Comparison of Limb Symmetry Index Values Across Different Knee Flexor Strength Testing Conditions in Healthy Male Recreational Athletes

Abstract

Background/Objectives: Restoring lower-limb strength and symmetry is crucial after ACL injury and reconstruction. The limb symmetry index (LSI) is often used to assess strength symmetry for return-to-sport decisions, but various assessment methods can influence outcomes. This study aimed to compare LSI across common knee flexor testing methods in healthy male athletes and to examine associations between absolute strength outcomes, thereby establishing baseline reference values for LSI in a healthy population. Methods: Twenty-two healthy recreationally active males participated in this prospective cross-sectional study. Knee flexor strength was assessed bilaterally using three force plate isometric tests, a static dynamometer-based test (isometric), and isokinetic dynamometer-based tests. Absolute strength values were normalized to body mass. LSI values were calculated for each testing condition. Differences in LSI across modalities were analyzed with repeated-measures ANOVA, and associations between normalized strength outcomes were assessed using Pearson correlation coefficients. Results: LSI values ranged from 96.69 to 101.83 across the testing conditions, with no significant differences observed between measures. Normalized absolute strength outcomes demonstrated very strong correlations within the same measurement category (r = 0.86–0.94 for force plate tests and r = 0.88–0.96 for isokinetic tests). In contrast, correlations between isometric and isokinetic strength outcomes were moderate (r = 0.41–0.67). Conclusions: LSI values were consistent across knee flexor strength testing modalities, suggesting that symmetry assessment was relatively consistent across different measurement methods in the studied group. In contrast, normalized absolute strength outcomes showed only moderate and variable associations across modalities, indicating that different testing approaches assess related but not interchangeable aspects of muscle strength.

1. Introduction

Anterior cruciate ligament (ACL) injuries are among the most common and impactful knee injuries in physically active populations and often require prolonged rehabilitation. Even after ACL reconstruction (ACLR), patients remain at substantial risk of subsequent knee injuries, including graft rupture and contralateral ACL tears [1,2,3]. Furthermore, many patients experience difficulties returning to their pre-injury level of sport participation, and recovery patterns are influenced by biomechanical and patient-specific factors [4,5,6].

Restoring lower-limb muscle strength and inter-limb symmetry is a critical goal of rehabilitation following ACLR and is commonly used to individualize rehabilitation programs, monitor recovery progress, and support decision-making regarding return to sports [7,8,9,10,11,12]. Various methods are used to evaluate knee flexor strength, raising questions about how strength and symmetry outcomes should be interpreted across different testing conditions [7,8,9,13,14,15]. Isokinetic dynamometry is widely regarded as a reference method due to its high degree of standardization and its ability to measure torque at controlled angular velocities [13,14,16]. In addition to peak torque, isokinetic testing enables the assessment of torque-derived parameters such as work and power, as well as other outcomes including position- and time-based metrics and fatigue indices, providing detailed insight into neuromuscular function [17,18]. However, its clinical use is often limited by high cost, time requirements, and restricted availability.

Consequently, alternative approaches using more portable devices such as force plates, handheld dynamometers, and sphygmomanometers are frequently used in both research and clinical settings [19,20,21,22]. These methods typically assess strength under static (isometric) conditions and are performed at different joint angles and body positions, thereby varying muscle length, moment arm, and mechanical demands [19,20,21,22].

To facilitate interpretation across limbs despite these methodological differences, indices of inter-limb symmetry are commonly used. Among these, the limb symmetry index (LSI) remains one of the most widely applied metrics. Despite ongoing debate regarding its interpretation and limitations as a standalone measure [23,24,25], LSI continues to be used to quantify inter-limb symmetry and to support clinical decision-making [7,10,14,15,16,26].

Differences in LSI across testing modalities may be expected due to distinct biomechanical and neuromuscular demands. Isometric assessments quantify strength under static conditions at specific joint angles, whereas isokinetic testing involves dynamic contractions at preset angular velocities through a predefined range of motion. These modality-specific characteristics may influence the magnitude of measured symmetry and result in non-comparable LSI values across tests. However, it remains unclear whether LSI values derived from different strength assessments yield comparable inter-limb symmetry results. This issue is particularly clinically relevant; as already mentioned, LSI is frequently used to guide rehabilitation progress and return-to-sport decisions [7,25,27].

Moreover, data on LSI behavior in healthy populations are also limited, making it harder to interpret individual results and highlighting the need for baseline reference values. That is why examining LSI in a healthy population is crucial to establishing a methodological reference for interpreting symmetry outcomes. Examining healthy individuals enables assessment of the natural behavior of symmetry indices without confounding factors such as pain, neuromuscular inhibition, or compensatory movement strategies. And again, it is important in light of the use of symmetry values as a reference in the clinical context.

Therefore, the primary aim of this study was to compare LSI values for knee flexor muscle strength across different testing conditions, including several isometric protocols and isokinetic assessment, in healthy male recreational athletes. The secondary aim was to examine associations between absolute strength outcomes across testing modalities to characterize how measures of knee flexor muscle strength vary under different mechanical conditions.

It was hypothesized that LSI values would differ across testing conditions, indicating that the specific strength assessment protocol may influence symmetry outcomes. It was further hypothesized that absolute strength values obtained across the different testing modalities would exhibit positive associations, although the magnitude of these associations would vary with the pair of tests compared.

The present study focused on male participants to ensure a controlled and homogeneous sample. This approach was adopted to reduce variability related to known sex-related differences in muscle strength and neuromuscular characteristics and is supported by evidence demonstrating differences between males and females in muscle mass, fiber composition, and neuromuscular function [28].

2. Materials and Methods

2.1. Ethics, Study Design, and Setting

The study was conducted in accordance with the Declaration of Helsinki and approved by the Bioethics Committee of Wroclaw Medical University, Wroclaw, Poland (approval no. KB 351/2025; 9 September 2025). Prior to participation, all volunteers received detailed information regarding the study procedures and provided written informed consent.

The study used an observational, cross-sectional, prospective design. It was therefore reported in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement guidelines and the relevant cohort study checklist [29]. Given the observational design of the study, the preregistration of the protocol was not applicable [30].

The study was carried out in the Physiotherapy Research Laboratory, University Centre of Physiotherapy and Rehabilitation, Faculty of Physiotherapy and Clinical Department of Orthopedics, Traumatology and Hand Surgery, Department of Orthopedics, Traumatology and Hand Surgery, Faculty of Medicine, Wroclaw Medical University, Wroclaw, since gaining the approval and by the end of November 2025. All measurements were performed during a single testing session per participant; thus, no exposure period or follow-up assessment was applicable.

2.2. Participants

Male volunteers were mainly recruited from students at Wroclaw Medical University. Due to insufficient volunteer participation from this group, additional volunteers were recruited from outside the university using the same eligibility criteria. All volunteers underwent a history review and physical examination before participating in muscle strength measurements. Only individuals meeting all inclusion criteria and none of the exclusion criteria were enrolled.

Participants were eligible if they were male; were aged 20–30 years; reported no previous injuries or disorders involving the lower limbs or spine; reported no current pain in the lower limbs or spine; declared good general physical and mental health; had no diagnosed systemic disease; presented a Physical Activity Level (PAL) between 1.6 and 1.8 (moderate to high recreational activity); performed physical activity exclusively for recreational purposes (not competitive or professional sport); had a body mass index (BMI) between 18.5 and 24.99 kg/m²; demonstrated full and symmetrical knee joint range of motion bilaterally; presented side-to-side knee circumference differences ≤ 2 cm and thigh circumference differences <3 cm; demonstrated normal muscle strength (Lovett scale grade 5) of muscles acting on the knee joint bilaterally.

In turn, the participants were excluded if they were female; were younger than 20 or older than 30 years; reported any previous lower-limb or spinal injury or disorder; reported current pain in the lower limbs or spine; presented poor general physical or mental health; reported a diagnosed systemic disease; had PAL < 1.6 or >1.8; performed competitive or professional sport; had BMI < 18.5 or >24.99 kg/m²; presented limited knee joint range of motion; presented asymmetry in limb circumference exceeding inclusion thresholds; demonstrated muscle strength < 5 on the Lovett scale in either limb.

The dominant limb was defined as the limb that the participant indicated would be used to kick a ball [31]. PAL was defined through a structured interview. Participants reported the frequency and duration of their regular weekly physical activity. Based on this information, PAL was categorized according to predefined criteria. A PAL of 1.6 indicated about 280 min of physical activity per week (2 to 3 sessions), while a PAL of 1.8 indicated approximately 420 min per week (3 to 4 sessions).

For the present analysis, only participants with complete data who successfully performed all strength tests under all planned testing conditions were included (complete-case analysis).

The broader research project from which the present dataset was derived included both male and female participants. However, to ensure sample homogeneity and to minimize variability related to known sex differences in muscle strength and neuromuscular characteristics, the present analysis was restricted to male participants. This approach allowed for a more controlled evaluation of the methodological behavior of LSI values across testing conditions.

2.3. Variables

The primary outcome variable was the LSI of knee flexor muscle strength, measured under each testing condition.

Secondary outcome variables included normalized-to-body-mass maximal strength values of the knee flexor muscles obtained from each testing modality. These comprised maximal force values recorded during isometric force plate testing (expressed in kgf/kg) and maximal torque values recorded during static and isokinetic dynamometry testing (expressed in Nm/kg).

The testing condition was treated as a within-subjects factor in the comparative analyses. No external exposures were investigated in this study. Diagnostic criteria were not applicable.

2.4. Measurements

Each participant attended a single measurement session comprising three groups of tests assessing knee flexor muscle strength. Rest intervals between successive groups of tests were approximately 30 min and were extended if the participant reported excessive fatigue.

All tests were performed bilaterally. The order of testing conditions and the starting limb were determined using a pre-generated randomization schedule for the 24 initially recruited participants. All six possible sequences of the three testing modalities were included and distributed as evenly as possible across the planned sample. For each of the six testing sequences, four slots were predefined, with two starting from the dominant limb and two from the non-dominant limb. Participants were assigned to these slots based on a pre-generated randomized list. This method aimed to minimize potential order, fatigue, and learning effects related to repeated measurements. Two participants were excluded before the final analysis because they did not meet the BMI eligibility criterion; as a result, the final sample was no longer perfectly balanced, although the original allocation schedule was designed to ensure an even distribution.

All measurements were performed by the first author (N. U.), a trained examiner who had been previously instructed in the standardized testing protocol, under the supervision of an experienced researcher (A. K.). Standardized verbal commands (“start” and “stop”) were used throughout.

At the beginning of the session, a standardized warm-up was performed. Participants were then familiarized with the testing procedures and completed practice trials prior to the recorded measurements. Standardized verbal instructions and encouragement were provided during all tests. After all measurements were completed, stretching exercises were performed for the tested muscle groups.

The applied protocols are based on established force plates and dynamometry procedures commonly used in research and clinical practice. The reliability of these protocols has been evaluated in the same laboratory where this study was conducted, across two separate projects using identical equipment and procedures; however, these results are currently under peer review and are not presented in detail in this manuscript.

2.4.1. Force Plate Isometric Tests

Three separate force plate isometric testing conditions were performed to assess knee flexor muscle strength under static conditions. These conditions differed in hip and knee joint positioning of the tested limb.

All tests were performed in the supine position. Force was measured using two force plates, one for the right and one for the left limbs (K-Force Plates, Kinvent, Montpellier, France). During testing, the measuring device was positioned under the heel of the tested limb.

During each testing condition, participants pressed their heel against the force plate with maximal effort, performing an isometric contraction of the knee flexor muscles in a stabilized position. Each contraction lasted 3 s. One familiarization trial was performed prior to data collection and was not recorded. Two maximal recorded trials were then performed for each condition. In accordance with the methodology used by other authors, rest intervals between repetitions were 120 s, and rest intervals between the three testing conditions were also 120 s [20,22].

The three force plate isometric testing conditions were:

• Hip and knee flexed to 90° with the force plate elevated (FP-IM-90/90) as shown in Figure 1;
• Force plate elevated 30 cm with the knee flexed to 30° (FP-IM-30-30cm) as shown in Figure 2;
• Force plate positioned on the floor with the knee flexed to 30° (FP-IM-30-Floor) as shown in Figure 3.

The ankle was kept in a neutral position (90° relative to the shank). The upper limbs were positioned alongside the trunk.

The highest maximal force value obtained from the two recorded trials for each limb in each condition was used for analysis. It was normalized to body mass and expressed in kgf/kg.

2.4.2. Static Dynamometer Isometric Test

A static dynamometer was used to perform isometric testing to assess knee flexor strength under static conditions. Isometric torque of the knee flexor muscles was measured using a static dynamometer (UPR-1, SUMER, Poland). Measurements were performed in the prone position with the tested knee flexed to 30° (SD-IM-30) as presented in Figure 4. The ankle was maintained in a neutral position. The dynamometer axis was aligned with the lateral femoral epicondyle. The lever arm length was standardized across participants at 40 cm. The pelvis and tested limb were stabilized using fixation straps. The head was supported on forearms.

Prior to data collection, one familiarization trial was completed and was not recorded. Subsequently, two maximal voluntary isometric contractions were performed, each lasting 6 s, with a 120 s rest interval between trials.

The highest maximal torque value obtained from the two recorded trials for each limb was used for analysis. It was normalized to body mass and expressed in Nm/kg.

2.4.3. Isokinetic Dynamometer Tests

An isokinetic dynamometer was used to assess knee flexor strength under isokinetic conditions. Peak torque of the knee flexor muscles was measured using an isokinetic dynamometer (Biodex System 4 Pro, Biodex Medical Systems, Inc., Shirley, NY, USA). Measurements were performed in the seated position as presented in Figure 5.

The dynamometer rotational axis was aligned with the lateral femoral epicondyle of the tested limb. The lever arm length was standardized for all participants and set at 40 cm. Testing was performed through the participant’s full available knee range of motion. Gravity correction was applied according to the manufacturer’s recommendations prior to data collection.

Participants were instructed to keep their arms crossed over their chests throughout testing. The trunk was stabilized with straps, and the tested limb was secured to the dynamometer using straps placed over the thigh and around the distal shank above the ankle.

Three sets of alternating knee extension–flexion repetitions were performed at angular velocities of 300°/s (15 repetitions), 180°/s (10 repetitions), and 60°/s (5 repetitions), corresponding to testing conditions ID-IK-300, ID-IK-180, and ID-IK-60, respectively. Rest intervals between sets were 120 s. Prior to each set, one familiarization trial was performed and was not recorded. A 10 s interval was provided between the familiarization trial and the recorded set.

For each velocity and each limb, the highest peak torque value obtained during the recorded repetitions was used for analysis. It was normalized to body mass and expressed in Nm/kg.

2.5. Bias

Several measures were implemented to minimize potential sources of bias.

To reduce selection bias, strict inclusion and exclusion criteria were applied, and all participants underwent a standardized screening procedure prior to enrollment. Only individuals who met all criteria and completed all testing conditions were included in the final analysis.

Measurement bias was minimized by using standardized testing protocols, calibrated equipment, and identical measurement procedures for all participants. All measurements were performed by a trained examiner under the supervision of an experienced researcher. Participants were familiarized with all testing procedures and completed practice trials prior to data collection to reduce learning effects.

To reduce order and fatigue-related bias, the sequence of testing modalities and the order of tested limbs were randomized, and standardized rest intervals were provided between trials and test modalities.

A homogeneous sample of healthy male recreational athletes without current or prior lower-limb pathology was selected to reduce potential confounding from injury history, training status, or sex-related differences.

2.6. Study Size

The minimum required sample size was calculated using G*Power software version 3.1.9.6. A significance level of α = 0.05, statistical power (1 − β) = 0.80, and a medium effect size (d = 0.5) were assumed, based on Cohen’s conventional criteria for effect size interpretation [32]. These benchmarks are widely used when prior data are unavailable, though they should be interpreted as general guidelines rather than strict thresholds [33]. In repeated-measures analysis of variance (ANOVA), this corresponds to an effect size of f ≈ 0.25. Additionally, a moderate correlation among repeated measures (ρ = 0.50) and a conservative nonsphericity correction (ε = 0.75) were assumed.

Accordingly, the following settings were applied in G*Power: test family: F tests; statistical test: ANOVA: repeated measures, within factors; type of power analysis: a priori. Input parameters were as follows: effect size f = 0.25, α = 0.05, power (1 − β) = 0.80, number of measurements = 7, number of groups = 1, correlation among repeated measures = 0.50, and nonsphericity correction ε = 0.75. The calculations indicated that a minimum of 22 participants was required. Two additional volunteers were recruited to account for potential exclusion, dropouts, or incomplete data.

2.7. Quantitative Variables

All quantitative variables, including LSI values and normalized body-mass-adjusted force and torque outcomes, were treated as continuous. For each testing condition, LSI was calculated from the maximal recorded strength values as the ratio of the non-dominant limb to the dominant limb multiplied by 100.

No categorization or grouping of continuous variables was performed. All analyses were conducted using the original continuous values. Testing condition was treated as a within-subject factor in comparative analyses of LSI and normalized strength outcomes.

2.8. Statistical Methods

Statistical analyses were performed using IBM SPSS Statistics version 30.0.0.0 (172). Continuous variables are presented as mean ± standard deviation (SD). The level of statistical significance was set at p < 0.05. The distribution of the data was assessed using the Shapiro–Wilk test.

To address the primary aim and compare LSI values across the seven testing conditions, a one-way repeated-measures ANOVA was performed. One variable deviated from a normal distribution; however, given the robustness of repeated-measures ANOVA to minor violations of normality and the absence of extreme outliers, parametric analyses were conducted. Mauchly’s test was used to assess the assumption of sphericity. When the sphericity assumption was violated, the Greenhouse–Geisser correction was applied. When appropriate, pairwise comparisons between testing conditions were conducted using a Bonferroni adjustment for multiple comparisons. Effect sizes were reported as partial eta squared (ηp²).

To address the secondary aim, associations between normalized maximal strength outcomes across testing conditions were analyzed separately for the dominant and non-dominant limbs using correlation analysis. It is worth adding that correlation analysis indicates the strength of association between variables and does not imply agreement or interchangeability between measurement methods.

For the dominant limb, six of the seven variables met the assumption of normality, whereas one deviated from normality. For the non-dominant limb, all variables met the normality assumption. Therefore, Pearson correlation coefficients were calculated.

Correlation strength was interpreted descriptively as negligible (<0.10), weak (0.10–0.39), moderate (0.40–0.69), strong (0.70–0.89), or very strong (≥0.90), according to Schober et al. (2018) [34]. Scatterplots were visually inspected to verify linear relationships and identify potential outliers.

Because the sample was homogeneous and the inclusion criteria were strict, no additional confounders were included in the statistical models. No subgroup or interaction analyses were performed.

Only participants who completed all testing conditions were included in the final analysis (complete-case analysis); therefore, no imputation of missing data was required. As all measurements were obtained during a single testing session, loss to follow-up was not applicable. No sensitivity analyses were performed.

3. Results

A total of 24 male recreational athletes volunteered and were screened. Twenty-two were confirmed eligible and enrolled in the final sample. Two were excluded due to excessively high BMI. All enrolled participants completed the full testing protocol during a single measurement session and were included in the final analysis (complete-case analysis). No participants withdrew or were excluded after enrollment, and no missing data were observed for any variable of interest. Because the study employed a cross-sectional, single-session design, follow-up was not feasible.

The final sample comprised 22 participants whose characteristics were as follows (mean ± SD; range): age 23.50 ± 2.32 years (21 to 28), body mass 80.18 ± 7.85 kg (61 to 100), body height 184.41 ± 7.01 cm (175 to 201), and BMI 23.56 ± 1.62 kg/m² (19.25 to 24.93). In all participants studied, the dominant limb was the right. It should be clarified that this distribution occurred by chance within the recruited sample, and no left-limb-dominant individuals were intentionally excluded.

As presented in

Table 1. Normalized knee flexor strength outcomes and limb symmetry index (LSI) across testing conditions in the studied participants.

Testing Condition	Dominant Limb (kgf/kg or Nm/kg) *	Nondominant Limb (kgf/kg or Nm/kg) *	LSI
FP-IM-90/90	0.28 ± 0.05	0.27 ± 0.05	96.93 ± 8.59
FP-IM-30-30cm	0.26 ± 0.04	0.26 ± 0.05	97.69 ± 10.03
FP-IM-30-Floor	0.25 ± 0.05	0.25 ± 0.05	98.31 ± 13.81
SD-IM-30	1.28 ± 0.40	1.23 ± 0.35	96.69 ± 10.02
ID-IK-300	0.86 ± 0.16	0.87 ± 0.18	101.83 ± 15.59
ID-IK-180	1.07 ± 0.21	1.03 ± 0.21	97.02 ± 12.22
ID-IK-60	1.39 ± 0.24	1.36 ± 0.27	98.05 ± 11.33

Values are presented as mean ± SD (n = 22). * Force plate outcomes are expressed in kgf/kg, whereas static and isokinetic dynamometer-based outcomes are expressed in Nm/kg. Abbreviations: LSI, limb symmetry index; FP-IM-90/90, force plate isometric test with hip and knee flexed to 90°; FP-IM-30-30cm, force plate isometric test at 30° knee flexion with the force plate positioned on a 30 cm support; FP-IM-30-Floor, force plate isometric test at 30° knee flexion with the force plate positioned on the floor; SD-IM-30, static dynamometer isometric test at 30° knee flexion; ID-IK-300/180/60, isokinetic dynamometer tests performed at 300°/s, 180°/s, and 60°/s, respectively.

, mean LSI values across testing conditions ranged from 96.69 to 101.83, indicating generally high inter-limb symmetry across all measurement modalities. Mauchly’s test indicated that the assumption of sphericity was violated (p = 0.017); therefore, Greenhouse–Geisser correction was applied. A one-way repeated-measures ANOVA revealed no significant differences in LSI values across the seven testing conditions (F = 0.52, p = 0.705, ηp² = 0.024). Bonferroni-adjusted pairwise comparisons revealed no significant differences among the testing conditions (all p = 1.000). The observed effect sizes (ηp²) were small, indicating negligible practical differences between testing modalities.

Correlation analyses of normalized maximal strength outcomes across testing conditions in the dominant limb revealed strong to very strong relationships within the same testing modalities (r = 0.660 to 0.886), whereas associations between modalities ranged from weak to moderate and were more variable (r = 0.106 to 0.650). Detailed results are presented in

Table 2. Correlation matrix for normalized knee flexor strength outcomes across testing conditions in the dominant limb.

	FP-IM-90/90	FP-IM-30-30cm	FP-IM-30-Floor	SD-IM-30	ID-IK-300	ID-IK-180	ID-IK-60
FP-IM-90/90	n/a	0.875 ***	0.733 ***	0.596 **	0.188	0.374	0.421
FP-IM-30-30cm	0.875 ***	n/a	0.832 ***	0.639 **	0.403	0.468 *	0.494 *
FP-IM-30-Floor	0.733 ***	0.832 ***	n/a	0.650 **	0.474 *	0.630 **	0.578 **
SD-IM-30	0.596 **	0.639 **	0.650 **	n/a	0.106	0.419	0.486 *
ID-IK-300	0.188	0.403	0.474 *	0.106	n/a	0.812 ***	0.660 ***
ID-IK-180	0.374	0.468 *	0.630 **	0.419	0.812 ***	n/a	0.886 ***
ID-IK-60	0.421	0.494 *	0.578 **	0.486 *	0.660 ***	0.886 ***	n/a

Values are presented as r, Pearson correlation coefficients (n = 22). * p < 0.05, ** p < 0.01, *** p < 0.001. Abbreviations: n/a, not applicable. FP-IM-90/90, force plate isometric test with hip and knee flexed to 90°; FP-IM-30-30cm, force plate isometric test at 30° knee flexion with the force plate positioned on a 30 cm support; FP-IM-30-Floor, force plate isometric test at 30° knee flexion with the force plate positioned on the floor; SD-IM-30, static dynamometer isometric test at 30° knee flexion; ID-IK-300/180/60, isokinetic dynamometer tests performed at 300°/s, 180°/s, and 60°/s, respectively.

.

As shown in

Table 3. Correlation matrix for normalized knee flexor strength outcomes across testing conditions in the nondominant limb.

	FP-IM-90/90	FP-IM-30-30cm	FP-IM-30-Floor	SD-IM-30	ID-IK-300	ID-IK-180	ID-IK-60
FP-IM-90/90	n/a	0.906 ***	0.820 ***	0.537 *	−0.074	0.230	0.176
FP-IM-30-30cm	0.906 ***	n/a	0.850 ***	0.381	0.054	0.269	0.209
FP-IM-30-Floor	0.820 ***	0.850 ***	n/a	0.427 *	0.042	0.167	0.125
SD-IM-30	0.537 *	0.381	0.427 *	n/a	−0.040	0.316	0.397
ID-IK-300	−0.074	0.054	0.042	−0.040	n/a	0.783 **	0.639 **
ID-IK-180	0.230	0.269	0.167	0.316	0.783 **	n/a	0.835 ***
ID-IK-60	0.176	0.209	0.125	0.397	0.639 **	0.835 ***	n/a

Values are presented as r, Pearson correlation coefficients (n = 22). * p < 0.05, ** p < 0.01, *** p < 0.001. Abbreviations: n/a, not applicable. FP-IM-90/90, force plate isometric test with hip and knee flexed to 90°; FP-IM-30-30cm, force plate isometric test at 30° knee flexion with the force plate positioned on a 30-cm support; FP-IM-30-Floor, force plate isometric test at 30° knee flexion with the force plate positioned on the floor; SD-IM-30, static dynamometer isometric test at 30° knee flexion; ID-IK-300/180/60, isokinetic dynamometer tests performed at 300°/s, 180°/s, and 60°/s, respectively.

, correlation analysis of normalized maximal strength outcomes across testing conditions in the non-dominant limb demonstrated a pattern similar to that observed for the dominant limb. Strong to very strong associations were found within the same testing modalities (r = 0.639 to 0.906), whereas correlations between modalities were generally weaker and more variable, ranging from negative to moderate (r = −0.074 to 0.537).

Overall, these findings indicate strong internal consistency within measurement categories, whereas cross-modality associations were moderate and less consistent.

4. Discussion

The primary aim of this study was to determine whether limb symmetry index (LSI) values differ across commonly used modalities for assessing knee flexor strength. Contrary to the initial hypothesis, LSI values were comparable across all tested conditions, with no significant differences and small effect sizes, despite substantial methodological differences among protocols, including body position, contraction type, and measurement device. These findings suggest that in healthy recreationally active males, inter-limb symmetry in knee flexor strength may represent a relatively consistent neuromuscular characteristic across different testing modalities

The secondary aim was to examine associations among absolute strength outcomes measured with different modalities. Absolute strength outcomes were related but not interchangeable. Very strong correlations were observed within the same measurement category, particularly among the force plate protocols and among the isokinetic dynamometer conditions, whereas cross-modality associations were only moderate and more variable. These results indicate that different testing modalities capture related but distinct aspects of knee flexor muscle strength. Importantly, correlation reflects association rather than agreement between measurement methods; therefore, the present findings should not be interpreted as evidence that strength values obtained using different testing devices can be used interchangeably. This distinction is critical, as strong correlations do not imply that results obtained using different devices can be used interchangeably in clinical or research settings.

In some areas of sports science literature, hamstring strength assessments are discussed within the broader concept of the “posterior chain” musculature, particularly when tests involve multi-joint movements or hip-extension–dominant tasks [20,22,35,36,37]. However, the present study specifically evaluated knee flexor muscle strength, as the applied testing modalities primarily quantify knee flexion torque rather than the integrated function of the entire posterior chain. This distinction is important for methodological clarity and allows for a more precise interpretation of the findings.

The main finding of this study is that LSI values may represent a relatively consistent neuromuscular characteristic across different testing conditions, although the present study does not assess temporal stability or intra-individual reliability. One possible explanation is that although absolute torque varies with mechanical conditions such as contraction type, joint angle, or testing velocity, both limbs may respond proportionally to these changes, thereby preserving the symmetry ratio [24,27]. Additionally, asymmetry in non-injured populations is typically small, which may further contribute to the relative consistency of LSI values across different testing modalities [38]. Notably, previous studies, particularly in clinical populations, have reported that inter-limb symmetry may vary depending on the testing protocol and population [23,24], suggesting that the consistency of LSI may not be universal across all conditions.

While theoretical models suggest that force and torque may scale differently with body size, contemporary literature commonly normalizes body mass across both force- and torque-based outcomes [39,40]. Therefore, a consistent normalization approach was adopted in the present study to facilitate comparability across testing modalities. Moreover, given the ratio-based nature of LSI, the influence of body mass is likely reduced.

Although mean LSI values were close to 100% across all testing conditions, the relatively large standard deviations observed in some conditions suggest considerable inter-individual variability in limb symmetry, even within a homogeneous group of healthy participants. This highlights the importance of interpreting LSI values at the individual level and suggests that some degree of asymmetry may be a natural characteristic rather than a deviation from normal function. This is particularly relevant when establishing reference values and interpreting symmetry indices in both research and clinical contexts.

These results are particularly relevant to rehabilitation following ACLR. Restoring inter-limb symmetry is often used as a criterion for returning to sport after ACLR [27,41,42,43]. Kodama et al. (2025) reported that an LSI threshold of ≥90% is the most frequently used functional metric in return-to-sports protocols after ACLR [44]. It should also be noted that LSI is a ratio-based metric, in which values below and above 100% reflect asymmetry in opposite directions rather than different magnitudes of asymmetry. Therefore, an LSI of 90% and 110% represent a similar degree of asymmetry but in opposite limbs. This characteristic should be considered when interpreting commonly used thresholds (e.g., ≥90%), as such cut-offs do not distinguish the direction of asymmetry.

However, the variability observed in the present healthy sample suggests that fixed symmetry thresholds should be interpreted with caution. Symmetry metrics such as LSI should be considered in the context of individual baseline characteristics and in combination with absolute strength and functional performance measures, rather than as universal criteria.

The findings of the present study suggest that symmetry-based measures may remain relatively consistent across different strength testing modalities, facilitating interpretation when athletes are assessed with different testing systems during rehabilitation. Still, it should be noted that symmetry measures may not fully capture underlying deficits in absolute strength or neuromuscular function.

In clinical practice, strength assessments following ACLR are frequently performed with different testing devices, depending on equipment availability across clinical and sports performance settings [7,8,9,27,45]. Laboratory-based testing often relies on isokinetic dynamometry, whereas rehabilitation clinics or field settings frequently use portable devices such as force plates or handheld dynamometers [7,8,9]. The findings of this study, therefore, suggest that symmetry-based indices such as LSI may remain interpretable even when different strength-testing modalities are used. However, absolute strength values should always be interpreted within the context of the specific testing protocol and device applied. This is especially important when transitioning between laboratory-based assessments and field-based or clinical testing environments, where access to specific equipment may vary.

Despite the widespread use of LSI in return-to-sports decision-making, its interpretation remains debated. Wellsandt et al. (2017) demonstrated that symmetry indices may overestimate knee function after ACL injury because deficits in the contralateral limb can mask persistent impairments in the involved limb [23]. Similarly, Simonsson et al. (2025) questioned the reliance on symmetry thresholds alone as indicators of readiness for return to sports after ACLR [24]. These observations highlight the importance of interpreting LSI alongside absolute strength outcomes and functional performance tests. The present findings contribute to this discussion by suggesting that although LSI values appear relatively consistent across testing modalities in this healthy sample, this does not imply equivalence of absolute strength outcomes across devices.

Indeed, although absolute strength values from isometric and isokinetic testing conditions were associated, they were not interchangeable. Previous studies comparing contraction modes have demonstrated that isometric and isokinetic testing assess related but mechanically distinct aspects of muscle function [20,27,35,46]. Differences in neural activation strategies, torque–angle relationships, and velocity-dependent properties of muscle contraction likely contribute to modality-specific strength outputs [17]. Consequently, absolute strength values obtained using different testing systems should not be directly compared, particularly in clinical contexts where accurate monitoring of rehabilitation progress is required.

Several limitations of the present study should be acknowledged. First, the study included only male participants in order to create a homogeneous sample and reduce variability associated with sex-related differences in muscle strength and neuromuscular characteristics. Consequently, the findings cannot be directly generalized to female populations. While these strict inclusion criteria increased internal validity by reducing variability, they may limit the generalizability of the findings to broader populations.

Second, the inclusion of healthy, recreationally active individuals should be interpreted primarily as a deliberate characteristic of the study design, as the aim of the study was to establish baseline behavior of LSI in a non-injured population. However, the findings should not be directly extrapolated to clinical populations, as neuromuscular inhibition, pain, and compensatory movement strategies following injury may influence both absolute strength outcomes and inter-limb symmetry.

Third, the relatively small sample size (n = 22) may have limited the ability to detect subtle differences between testing modalities.

Fourth, the cross-sectional design of the study does not allow conclusions regarding temporal changes or intra-individual variability over time.

Finally, the study did not include functional or performance-based outcomes, which may provide additional context for interpreting inter-limb symmetry in applied or clinical settings. Future studies should therefore investigate the relationship between LSI values and functional performance measures.

Understanding how LSI values and absolute strength outcomes behave across different testing conditions is important for both research and clinical interpretation. If symmetry indices were strongly dependent on testing modality, comparisons across studies or assessments using different protocols would be problematic. In the present study of healthy, recreationally active males, LSI values were comparable across testing modalities, suggesting that inter-limb symmetry can be interpreted consistently when the studied protocols and devices are used consistently.

In contrast, associations between absolute strength outcomes across modalities were only moderate and variable, indicating that these tests capture related but not identical aspects of knee flexor strength. Accordingly, absolute strength values should ideally be monitored using the same testing modality over time. Overall, the current findings suggest that symmetry-based metrics might be more adaptable across different testing conditions than absolute strength outcomes, which may be particularly relevant in rehabilitation settings where equipment availability varies. This distinction between the comparability of symmetry indices and the non-interchangeability of absolute strength outcomes represents a key practical implication of the present findings, particularly in clinical and applied settings where different testing devices are used.

5. Conclusions

LSI values for knee flexor strength were consistent across multiple isometric protocols and isokinetic assessments in healthy, recreationally active males, reflecting baseline symmetry indices in a non-injured population.

In contrast, absolute strength outcomes showed only moderate, variable associations across different testing modalities. Although strong relationships were observed within the same measurement category, cross-modality correlations were weaker, indicating that different testing approaches evaluate related but not interchangeable aspects of knee flexor muscle strength.

These findings emphasize the importance of using consistent testing methods when monitoring changes in absolute strength, while symmetry-based parameters may provide a more transferable measure across various testing conditions. At the same time, LSI values should be interpreted with consideration of individual variability and contextual factors, particularly when applied in clinical or return-to-sport settings.

From a practical perspective, this study supports using LSI as a supplementary metric across different testing methods, while highlighting that absolute strength outcomes should be consistently measured within the same testing system. Future research should explore whether these findings apply to injured populations, females, and longitudinal rehabilitation settings.