Intra-Session Reliability and Predictive Value of Maximum Voluntary Isometric Contraction for Estimating One-Repetition Maximum in Older Women: A Randomised Split-Sample Study

Reviewer 1

We are very grateful for the time and attention the reviewer has devoted to reading our manuscript, and for the valuable corrections, clarifying suggestions, and constructive recommendations for future research.

Point-by-point response to Comments and Suggestions for Authors

Comments 1: [The authors mentioned the positive aspects of testing MVIC instead of 1 RM for practical applications, however, I don’t think the one downside was mentioned, which is in most gyms in most practical situations there is not an isometric dynamometer. This is usually only in research. This issue should be discussed in the Discussion along with maybe using prediction equations to determine 1 RM from the max reps done at a given weight]

Response 1:

We appreciate the reviewer’s insightful comment. Indeed, the availability of isometric dynamometers based on load cells is still uncommon in most commercial gym settings. However, load-cell dynamometric devices represent a cost-effective option, and their presence is becoming increasingly common in rehabilitation and physical reconditioning centers. This trend is supported by several studies that have examined the validity of simple and portable devices for measuring isometric strength (e.g:, DOI: 10.1123/jsr.2013.TR7; DOI: 0.1016/j.ptsp.2024.04.008; DOI: 10.1371/journal.pone.0301872; DOI: 10.1186/s12891-019-2703-0). Therefore, we believe our study is timely and addresses an emerging need.

Moreover, one of the main aims of our study was to emphasize that, in light of concerns often expressed by physical trainers and caregivers regarding the safety of conducting 1RM testing in older adults, the assessment of Maximum Voluntary Isometric Contraction (MVIC) may represent a safer, more accessible, and less time-consuming alternative in terms of both equipment and logistical requirements.

With regard to the reviewer’s relevant comment on the use of prediction equations for 1RM, we had already addressed this point in the introduction: “…some trainers and researchers opt to estimate 1RM based on the statistical relationship between certain load, the maximum number of repetitions possible with that load, and the 1RM [8]” (lines 77-80).

Comments 2: [The lack of a typical washout period of a week between sessions. Most tDCS studies use a week but this study conducted the three conditions/experiments on 3 consecutive days. It was counterbalanced however. Perhaps the authors should do a statistical analysis for an order effect. I am sure that performance would like show an order effect but the important issue would be if there was a differential order effect due to one or more of the stimulation conditions eliciting retention effects that could last a day.]

Response 2:

We appreciate the reviewer’s insightful comment regarding the interval between sessions. Indeed, there is a lack of uniformity in the literature concerning the use of terminology such as precision, repeatability, reproducibility, test–retest reliability, and accuracy. However, we would like to clarify that the primary objective of our study was to estimate the precision of the measurement instrument by assessing intra-session reliability—that is, the random error within the same session—rather than to evaluate test–retest reliability over time.

For this reason, we deliberately chose to conduct all measurements within a single day, rather than across multiple days, in order to minimize the influence of physiological or environmental factors that may fluctuate across separate testing sessions. It is important to note that the task involved—a brief maximal isometric contraction lasting only five seconds—did not impose a substantial physiological load, and thus was unlikely to produce residual fatigue, especially after the five-minute rest intervals we provided between conditions. In fact, prior studies have shown that in healthy adults, complete neuromuscular recovery following maximal isometric efforts typically occurs within just a few minutes (see: https://bmcgeriatr.biomedcentral.com/articles/10.1186/s12877-023-04552-3).

Nonetheless, in response to the reviewer’s valuable feedback and in order to improve the clarity of our manuscript, we have revised the terminology throughout the text, including the title, to explicitly state “intra-session reliability” instead of the more general term “reliability.” This modification clarifies that our focus was on evaluating measurement precision under controlled conditions rather than on the temporal stability of the measurement.

Regarding the reviewer's suggestion to check for a possible order effect, given the short duration and low intensity of the task, along with the relatively short intervals between measurements, we consider the likelihood of a significant carryover effect to be minimal. However, we are open to exploring this possibility in future studies that employ longer washout periods and formal tests to detect order effects.

Comments 3: [Some of the formatting of the Tables could be improved. For instance in Table 2 the headings are smaller in font than the values in the table. In addition, all of the tables seem to be of different sizes using different fonts in different places relative to each other.]

Response 3: Thank you very much for pointing out the formatting issues in the tables. We have standardized the table format to improve consistency in table size and style.

Comments 4: [Bibliogaphy has some inconsistencies in formatting sometimes the journal name is completely spelled out such as in reference 4 and other times in is abbreviated such as reference 6 as just one example.]

Response 4: We appreciate the reviewer for pointing out this formatting inconsistency. We have corrected the bibliography by standardizing the way journal names are presented throughout the reference list.

Comments 5: [Maybe the authors should discuss in the Discussion how the results could be different or how applicable they are to more complex free weight exercises such as bench press and squat compared to the single joint simple leg extension, which is constrained.]

Response 5: We understand the reviewer's comment; however, we believe that this point goes beyond the scope of our study, which did not aim to analyze the biomechanics of the movement itself. Furthermore, we do not have a bibliography that would allow us to make a meaningful comparison between our results from the leg extension exercise and those from more complex, multi-joint exercises, such as bench presses or squats, in a similar population. Nevertheless, we acknowledge the relevance of this consideration and will take it into account in future research.

Reviewer 2

We are very grateful for the time and attention the reviewer has devoted to reading our manuscript, and for the valuable corrections, clarifying suggestions, and constructive recommendations for future research.

Point-by-point response to Comments and Suggestions for Authors

Comments 1: [Title :Emphasizing the type of reliability (inter-day, inter-session, etc.) is necessary, considering that specifying the type of test applied is essential for testing reliability. It seems much more adaptable to conduct it on different day]

Response 1:

We appreciate the reviewer's observation regarding the need to specify the type of reliability assessed. In response to this pertinent comment, also noted by reviewer 1, and in order to improve the clarity and precision of the manuscript, we have replaced reliability throughout the text, including in the title, with intra-session reliability to more explicitly reflect the scope and primary objective of the study, which was to assess the accuracy of the instrument under controlled conditions and on the same day, rather than its temporal stability over different days. which was to evaluate the accuracy of the instrument under controlled conditions and on the same day, rather than its temporal stability over different days.

While we acknowledge that between-day or between-session designs may offer additional information, particularly with regard to long-term reliability, our methodological choice to focus on intra-session reliability was intentional, as it is consistent with our primary objective of assessing random error within a single test session, with minimal physiological variability.

Comments 2: [Abstract 33-35 "Moreover, the predictive equation (1RM = 0.932, MVIC – 3.852) showed no significant differences between estimated and measured 1RM values (p = 0.791), albeit with a prediction error of 13.4%." This sentence seems unclear. Please check the meaning of this sentence. It can be unclear to the reader. The predictive equation is also unclear. This part should be rephrased.]

Response 2:

Thank you for this valuable observation. We agree that the original phrasing and the mathematical expression were not sufficiently clear. We have revised the sentence for clarity, and it now reads as follows in the abstract:
“Although the predictive equation 1RM = [(0.932 × MVIC) – 3.852] did not yield statistically significant differences between the estimated and actual 1RM values (p = 0.791), it exhibited a prediction error of 13.4%." (lines 34-36)

Comments 3: [46-48: Such claims should be supported by appropriate and original references. "It is estimated that, from the age of 50–60, older adults experience a progressive decline in both muscle strength (approximately 1.5–5% per year, and muscle mass (around 1% per year. ]

Response 3: Thank you for this observation. You are correct that such statements should be supported by appropriate references. We have now added relevant sources to substantiate the reported rates of decline in muscle strength and muscle mass. The revised sentence now reads: “It is estimated that, from the age of 50–60, older adults experience a progressive decline in both muscle strength (approximately 1.5–5% per year) [2] and muscle mass (around 1% per year)” (lines 58-60)

We believe this correction improves the scientific accuracy and credibility of the statement.

Comments 4: [57-73: Regarding equipment availability, it is unclear that MVIC is more accessible for testing than 1RM. On the contrary, MVIC requires a special setup and software for testing compared to 1RM, where classic gym equipment is necessary. Additionally, it was previously mentioned that the technique is more complex when performing 1RM than MVIC. However, it is a simple one-joint movement and leg extension, so this conclusion does not apply.]

Response 4: We appreciate the opportunity to clarify this point regarding the availability and feasibility of the instruments used to assess MVIC versus 1RM. While the reviewer's perspective is certainly valid, we would like to point out that, especially in older populations, the recommended procedure for determining 1RM typically involves the use of machines with weights and guided resistance. The most appropriate method is to apply progressively increasing loads until the maximum load that can be moved once is identified, which may also require the inclusion of intermediate weights beyond those provided by the standard machine (DOI: 10.1186/s40798-020-00260-z).

In contrast, MVIC assessment can be performed in a much simpler and more accessible way using only a load cell, a stable seat (e.g., a stretcher or a sturdy chair) and a basic attachment system, such as a Velcro strap attached to a rigid structure. As we also pointed out in our response to reviewer 1, dynamometer devices with load cells currently represent a cost-effective and inexpensive solution (between £150 and £300) and are increasingly available, especially in rehabilitation and physical reconditioning settings. This trend is supported by multiple studies validating the use of simple, portable devices for measuring isometric strength (DOI: 10.1123/jsr.2013.TR7; DOI: 10.1016/j.ptsp.2024. 04.008; DOI: 10.1371/journal.pone.0301872; DOI: 10.1186/s12891-019-2703-0).

Therefore, we believe that our study is timely and in line with emerging practices in this field. Our work aims to highlight that, especially in older adults, MVIC tests offer an accessible and less laborious alternative to traditional 1RM tests, both in terms of equipment and material and in terms of the time required for assessment.

Comments 5: [Accordingly, when designing the experiment, did you consider other scientific methods for the indirect estimation of 1RM, such as force-velocity relationships, and do you have any explanations for why you did not include them?.]

Response 5: There are certainly different methods for estimating 1RM, such as those based on the force-velocity relationship (load-velocity of load mobilisation), which do require performing the 1RM assessment procedure by monitoring the load using more sophisticated and expensive instruments, such as linear position transducers (encoders) or video recordings, which also require more complex interpretation of the data. Although this is a very promising and relatively recent methodology with many publications in recent years, there is still no consensus on its practical applicability outside the research context, especially with regard to strength training prescription for health in older populations.

Our study was not designed to compare different 1RM estimation methodologies, but rather to explore the relationship between isometric strength (MVIC) and the most accepted and validated form of dynamic strength assessment, i.e., the traditional 1RM test.

However, in light of the reviewer's insightful comment, we have added a clarification to the manuscript, acknowledging that there are alternative methods for estimating maximum strength, such as calculations based on movement speed or loss of speed as loads increase. The paragraph now reads as follows: “Consequently, some trainers and researchers choose to estimate 1RM based on the statistical relationship between a given load, the maximum number of repetitions performed with that load, and the corresponding 1RM. In addition, emerging methods propose estimating 1RM from the relationship between the load and the speed at which it is lifted, offering an alternative approach that may be less stressful ” (lines 77-82)

Comments 6: [91-94: The scientific foundation of why the unilateral and bilateral factors influenced the measurement results and study effects is not clearly emphasized. It is necessary to elaborate further and highlight these factors if they affected the design of the experiment in the context of whether the movement was unilateral or bilateral. Additionally, I could not find a description in the methods of whether unilateral or bilateral movement was performed.]

Response 6: We appreciate your insightful observation. Indeed, ‘typically in a unilateral context’ introduces an assessment that is not relevant to the context of our work, and the articles referred to do not address unilateralism. We have deleted the phrase: ‘evaluations in older adults more commonly utilise the knee extension test (leg extension)” (lines 155)

Comments 7: [Considering that strength decreases with age and that part of this effect is due to the reduction of neuromuscular function, was it essential for practical and scientific purposes to also monitor other variables in the domain of isometric testing, such as the Rate of Force Development, given that these variables more sensitively describe the neuromuscular characteristics of individuals? In the context of the goal of this study, it would be interesting to examine the reliability of other variables that describe isometric characteristics.]

Response 7: We appreciate your insightful comment, which undoubtedly raises a very relevant and enriching line of research, as variables such as rate of force development (RFD) would provide valuable information on neuromuscular characteristics, especially in the context of ageing and loss of function. However, the aim of our study was not to analyse the physiological determinants of isometric strength, but to evaluate the practical usefulness of a conventional measurement of maximum isometric strength from the perspective of its intra-session reliability and its predictive capacity of 1RM. Therefore, although relevant, the inclusion of additional variables such as RFD exceeded the defined scope of the present study. We appreciate and take note of this valuable suggestion for future studies in which we will implement both RFD and fatigue and loss of isometric strength throughout sustained isometric contraction. Thank you very much for the suggestion.

Comments 8: [Is there any reason why authors did not emphasize hypotheses, considering that a reasonable number of results could predict the outcomes of this study?]

Response 8: We appreciate the reviewer's comment on the absence of explicit hypotheses. Our main objective was to evaluate the intra-session reliability of a measurement instrument, a type of analysis that is fundamentally descriptive and methodological. As this is not a study designed to test a specific theoretical hypothesis, it does not usually require a traditional null hypothesis (H₀) versus alternative hypothesis (H₁) framework, since it is not designed to establish statistical differences, but rather to assess the consistency of measurements under the same conditions.

As for our second secondary objective, which was to explore the predictive value of one measure (MVIC) for estimating another (1RM), our approach was also not inferential or explanatory in nature. The analysis aimed to examine the strength of the association and the estimation potential. For this reason, we did not consider it necessary to formulate a dichotomous hypothesis (H₀ versus H₁), which is not usually formulated in this type of study.

Comments 9: [It is not emphasized that this is inter-session reliability. This should be highlighted. Accordingly, the reliability values would be even more significant if we had data on inter-day reliability.]

Response 9: We thank the reviewer for this observation. However, we would like to clarify that the reliability analysed in our study is intra-session reliability, not inter-session or inter-day reliability. In response to your comment, we have thoroughly revised the manuscript, including the title and all relevant sections, to explicitly state ‘intra-session reliability’ rather than the more general term ‘reliability,’ to ensure clarity and accurately reflect the scope and methodology of our analysis.

We agree with the reviewer that inter-day or inter-rater reliability may provide additional information, and we acknowledge that this is a potential area for future research. However, given the objectives of the present study, we have focused specifically on assessing random error within the same testing session under controlled conditions.

Comments 10: [I carefully read the methods, but could not find the methodology for the estimated 1RM. Please highlight that part.]

Response 10: We believe this may have been due to an error in the file received or in the viewing process, as the methodology for estimating 1RM was clearly described in Section 2.4, titled “One-Repetition Maximum (1RM) Assessment.” In the current version of the manuscript, this information can be found on lines 227–243.

Comments 11: [146-150: Considering that previous muscle activation affects the level of force expressed in isometric tests, provide appropriate references showing that one repetition was sufficient to express maximal isometric force. For these purposes, it may be helpful to show the t-test results for dependent samples between the first and second attempts to determine if the first contraction affects the second. In any case, the t-test can contribute to the reliability analysis..]

Response 11: We thank the reviewer for this detailed and thoughtful comment. It is true that prior muscle activation can influence the force generated in isometric tests (as well as in dynamic tests). However, our testing protocol is similar to protocols commonly used in studies evaluating maximum isometric strength in older adults. Specifically, participants first performed a standardised warm-up (in our case, cycling at 60 rpm with an approximate workload of 25 W), followed by two maximum voluntary isometric contractions. The highest value obtained was recorded for analysis.

Although we did not perform a t-test for paired samples between the two trials (it should be noted that the p-value is not an important value, since, as is normal due to the low variability, it can show statistically significant values when in reality what is important is the magnitude of the difference between the measurements), we did perform and show the key indicators of our study, such as the magnitude of variability between the first and second determinations, using the intraclass correlation coefficient (ICC) of 0.960 and the coefficient of variation (CV) of 3.2%, which demonstrates excellent consistency and minimal variability between attempts. These results suggest that the second contraction was not influenced by previous activation or fatigue, and that maximum voluntary isometric strength was reliably expressed in both tests.

Comments 12: [168-169: It states "proper technique." What does that mean in the context of the leg extension exercise? The criteria for proper technique are not described anywhere. This is important if we want to achieve reproducibility in the study.]

Response 12: We thank the reviewer for the insightful comment regarding the need to clarify the definition of “proper technique.” This criterion follows the standard description commonly used in the literature for leg extension procedures (e.g., doi: 10.1519/JSC.0b013e318220db2c). In our specific study, proper technique was defined as completing at least approximately 75% of the knee extension range of motion achieved with the initial load.

In response to the reviewer’s comment, we have now added this explanation to the 1RM methodology section, which now reads: “This process was repeated until the maximum load that each participant could lift once with proper technique—defined as completing at least approximately 75% of the knee extension range of motion achieved with the initial load—was determined and recorded as their 1RM” (lines 240-243)

We have also added a clarification in the limitations section, noting that the movement range was not monitored using a device, but rather supervised and assessed by an experienced evaluator. The paragraph shall now read as follows: “In addition, the load displacement during the 1RM assessment was not monitored using a linear position transducer, but was instead determined through the supervision of an experienced evaluator..” (lines 440-442)