Introduction
Over the past half-century, physiotherapy has undergone dynamic development in both scientific and clinical domains. Beyond clinical advancements, physiotherapy is gaining recognition in scientific research, as evidenced by the growing number of high-impact professional journals and influential studies worldwide.
Despite this progress, scientifically oriented physiotherapists face certain challenges, particularly regarding diagnostic and measurement tools. One issue is the lack of precise instruments that can be seamlessly integrated into research. This has led to the adoption of technologies from the field of medicine. Physiotherapists have successfully incorporated ultrasound for muscle morphology and activity assessment [1-7], electromyography in various applications [8, 9], functional magnetic resonance imaging for motor tasks [10, 11], motion analysis for spinal mobility, pelvic configuration [12-16], as well as myotonometry [17, 18] and elastography [19, 20] to evaluate tissue biomechanics and therapeutic effects [21].
Nevertheless, an equally significant issue arises when advanced technology proves insufficient. While sophisticated tools provide highly precise data, they may fail to capture broader functional aspects of human movement, an essential focus in physiotherapy. In some cases, simpler, more practical clinical assessments are needed to provide a holistic view. To make these widely used tests suitable for scientific research, they must first be refined to meet rigorous methodological standards. This raises a key question about the reliability of refined assessment techniques.
Assessing the reliability of clinical tests is crucial to ensure their accuracy, consistency, and applicability in both research and practice. Reliable tests allow clinicians to track changes in muscle flexibility and function over time, ensuring that observed differences are due to actual physiological changes rather than measurement errors. In a clinical setting, this enhances decision-making regarding diagnosis, treatment planning, and rehabilitation progress. In research, high reliability is essential for producing reproducible and comparable findings, which strengthens the validity of scientific conclusions and supports evidence-based practice. Without proper reliability testing, even widely used assessments may yield inconsistent results, leading to potential misinterpretations and ineffective interventions.
This research focuses on the adaptation of two widely utilized clinical muscle length assessment techniques, one for the knee flexors (hamstrings) and another for the knee extensors (quadriceps femoris). These muscle groups play a crucial role in postural compensation mechanisms and are often affected by sedentary lifestyles in children and adolescents [22]. Shortened muscles and strength imbalances are frequently observed in inactive youth and young athletes [23-25]. As a result, these tests are integral to postural assessments during development.
Various versions of knee flexor and extensor length tests exist in the literature. The knee flexors test, in particular, has been widely studied, with researchers implementing different measurement techniques, yielding varying degrees of reliability [22, 26-35]. However, there remains a need for simplified, time-efficient, and user-friendly versions that still meet scientific research standards. Additionally, certain reliability aspects, particularly longer intervals between test-retest measurements, have been insufficiently explored in previous studies.
Accordingly, this study assesses the intra- and inter-rater reliability of modified knee flexor and knee extensor length tests, examining their reliability within the same day as well as at 2-day and 5-day intervals in children aged 10-15 years. The findings determine whether modified versions of tests can be effectively implemented in scientific research settings.

Materials and Methods

Study participants
A total of 82 children from three local primary schools volunteered, including 42 boys and 40 girls from classes 4-8 (10-15 years). The participants were generally healthy children presenting various levels of day-to-day physical activity (rather than sitting, recreational activity, and sport activity). The inclusion criteria were as follows: age between 10 and 15 years (to cover the whole puberty spurt period); typical neurologic and motor development (no medical diagnoses of any illnesses or dysfunctions, between 25-75 centiles of the normative body mass and height according to age); and ability to comply with verbal commands. The participants were excluded if they had a history or current diagnosis of any significant orthopedic or neurological disorders (such as fractures, congenital abnormalities, cerebral palsy, or musculoskeletal pain and or dysfunction lasting more than two weeks). Additional exclusion criteria were having a history of surgical procedures, recent episodes of minor musculoskeletal issues within one month before the study, or experiencing minor health complaints (e.g. cold, headache) on the day of assessment. Two girls were excluded due to a history of surgical interventions within the abdominal area, one girl and three boys, due to a history of fractures within the lower limb.
The chosen age range was selected for several reasons. Firstly, it encompasses the entire puberty growth spurt, a period during which clinicians routinely conduct postural assessments in children, including the two muscle length tests under investigation, and when numerous postural abnormalities are commonly identified. The variability in participants’ postural characteristics contributed to diversity and posed a challenge for measurements, while also enhancing the potential for generalizing the results. For this reason, the participants with varying levels of daily physical activity were also included.
Finally, 76 children (37 girls) who qualified for the study (mean age=13.24 years [age range=10-15 years], average body mass=51.27 kg [body mass range=30-72 kg], and average body height=1.534 m [body height range=1.35-1.745 m]) together with their parents received detailed information on the objectives and procedures. The parents granted their informed consent. The measurements took place in one of the local academic centers, in the Motion Analysis Laboratory. During the procedure, no cases of resignation were recorded. 
The minimal sample size was estimated using the sample size calculator [36]. The highest number of participants (n=53) was required for calculations concerning the knee extensors length test based on one repeated measurement with an assumed minimal intra-class correlation coefficient (ICC) of 0.70 and an assumed expected ICC of 0.85.

Study design
This was a technical study with repeated measurements of the variables of interest performed by the two assessors on the three measurement days separated by one-day pauses (Figure 1). To capture all considered comparisons, the narrow linear arrows (representing inter-rater reliability) connecting the blocks of rater A and rater B should be mirrored to the opposite side. Likewise, the solid block arrows (depicting intra-rater reliability) linking successive blocks for rater B should also be mirrored accordingly. To avoid redundancy during data analysis, all comparisons indicated by the same type of arrows in Figure 1 were pulled together. The intra-rater reliability 2/5 refers to reliability across measurements taken by this same rater with 2 and 5-day intervals; the inter-rater reliability 0/2/5 denotes the reliability for measurements conducted by the different raters on the same day as well as after 2 and 5 days, respectively. 



Raters
To establish a more demanding testing environment, the study involved two raters with relatively limited professional experience. Both were licensed physiotherapists, each with 2.5 to 3.5 years of clinical practice. The raters completed a two-week intensive training program (3 h×3 times per week) to develop sufficient proficiency in performing the two-muscle length test. The training was supervised by an experienced specialist who was not otherwise involved in the study. Upon completion, the specialist evaluated and confirmed that the raters had acquired the necessary skills to competently perform their roles in the research. Raters’ skills were also tested in the minor pilot study conducted on 15 adult volunteers, during which the lowest recorded ICC (3,3) equaled 0.91 (inter-rater ICC for knee extensor length test). For blinding reasons, an assistant was also employed whose task was to read and record measurement devices’ readouts, which were hidden from the rates themselves.

Knee flexors length test
The subject lay supine on the couch (Figure 2), with the hip and knee of the tested leg flexed to 90 degrees and supported by a small stool held in place by the subject’s hands for stability. Two reference points were marked on the anterior lower leg, 5 cm and 15 cm below the patella (measured with the knee extended). Flat neodymium magnets (1.5 cm diameter) were attached with adhesive tape, aligning their proximal edges with these marks to streamline measurement setup and minimize manual error. A Baseline 12-1057 digital inclinometer (Baseline Products, USA) was calibrated vertically and positioned so its proximal edge aligned with the upper magnet. The readout was kept out of the rater’s view. A horizontal line was also drawn on the skin between the malleoli. Passive knee extension force was applied using a Steinberg SBS-KW-300A force gauge, placed on the posterior lower leg at the marked line. The rater held the gauge perpendicular to the leg, ensuring consistent force application. The verbal instructions were as follows: “Inhale, exhale, let your leg rise smoothly. Say ‘Stop’ when you feel a strong but tolerable stretch in the back of your thigh.” An initial trial determined the force required to reach the “Stop” point. This force was replicated in three test repetitions. Control for the weight of the lower leg was neglected following the findings of Guex et al. [30]. At the following “Now” command, the assistant recorded the inclinometer reading, ensuring blinding of the rater. The procedure was repeated for the opposite leg.




Knee extensors length test
The subject lay prone, positioned diagonally on the couch (Figure 3). The untested leg hung freely off the edge of the table, while the knee (flexed to approximately 90 deg.) was supported on a stool. The couch height was adjusted for comfort. The inclinometer was set up in the same manner as described previously. Similarly, the force gauge was used as before but was applied to the anterior aspect of the lower leg to induce passive knee flexion. The rater gave the following instructions: “Inhale, exhale, and let your leg move smoothly upwards. At the end, say ‘Stop’ when you feel a strong but tolerable stretch, not pain, in the front of your thigh.” As in the flexors test, an initial trial determined the force required to reach the ‘Stop’ point, ensuring consistency in subsequent repetitions. The actual muscle length test consisted of three repetitions, during which the rater replicated the previously recorded force level. At the following “Now” command, the assistant recorded the inclinometer reading, which was, again, obscured from the rater. After testing one leg, the procedure was repeated on the opposite side.



Study procedure
After the introductory interview and verification of the selection criteria were completed, the convenient week for measurements was chosen by the parents and children. They showed up in the laboratory in the afternoon after at least 3-h post-school rest and at least 2-h after the last meal. The children were asked to wear non-restrictive clothes and perform 10-min low-load warm-up using the stationary bike. Then, subjects received information on how to cooperate during the test performance, and 2-3 attempts of each test were introduced; however, they were fulfilled without exploring the full range of the subjects’ motion. The order of the tests for the given participant was randomly established on the first measurement day (Monday) and stayed unchanged on the consecutive measurement days (Wednesday, Friday of the same week). The order of body sides was always right-left. Letters A and B were randomly assigned to the raters. After this, the raters’ order always remained A–B. On Monday, after the subjects’ proper preparation, the two muscle length tests were repeated by Rater A, three times each. Then, all markings on subjects’ skin, attached magnets, etc., were removed, and subjects rested in a relaxed half-seated position in an armchair for 15 min. Subsequently, Rater B prepared subjects again for their measurements, and two tests were repeated once more, three times each. The whole procedure was identically repeated on Wednesday and Friday (Figure 1).
The participants were instructed to maintain their usual level of physical activity throughout the entire measurement week and avoid introducing any sudden changes to their routine activity.

Data processing
Data recorded by the two raters was gathered in the Statistica 13 (Statistica, Tulsa, USA) spreadsheet. To calculate reliability indices for different types of reliability (Figure 1) and to avoid redundancy of results, this database was specifically restructured. Planned comparisons were pooled together following the following scheme:
Intra-rater reliability, 2-day break between consecutive measurements: rater A on Monday vs rater A on Wednesday+rater A on Wednesday vs rater A on Friday+rater B on Monday vs rater B on Wednesday+rater B on Wednesday vs rater B on Friday (304 records in total); Intra-rater reliability, 5-day break between consecutive measurements: Rater A on Monday vs rater A on Friday+rater B on Monday vs rater B on Friday (152 records in total); Inter-rater reliability, measurements taken on the same day: Rater A on Monday vs rater B on Monday+rater A on Wednesday vs rater B on Wednesday+rater A on Friday vs rater B on Friday (228 records in total); Inter-rater reliability, 2-day break between consecutive measurements: Rater A on Monday vs rater B on Wednesday+rater A on Wednesday vs rater B on Friday+rater B on Monday vs rater A on Wednesday+rater B on Wednesday vs rater A on Friday (304 records in total); Inter-rater reliability, 5-day break between consecutive measurements: Rater A on Monday vs rater B on Friday+rater B on Monday vs rater A on Friday (152 records in total).

 Statistical analysis
A factorial analysis of variance model was employed to compare the mean values of the muscle length tests obtained by the two raters across the successive measurement days, with measurement day, rater, and body side treated as independent factors.
For the calculation of ICC, a mixed-model analysis of variance was used, where the repeated factor was the consecutive measurements and the independent factor was the subjects. The reliability of the same-day measurements conducted by a single rater is not reported separately, as it is inherently encompassed within other forms of intra-rater reliability (Figure 1). In Model 2, k of the ICC was applied to allow for generalization into the whole population of similar raters. The ICCs were separately calculated for the results of a single repetition of the given muscle length test, two repetitions (mean value), and three repetitions (mean value). Additionally, standard errors of measurement (SEM) were calculated using the Equation 1, along with the smallest detectable differences (Equation 2). Interpretation of the reliability of ICCs values was as follows: Poor=0.00-0.50; moderate=0.50-0.75; good=0.75-0.90; and excellent=0.90-1.00 [37].



Results
In Table 1, presented are descriptive statistics for the results of the two muscle length tests of interest, as recorded by both raters across consecutive measurement days. For all main effects of analysis of variance (measurement day, rater, side of the body), as well as their interactions, no significant differences were revealed (all P<0.05).



The two muscle length tests of interest demonstrated good to excellent reliability, regardless of whether measurements were performed by the same rater with a 2- or 5-day interval (Table 2) or by different raters on the same day as well as with a 2- or 5-day interval (Table 3). The lowest calculated ICC of 0.79 was the one for the knee flexors length test on the left side of the body (SEM=5.47, SDD=15.17 degrees; inter-rater reliability based on one repeated measurement taken with a 5-day break). In all remaining cases, ICCs higher than 0.80 were obtained.
In case of intra-rater reliability (Table 2), even one repeated measurement proved enough to obtain ICCs higher than 0.87. An increasing number of repeated measurements caused a further rise in the ICC values. Across the three repeated trials, ICC values reached or exceeded 0.94. In parallel, a gradual decrease in SEMs and SDDs values was recorded.



For inter-rater reliability (Table 3), lower ICC values were obtained, especially with the longer time break between the measurements. It was frequently the case that one repeated measurement returned ICCs lower than 0.90 (lower than 0.80 in one case mentioned earlier in this section). An increasing number of repeated measurements caused the noticeable rise of the ICC values. With three repeated measurements, all ICCs, even those calculated for measurements taken with the 5-day break, proved higher than 0.90. Again, all SEMs and SDDs gradually decreased.



For each reliability type under investigation, the increasing number of repeated measurements caused the gradual narrowing of the ICCs’ confidence intervals, even when the ICC value was no longer increasing markedly. 

Discussion
The results of our study indicate that the two modified muscle length tests, for the knee flexors and knee extensors tests, demonstrate a very satisfying level of reliability (for 3 repeated measurements, all calculated ICCs proved higher than 0.90). This finding supports the use of these modifications in scientific research settings, providing a reliable alternative to highly sophisticated technological tools. These tools may sometimes fail to capture the broader functional aspects of human movement that clinical tests can address. The findings are particularly beneficial for researchers aiming to implement clinically-oriented measurements in their studies, where simpler, more accessible methods are preferable.
In terms of absolute reliability, our SDD for three repeated measurements was a maximum of 9.93 degrees for the knee flexors and 4.48 degrees for the knee extensors. While the clinical or scientific implications of these values depend on the context, we observed that the differences between raters or on different days were smaller than the SDDs, suggesting that the measurements remained stable and consistent over time. This highlights the robustness of our modified procedures and supports their use for reliable clinical and scientific assessments.
This study was conducted under challenging conditions that might negatively affect reliability. The subjects were in the middle of puberty, a period marked by rapid and unpredictable changes in both morphology and function. Additionally, we incorporated long intervals between tests (up to 5 days) and used two relatively inexperienced raters who underwent a thorough training procedure. Despite these potential challenges, our findings showed excellent reliability, encouraging the use of these modified tests in both clinical practice and research applications.
The knee flexor length test is frequently of interest to researchers worldwide. Our results align with those of previous studies; however, we could address aspects of test reliability that have not been explored before. While many studies focus on intra-rater reliability and measurements taken on the same day, there is a lack of data on inter-rater reliability and longer test-retest intervals. To our knowledge, Hamid et al. [38] are the only authors who have reported inter-rater reliability for this test with a one-week interval. They recorded an ICC (2, 1) of 0.81-0.87, which is comparable to our results (inter-rater at 5 days), although their confidence intervals were much broader, ranging from 0.32 to 0.92. This discrepancy is likely due to their small sample size (n=14) and the lack of repeated measurements (they only performed single measurements). As a result, we can assert that our approach provides more precise estimations of the true ICCs for this measurement, which is particularly important in scientific research settings. Furthermore, our team has previously reported the inter-rater reliability of the knee flexor test [29], though those measurements were conducted by two raters on the same day. Since then, we have refined our methodology by introducing a more “user-friendly” strain gauge and force application technique, as well as a different method for determining the motion endpoint. Additionally, we employed a simple tool to support the subjects’ lower legs, which appears even more straightforward compared to the devices used in other studies [33, 38]. Moreover, the available literature data are frequently gathered from small samples [26, 27, 29, 30, 38]. Authors use inconvenient ways of motion endpoint determination [33] or implement complicated instrumentation for pelvic motion control [26, 31]. Such measures do not seem to be necessary to obtain a reliable angular measurement in the knee flexors length test. 
The available data on the knee extensor length test is somewhat limited. Gajdosik [28] demonstrated excellent reliability using goniometric measurements; however, his study involved a single rater taking measurements on the same day with only 15 subjects. Our team [29] previously provided data on both intra- and inter-rater reliability for this test, but again, all measurements were taken on a single day with a small sample size of 14 subjects. In this study, we addressed these limitations by using a different strain gauge and, again, a refined method for determining the motion endpoint. We also opted not to use external pelvic stabilization, as we found that stabilization provided by the counter-rotation of the two pelvic bones was sufficient (Figure 3). All other data regarding the knee extensor length test presented in this study are novel.
To provide recommendations for the use of the two muscle length tests, we suggest that in scientific settings, it would be beneficial to perform three repeated measurements of the test, particularly when there is a longer time separation between tests and retests (up to 5 days in this study) or when the tests and retests are conducted by different raters. In situations where the time separation is shorter and or a single rater is involved, two repeated measurements may be sufficient. We do not recommend reducing the number of repeated measurements to fewer than two. While this approach might be acceptable in a clinical setting (assuming the clinician has received proper training and adheres strictly to the procedure), where reliability requirements are slightly more flexible, we note that for a single repeated measurement, the lowest ICC we recorded was 0.79 (for the knee flexors length test, inter-rater reliability with a 5-day interval), which is still a satisfactory result for clinical use. Meanwhile, beyond a certain point, increasing the number of repeated measurements does not result in a further increase in the ICC value but does help narrow the confidence interval of the ICC. This phenomenon is advantageous from a scientific perspective, as it enhances the precision of the reliability estimates.

Conclusion
After appropriate specific modifications, there is a possibility of obtaining a very good and excellent level of reliability of the knee flexors and knee extensors length tests. Using the three repeated measurements, all recorded ICCs proved higher than 0.90. Such a high reliability level justifies the utilization of the modified versions of the tests in scientific settings. No sophisticated equipment or high time, personal, or economic costs are needed to achieve this goal.

Study limitations
One limitation of our study is its restricted external validity, as the sample only included teenagers. The knee flexors and knee extensors tests are often used in postural assessments during this developmental stage, but caution is needed when attempting to generalize these findings to other age groups. Nonetheless, the modifications we employed offer advantages, including low cost, minimal equipment (digital goniometer and force gauge), and reduced personnel requirements, which make them accessible for use in various settings. Furthermore, our results suggest that the modified tests can be implemented without the need for complex or expensive tools like X-rays or motion analysis systems. Finally, we chose to present ICCs for model 2, k, which allows for broader generalization across different raters, though model 3, k would likely show higher reliability. We did not provide intra-rater reliability for same-day measurements, as it is inherently captured in the higher-order reliabilities, where we observe higher ICC values due to reduced variance.
The promising results of this study open up several avenues for future research on the reliability and application of modified clinical muscle length tests, particularly for use in both clinical practice and scientific research. The key areas of the future research may include: broader age range of participants and different populations (e.g. young athletes), diverse clinical conditions (e.g. cerebral palsy, muscular dystrophies, etc.), longer follow-up periods, investigations on validity of the tests, and finally incorporating them into experiments in the role of measurement tools.

Ethical Considerations

Compliance with ethical guidelines
The study was approved by the institutional Biomedical Research Ethics Committee (Code of Ethics: 18/2020).

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
Conceptualization, Resources, Writing – Original Draft Preparation and Writing – Review & Editing: All authors; Methodology and Supervision: Rafał Gnat, Anna Gogola, Tomasz Wolny; Software: Rafał Gnat; Validation, Formal Analysis and Project Administration: Rafał Gnat, Anna Gogola; Investigation: Anna Gogola, ;Data Curation: Agnieszka Polaczek, Piotr Woźniak; Visualization: Rafał Gnat, Agnieszka Polaczek, Piotr Woźniak; Funding Acquisition none

Conflict of interest
The authors declared no conflict of interest.



 

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