I think the paper basically points out the main limitations of the study that have to be taken into account when reading and trying to guess the scope of their work. The improvements I think are sufficient.