Hamstring strength and hip mobility associated with pain and disability in lumbar instability

Abstract

Chronic low back pain (LBP) has been hypothesized to involve lumbar instability, yet the roles of hamstring strength and hip mobility remain underexplored. While earlier studies focused on hamstring tightness, this study examined how these factors are strongly associated with pain and disability in young adults with lumbar instability. A case-control study was conducted on 129 participants (67 symptomatic, 62 asymptomatic) aged 18–35 years. Hamstring strength was assessed using the NordBord dynamometer, while hip mobility was measured using a goniometer. Pain intensity and disability were evaluated using the Numeric Pain Rating Scale and Oswestry Disability Index. Hamstring strength showed a strong negative correlation with pain (r = -0.71, p < 0.001) and disability (r = -0.72, p < 0.001). Regression analysis confirmed hamstring strength and hip mobility as key correlates of LBP severity. Cutoff values were determined for hamstring strength (Female: 178.0 N; Male: 197.5 N), hip internal rotation (Female: 23°; Male: 18°), and hip extension (Female: 21°; Male: 19°). Hamstring strength and hip mobility are key correlates of pain and disability in young adults with clinically defined lumbar instability. Candidate thresholds showed apparent in-sample discrimination and should be interpreted as hypothesis-generating rather than diagnostic.

Introduction

Recent research increasingly examines the relationship between low back pain (LBP) and lumbar spine instability^1–3. Lumbar instability is commonly described as the spine’s inability to maintain controlled displacement under physiological loads, which may contribute to pain and disability^4,5. Building on this framework, growing attention has turned to peripheral factors such as hamstring strength and hip mobility, which may influence symptoms and functional outcomes in young adults with LBP. A growing body of evidence highlights the burden of chronic LBP, with lifetime prevalence estimates as high as 57.8%⁶. Among the many contributing factors, altered muscle strength, flexibility, and overall physical function have received increasing attention^7–10. In particular, impairments in hamstring performance and hip mobility are thought to play an important role in functional limitations and disability, making them relevant clinical targets for investigation^11–13. LBP has multifactorial underpinnings, with disc-degenerative changes and altered trunk-hip muscle performance frequently reported in association with symptoms^14,15. In addition, previous studies have noted links with degenerative changes, muscle morphology, and specific clinical tests^2,16. Together, this literature highlights the importance of considering both structural and functional aspects, while drawing attention to measurable factors such as hamstring strength and hip mobility that may be clinically relevant in young adults with LBP.

Research indicates that lumbar instability can manifest across diverse populations, with risk factors such as age, sex, and specific physical conditions playing a role¹⁷. Targeted exercise programs, particularly core stability and corrective strengthening routines, have been associated with modest but clinically meaningful reductions in excessive lumbar lordosis, although effect sizes remain heterogeneous across trials^18,19. Observational and interventional studies also suggest that individuals with chronic LBP often exhibit reduced hip range of motion (ROM) and that programs emphasizing core stability and hip strength can improve functional abilities compared with controls^20–22. These findings support the clinical relevance of hip mobility and related muscular performance in the context of lumbar stability.

The role of hamstring function in lumbopelvic mechanics has also been discussed. Tight hamstrings may influence pelvic tilt and lumbar posture^23,24, although it remains unclear whether these changes are causal or compensatory. Beyond flexibility, broader hamstring performance, including strength, has been linked to functional outcomes. For example, targeted hamstring exercises have been associated with improvements in balance and stability in individuals with LBP²⁵. These observations support examining hamstring strength as a clinically relevant factor in relation to pain and disability.

Despite growing interest in the relationship between muscle properties and LBP, significant gaps remain. While some studies have examined the effects of hamstring tightness on lumbar mechanics, the role of hamstring strength in relation to pain and disability is less clear. For instance, hypotheses that hamstring shortness could weaken the gluteus maximus and influence lumbopelvic function have not been substantiated with direct empirical data²⁶. Other studies have described compensatory mechanisms of hamstring tightness in relation to lumbar motion, though clear relationships remain uncertain^27,28. In addition, cut-off values for hamstring strength and hip mobility measures have not been established. These gaps highlight the need for further investigation into how hamstring strength and hip ROM relate to pain and disability in young adults. Therefore, the primary aim was to compare hamstring strength and hip range of motion between young adults with non-specific LBP and age-matched asymptomatic controls. The secondary aim was to examine associations of hamstring strength and hip range of motion with pain intensity and disability. The exploratory aim was to derive candidate cut-off values for hamstring strength and hip range of motion that may differentiate symptomatic from asymptomatic individuals, treated as hypothesis generating and not as evidence of diagnostic validity.

Methods

Study design

This case–control study was conducted between July and December 2024 in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of the Wroclaw University of Health and Sport Sciences, Poland (No. 19/2024). Written informed consent was obtained from all participants. Before enrolment, participants were informed about the study procedures and their right to decline or withdraw at any time. Reporting followed the STROBE checklist²⁹.

Participants

Participants were assigned to either the symptomatic group (SYM), comprising patients with low back pain and clinical signs of lumbar instability, or to the control group (CON), consisting of age-matched healthy individuals recruited from the university population. The SYM group included patients attending a private physiotherapy clinic who exhibited positive results on both the Passive Lumbar Extension (PLE) test and the Painful Catch sign^30,31. Lumbar instability was operationally defined as a positive outcome on both tests, and only individuals meeting this criterion were classified as symptomatic. In contrast, the CON group consisted of asymptomatic individuals without a history of low back pain, matched by age.

To be eligible, participants had to be between 18 and 35 years old and report non-specific low back pain that had persisted—or recurred without a pain-free interval—for at least six consecutive weeks. They were required to have experienced an average pain intensity of 3 or higher (on a 0–10 Numeric Pain Rating Scale) during the week prior to screening, and to score at least 12% on the Oswestry Disability Index (ODI), indicating minimal disability across daily living domains such as lifting, sitting, standing, and personal care. Exclusion criteria encompassed, degenerative disc disease symptoms, radiculopathy or radicular pain, severe spinal issues (e.g., metastatic disease, fractures), significant spinal deformities, previous surgeries in the spinal, hip, or knee regions, lower limb muscle conditions (e.g., trauma), tumors, or any other disorders that could affect the results. Additionally, individuals involved in professional sports or currently undergoing treatment were excluded.

Outcome measures and procedures

A licensed physiotherapist with over 12 years of experience in manual therapy, adhering to the International Federation of Orthopaedic Manipulative Physical Therapists (IFOMPT) standards, conducted all assessments throughout the study.

Hamstring strength

All participants underwent hamstring strength evaluation, using the noninvasive NordBord device (VALD Performance, Australia). The NordBord is a validated tool for measuring eccentric knee flexor strength, with studies reporting high reliability (coefficients of variation: 5.8%–8.5%, intraclass correlation: 0.83–0.90)^32–34. It has been validated against traditional isokinetic dynamometers, offering a reliable and accessible option for assessing hamstring function^35,36.

The Nordic hamstring exercise (NHE) was used for assessment, with participants kneeling on a padded board and their ankles secured above the lateral malleolus^37,38. This setup immobilizes the lower limbs, ensuring accurate force measurement. Participants leaned forward in a controlled descent, engaging their hamstring muscles to resist the motion^39,40. The NordBord recorded eccentric force output, enabling precise strength analysis.

For data collection, participants completed a warm-up set, followed by three consecutive maximal bilateral NHE repetitions. They maintained a neutral trunk and hip position, with hands crossed over the chest. Peak force values from each repetition were averaged for statistical comparisons. To minimize bias, no verbal encouragement was provided, and pain levels were assessed using a visual analog scale (VAS), with no reported discomfort.

Test–retest reliability for the NordBord has been reported with intraclass correlation coefficient (ICC) values between 0.83 and 0.90⁴¹. The standard error of measurement is approximately 20 Newtons, and the minimal detectable change at the 95% confidence level is estimated at around 60 Newtons. Currently, no minimal clinically important difference (MCID) has been established for hamstring strength in this population.

Oswestry disability index questionnaire (ODI)

The Polish version of the ODI was used to assess disability⁴². The ODI has ten sections scored 0 to 5. The summed raw score 0 to 50 is converted to a 0 to 100% score by multiplying by two, so 1 raw point equals 2% points. We report ODI on the 0 to 100 scale. In the Polish validation, test–retest reliability was excellent, with ICC of 0.97, standard error of measurement approximately 3.5% points, and minimal detectable change at the 95% level (MDC95) of about 10% points⁴³. For chronic low back pain, we use an absolute MCID of 10% points on the 0 to 100 scale, equivalent to 5 raw points⁴⁴, acknowledging method-dependent variability⁴⁵.

Pain intensity measurement

The Numeric Pain Rating Scale (NPRS Pain) is an 11-point scale used to measure pain intensity in participants from the SYM group during ADLs on the same day as other procedures. The NPRS scale was applied according to the procedure^46,47. The NPRS has excellent reliability, with ICC values reported around 0.96. The standard error of measurement is approximately 0.7 points, and the minimal detectable change at the 95% confidence level is about 2 points⁴⁸. The accepted MCID for chronic low back pain is also 2 points.

Hip range of motion

Each patient’s hip range of motion (internal rotation, external rotation, and extension) was assessed with a goniometer. Hip flexion was not measured since no pathology is typically observed in young individuals. After the examiner demonstrated the movement, each participant performed two un-recorded practice repetitions through their full comfortable range; hip IR, ER and EXT were then measured three times with a handheld goniometer. The mean of the three values was used for analysis. The goniometer’s reliability for measuring hip range of motion is deemed very high, with Cronbach α values reaching a minimum of 0.90⁴⁹. Reported test–retest reliability for goniometric hip range-of-motion measurements ranges from 0.80 to 0.91. The standard error of measurement is typically 2 to 4 degrees, with a minimal detectable change of 6 to 11 degrees depending on the specific motion assessed. No minimal clinically important difference has been established for hip range of motion.

Power analysis

To evaluate the statistical robustness of the primary group comparisons, a post hoc power analysis was conducted using G*Power 3.1 software. The analysis focused on the between-group difference in hamstring strength, which represented the primary outcome of interest. Based on the observed means (SYM: 158.84 ± 38.55 N; CON: 245.23 ± 27.21 N), the calculated standardized effect size (Cohen’s d) was 2.58, indicating a very large effect. Given the sample sizes of 67 (SYM) and 62 (CON) participants, and assuming a two-tailed α level of 0.05, the achieved power (1 – β) exceeded 0.99. This result confirms that the study was highly powered to detect the observed group difference in hamstring strength. Similarly, post hoc analyses of the correlation (r = − 0.72) between hamstring strength and disability and the multiple regression model of pain intensity (R² = 0.64) also demonstrated power levels exceeding 0.99.

Data analysis

Data were analyzed using JASP version 0.19.3 (University of Amsterdam, The Netherlands), and R (R version 4.4.2, The R Foundation for Statistical Computing Platform). Continuous variables were reported as mean and standard deviation (SD). The Shapiro-Wilk test was used to assess the distribution of quantitative data. Independent samples t-tests were applied for normally distributed variables, while non-normally distributed variables were analyzed using Mann-Whitney U tests. Correlation analysis was conducted using Pearson’s correlation for normally distributed variables and Spearman’s rank correlation for non-normally distributed variables.

A multiple linear regression analysis was performed to estimate cross-sectional associations of hamstring strength (and asymmetry) with pain (NPRS) and disability (ODI). These models estimate associations and do not imply prediction or causality. Pairwise correlation tests were used to evaluate the assumption of a linear relationship between the independent and outcome variables. Receiver operating characteristic (ROC) analyses were conducted to derive in-sample candidate thresholds for hamstring strength, hip internal rotation, and hip extension that separated symptomatic from asymptomatic participants. The area under the ROC curve (AUC) quantified discrimination within this dataset, and Youden’s J was used to identify candidate cutoff values. These analyses are hypothesis-generating, no external reference standard was used, and the thresholds are not clinically validated. Accordingly, ROC analyses are presented as apparent in-sample discrimination only.

Structural equation modeling (SEM) was used to estimate contemporaneous associations among clinically measured variables and outcomes in lumbar instability. The model included bilateral hamstring strength (Newtons), hip mobility (internal rotation, external rotation, extension; degrees), pain intensity (Numerical Pain Rating Scale, 0–10), and functional disability (Oswestry Disability Index). We specified paths (associations) from hamstring strength and hip mobility to pain and disability, and from pain to disability. Standardized coefficients (β) were interpreted as cross-sectional associations; where relevant, we report model-implied indirect associations via pain intensity (i.e., products of standardized paths). No causal or mediated-effect inferences were drawn.

Correlation, regression, and ROC analyses were conducted in the full sample (n = 129; symptomatic and asymptomatic) to examine continuous relationships between physical measures (hamstring strength, hip mobility) and patient-reported outcomes (pain, disability). SEM was performed in the symptomatic subgroup (n = 67) to enhance model stability and clinical relevance. Statistical significance was set at α < 0.05.

Results

Participants characteristics

The study comprised 129 participants, including 65 females (50.4%) and 64 males (49.6%). The symptomatic group consisted of 67 individuals, while the control 62. The descriptive statistics of the study participants are summarized in Table 1. The mean age of participants was 27.57 ± 4.64 years (range: 18–35 years), with a mean height of 173.02 ± 9.34 cm and weight of 73.21 ± 12.70 kg. The mean BMI was 24.41 ± 3.44 kg/m², indicating that the majority of participants were within the normal to overweight range. Regarding physical measures, the mean hamstring strength was 200.36 ± 54.74 N, with values ranging from 95.5 N to 321.5 N. Hip mobility measurements showed mean internal rotation of 23.08 ± 9.76 degrees and mean hip extension of 20.00 ± 5.37 degrees. Clinical outcomes revealed a mean pain intensity (NPRS) of 3.57 ± 2.97 points on a 0–10 scale, while the mean disability score (ODI) was 33.13 ± 26.63, indicating mild to moderate functional limitations in the study population.

Table 1.

Participants characteristics.

Variable	Total (n = 129)	SYM (n = 67)	CON (n = 62)	p value
Age	27.57 ± 4.64	27.30 ± 4.67	27.87 ± 4.63	0.48
Height [cm]	173.02 ± 9.35	170.18 ± 8.09	176.08 ± 9.70	< 0.001
Body mass [kg]	73.21 ± 12.70	66.72 ± 9.06	80.23 ± 12.38	< 0.001
BMI	24.41 ± 3.44	23.07 ± 2.92	25.85 ± 3.39	< 0.001
Hamstrings L [N]	203.64 ± 56.02	161.66 ± 38.09	249.02 ± 31.41	< 0.001
Hamstrings R [N]	197.08 ± 56.02	156.03 ± 44.03	241.44 ± 25.23	< 0.001
Hamstrings asymmetry	2.59 ± 13.18	2.61 ± 17.15	2.56 ± 6.80	0.21
Hamstrings mean [N]	200.36 ± 54.74	158.84 ± 38.55	245.23 ± 27.21	< 0.001
Hamstrings [N/kg]	2.75 ± 0.68	2.40 ± 0.53	3.13 ± 0.61	< 0.001
Hip IR [°]	23.08 ± 9.76	16.43 ± 6.58	30.26 ± 7.23	< 0.001
Hip ER [°]	36.20 ± 8.74	37.33 ± 7.98	34.98 ± 9.41	0.12
Hip EXT [°]	20.00 ± 5.37	15.81 ± 3.00	24.53 ± 3.25	< 0.001
NPRS [0–10]	3.57 ± 2.97	6.02 ± 1.95	0.92 ± 0.86	< 0.001
ODI	33.13 ± 26.63	55.25 ± 17.61	9.22 ± 5.85	< 0.001

As shown in Table 1, between-group comparisons (SYM vs. CON) revealed significant differences in hamstring strength and hip mobility, with lower values observed in the symptomatic group. These comparisons provide the basis for subsequent within-group analyses in the SYM cohort.

BMI: Body Mass Index; L: left; R: Right; IR: Internal Rotation; ER: External Rotation; EXT: extension; NPRS: Numeric Pain Rating Scale; ODI: Oswestry Disability Index; Values presented as mean ± Standard Deviation; p value as a result of U Mann-Whitney test.

Correlation analysis

Correlation analysis (Fig. 1) revealed strong associations between strength parameters and clinical outcomes in individuals with low back pain. Hamstring strength demonstrated the strongest negative correlations with both pain intensity (r = −0.71, p < 0.001) and disability (r = −0.72, p < 0.001), indicating that lower hamstring strength was associated with higher levels of pain and disability. Similarly, hip extension mobility showed strong negative correlations with both pain intensity (r = −0.71, p < 0.001) and disability (r = −0.69, p < 0.001), while hip internal rotation also demonstrated substantial negative correlations with pain intensity (r = −0.59, p < 0.001) and disability (r = −0.64, p < 0.001). The analysis also revealed significant intercorrelations among physical parameters, with moderate positive correlations between hamstring strength and both hip internal rotation (r = 0.53, p < 0.001) and hip extension (r = 0.61, p < 0.001), suggesting a potential interaction between muscle strength and joint mobility. Additionally, a strong positive correlation was observed between pain intensity and disability (r = 0.82, p < 0.001), supporting the close relationship between these clinical outcomes. These findings collectively suggest that impairments in hamstring strength and hip mobility are strongly associated with both pain and functional limitations in individuals with low back pain, highlighting the potential importance of these parameters in clinical assessment and treatment planning.

Fig. 1.

This heatmap visualization represents the strength and direction of relationships between variables, with darker blue colors indicating stronger negative correlations and darker red colors indicating stronger positive correlations. Note: Analyses shown were conducted on the full sample (n = 129).

Physical measures across pain categories

The distribution of physical measures across different pain severity categories revealed distinct patterns, as illustrated in Fig. 2. Participants were stratified into three pain categories based on their NPRS scores: mild (0–3), moderate (4–6), and severe (7–10). Analysis showed that individuals with severe pain had significantly lower hamstring strength (mean difference: 84.72 N, p < 0.001) compared to those with mild pain. Similarly, hip mobility measures demonstrated a gradient across pain categories, with the severe pain group showing reduced hip internal rotation (mean difference: 14.02 degrees, p < 0.001) and hip extension (mean difference: 8.25 degrees, p < 0.001) compared to the mild pain group. These findings strongly support the hypothesis that physical impairments are more pronounced in individuals experiencing greater pain intensity.

Fig. 2.

Distribution of physical measures across pain intensity (NPRS) and disability (ODI) categories. Note: Analyses shown were conducted on the full sample (n = 129).

The boxplots in Fig. 2 illustrate the relationships between physical parameters and clinical outcomes, specifically pain intensity and disability categories. The left column of the figure shows the relationships with pain intensity categories (Mild: NPRS ≤ 3, Moderate: NPRS 4–7, Severe: NPRS > 7), while the right column depicts the relationships with disability categories (Minimal: ODI ≤ 20%, Moderate: ODI 21–40%, Severe: ODI > 40%). Statistical analysis revealed significant differences across both pain and disability categories for all physical measures. For hamstring strength, significant differences were observed across pain categories (F = 64.58, p < 0.001), with mean values decreasing from mild (236.39 N) to moderate (162.12 N) and severe (150.85 N) pain. A similar pattern was found across disability categories (F = 133.35, p < 0.001), with means declining from minimal (246.35 N) to moderate (157.87 N) and severe (156.05 N) disability. Hip internal rotation also showed significant differences across pain categories (F = 36.62, p < 0.001), with means decreasing from mild (28.49°) to moderate (17.79°) and severe (14.47°) pain, and across disability categories (F = 59.66, p < 0.001), with means declining from minimal (29.95°) to moderate (18.93°) and severe (15.80°) disability. Similarly, hip extension exhibited strong differences across pain categories (F = 107.32, p < 0.001), with means decreasing from mild (23.96°) to moderate (15.35°) and severe (15.71°) pain, and across disability categories (F = 110.79, p < 0.001), with reductions from minimal (24.37°) to moderate (15.33°) and severe (15.98°) disability. These findings demonstrate that participants with higher pain intensity and greater disability consistently exhibited lower hamstring strength and reduced hip mobility. The relationships were statistically significant for all physical parameters, with particularly strong effects observed for hamstring strength and hip extension. The parallel patterns observed across both pain and disability categories suggest that physical impairments are strongly associated with both symptoms and functional limitations.

β: unstandardized regression coefficient; SE: standard error; CI: confidence interval; NPRS: Numerical Pain Rating Scale; ODI: Oswestry Disability Index. Analyses were cross-sectional (n = 129). Coefficients reflect in-sample associations and do not imply prediction or causality.

Table 2 presents the results of multiple regression analyses examining associations of physical measures with pain intensity (NPRS) and disability (ODI). For pain intensity, the model explained 64.4% of the variance (R² = 0.64, p < 0.001), with significant negative explanatory variables including hamstring strength (β = −0.02, p < 0.001), hip internal rotation (β = −0.06, p = 0.008), and hip extension (β = −0.208, p < 0.001). Similarly, for disability, the model accounted for 66.2% of the variance (R² = 0.66, p < 0.001), with hamstring strength (β = −0.002, p < 0.001), hip internal rotation (β = −0.01, p < 0.001), and hip extension (β = −0.02, p < 0.001) showing significant inverse associations. These findings highlight strong cross-sectional associations of reduced hamstring strength and hip mobility with both pain and disability, with hip extension showing the largest effect sizes in both models.

Table 2.

Multiple linear regression: variables associated with pain intensity and disability.

Independent variables	β	SE	t	p	95% CI	R²	F
NPRS						0.64	75.5
Hamstring strength (N)	−0.02	0.004	−5.408	< 0.001	[−0.028, −0.013]
Hip internal rotation (°)	−0.06	0.02	−2.692	0.008	[−0.095, −0.014]
Hip extension (°)	−0.21	0.04	−5.27	< 0.001	[−0.287, −0.130]
ODI						0.66	81.65
Hamstring strength (N)	−0.002	0.01	−5.481	< 0.001	[−0.003, −0.001]
Hip internal rotation (°)	−0.01	0.002	−4.135	< 0.001	[−0.011, −0.004]
Hip extension (°)	−0.02	0.003	−4.536	< 0.001	[−0.022, −0.009]

Relationship between physical measures and pain

Further analysis of the relationship between physical measures and pain intensity revealed clear trends, as shown in Fig. 3. The scatterplot demonstrates a significant inverse relationship between hamstring strength and pain intensity, with a correlation coefficient of r = −0.38 (p < 0.001). Similarly, hip extension showed a strong negative correlation with pain (r = −0.38, p < 0.001). The relationship remained significant even after adjusting for potential confounders such as age and BMI (adjusted R² = 0.34, p < 0.001).

Fig. 3.

Relationship between hamstring strength, hip extension, and pain intensity. This scatterplot illustrates the complex interplay between physical measures and pain. The color gradient represents pain intensity levels, with darker colors indicating higher pain scores. The surface plot visualizes the combined association of hamstring strength and hip extension with pain intensity, highlighting the importance of both factors in pain presentation. Note: Analyses shown were conducted on the full sample (n = 129).

SEM results

To better understand the direct and indirect relationships between physical factors and clinical outcomes, a comprehensive SEM analysis was conducted. The results, visualized in Fig. 4, revealed significant associations from physical measures to both pain and disability. In the cross-sectional regression model, hamstring strength showed the largest magnitude independent association with pain intensity (β = −0.38, p < 0.001), with a comparable association for hip extension (β = −0.38, p < 0.001) and hip internal rotation (β = −0.18, p = 0.008). The model also demonstrated significant direct effects on disability, with pain intensity showing the strongest relationship (β = 0.47, p < 0.001). The total effects on disability, which include both direct and indirect associations, were substantial for all physical measures: hamstring strength (−0.38), hip internal rotation (−0.27), and hip extension (−0.32). The model’s fit indices indicated good overall fit (CFI = 0.97, RMSEA = 0.06), supporting the validity of the proposed relationships.

Fig. 4.

SEM analysis model with standardized coefficients. This comprehensive diagram illustrates the complex network of relationships between physical factors, pain intensity, and disability. Arrows represent directional relationships, with standardized path coefficients indicating the strength and direction of each relationship. Solid lines represent statistically significant paths (p < 0.05), while the thickness of the lines corresponds to the magnitude of the relationship. Note: Analyses shown were conducted on the SYM group only sample (n = 67).

AUC: Area Under Curve; IR: Internal Rotation; EXT: Extension. Note: Analyses shown were conducted on the full sample (n = 129). Apparent performance, no external validation; thresholds are candidate values only.

Table 3 presents apparent in-sample discrimination for symptom status using three physical measures, stratified by sex, with candidate threshold values. For hamstrings strength, the general cutoff was established at 197.5 N (sensitivity = 0.91, specificity = 0.97, AUC = 0.96), with females showing a lower optimal threshold at 178.0 N and males matching the general cutoff at 197.5 N. Hip internal rotation demonstrated varying thresholds, with a general cutoff of 23.0° matching the female-specific value, while males showed a lower threshold at 18.0°. Hip extension revealed a general cutoff of 19.0° (matching the male-specific value), while females demonstrated better discrimination at 21.0°.

Table 3.

In-sample discrimination (ROC) for symptom status.

Measure	Cutoff	Sensitivity	Specificity	AUC	Youden’s J	p
Total
Hamstrings Strength	197.5 N	0.91	0.97	0.96	0.88	< 0.001
Hip IR	23.0°	0.87	0.77	0.91	0.64	0.30
Hip Extension	19.0°	0.82	1.00	0.98	0.82	0.11
Female
Hamstrings strength	178.0	0.95	1.00	0.99	0.95
Hip IR [°]	23.0	0.86	0.87	0.95	0.73
Hip EXT [°]	21.0	0.98	0.87	0.98	0.85
Male
Hamstrings strength	197.5	0.84	1.00	0.91	0.84
Hip IR [°]	18.0	0.68	1.00	0.91	0.68
Hip EXT [°]	19.0	0.88	1.00	0.98	0.88

Figure 5 presents the ROC curves stratified by sex for each physical parameter. The curves indicate apparent in-sample discrimination (AUCs > 0.90) across measures. Hamstrings strength showed the highest discriminative ability in females (AUC = 0.99) with a nearly perfect ROC curve shape, while maintaining excellent accuracy in males (AUC = 0.91). Hip extension demonstrated apparent in-sample discrimination in both sexes (AUC = 0.98 for females, AUC = 0.98 for males), with curves showing steep initial slopes indicating optimal sensitivity-specificity trade-offs. Hip internal rotation, while still showing excellent discrimination (AUC = 0.95 for females, AUC = 0.91 for males), demonstrated slightly lower but clinically meaningful diagnostic accuracy. Notable sex-specific patterns emerged: males consistently showed perfect specificity (1.00) across measures, while females demonstrated more balanced sensitivity-specificity trade-offs. These patterns suggest that sex-specific candidate cutoffs may provide more precise reference points for future hypothesis-generating work.

Fig. 5.

ROC curves for hamstring strength and hip range of motion. Note: Analyses shown were conducted on the full sample (n = 129). Apparent in-sample performance; no external validation.

Discussion

This study compared hamstring strength and hip ROM in young adults with and without LBP. Individuals with LBP exhibited lower hamstring strength and reduced hip mobility, and these measures were significantly associated with higher pain and disability. These are cross-sectional, in-sample findings and do not establish diagnostic validity or causality. Together, the results support hamstring strength and hip mobility as clinically relevant correlates of pain and disability. Hamstring strength showed a strong inverse association with both outcomes, and hip extension and internal rotation discriminated LBP status in ROC analyses. These observations align with prior work implicating muscle performance and hip biomechanics in LBP⁵⁰.

The relationship between hamstring function and lumbopelvic mechanics is well established, though most prior studies have focused on flexibility rather than strength. The present findings extend this evidence by showing that reduced hamstring strength is significantly associated with higher pain and disability, providing complementary insight beyond flexibility measures. Prior studies on hamstring flexibility and lumbopelvic posture report mixed associations with alignment and motion, while links to pain are inconsistent; our results complement this literature by focusing on hamstring strength and hip ROM and their associations with pain and disability^51–53.

Hamstring activity and length have been linked to changes in pelvic orientation and lumbar posture. For example, Park and Jung (2021) reported that individuals with shortened hamstrings showed greater anterior pelvic tilt and altered spinal curvatures⁵⁴, while Han et al. (2016) found associations between limited hamstring length and increased posterior pelvic tilt, affecting sagittal alignment^55,56. Such findings highlight the influence of hamstring properties on lumbopelvic posture, although their direct relationship to pain outcomes remains debated. In this context, our results add to the literature by focusing on hamstring strength as a factor associated with pain and disability, complementing previous flexibility-focused research. Collectively, flexibility studies suggest links between hamstring properties and lumbopelvic posture, although direct relationships with pain remain debated. Our findings complement this literature by showing that hamstring strength – alongside hip ROM – was associated with pain and disability in young adults.

The concept of hip–spine syndrome highlights the close interaction between hip and lumbar function. Limitations in hip motion have been associated with compensatory movement patterns in the lumbar spine, which may influence symptoms of LBP. Redmond et al., in a systematic review, reported frequent alterations in hip ROM among patients with LBP and noted that treatments addressing hip pathology were often accompanied by improvements in back pain⁵⁷. Similarly, Kim et al. found that hip mobilization exercises reduced symptoms in individuals with chronic non-specific LBP, underscoring the potential clinical relevance of hip impairments⁵⁸. Supporting evidence also shows that patients with LBP commonly demonstrate reduced hip internal rotation and hip abductor weakness⁵⁹. Nishimura and Miyachi observed that increased lumbar mobility in the presence of hip restriction was associated with pain sensitivity⁶⁰, and Khoury et al. described abnormal hip conditions as leading to compensatory pelvic and lumbar adaptations⁶¹. Taken together, these studies reinforce the importance of considering hip mobility when evaluating LBP. In line with this evidence, the present results showed that reduced hip extension and internal rotation were associated with pain and disability in young adults.

Clinical implications

This study provides valuable insights for clinicians, highlighting the importance of assessing and addressing hamstring strength and hip mobility in individuals with LBP. Given the observed associations between these physical measures and clinical outcomes, rehabilitation protocols may benefit from emphasizing strength and mobility-based exercises rather than relying on passive stretching alone. By incorporating targeted strength training and hip mobilization strategies, clinicians can develop more comprehensive treatment plans tailored to young adults with LBP.

Limitations and future research

While this study provides valuable insights, several limitations should be acknowledged. The cross-sectional design precludes causal inferences, and the sample characteristics may limit generalizability. Hamstring weakness and limited hip mobility may either contribute to, or arise as consequences of, pain and disability in individuals with lumbar instability; in this study, lumbar instability was operationally defined using clinical screening tests (PLE/painful catch), and no imaging or other gold-standard reference was used. The sample consisted of volunteers from a physiotherapy clinic, which may not represent the general population. Identifying reliable strength and ROM threshold values would require larger and more diverse cohorts, and ROC-derived cut-offs should be regarded as exploratory rather than validated diagnostic criteria. Minimal clinically important differences (MCIDs) for hamstring strength (NordBord) and goniometric hip ROM have not been established; therefore, we emphasize reliability indices (SEM/MDC) and interpret thresholds as exploratory. Future research should use longitudinal designs to clarify causal relationships, evaluate the effectiveness of interventions targeting hamstring strength and hip mobility, and consider additional variables (e.g., core strength, flexibility). The ODI score may also be analyzed in more detail when stratified into specific sections. Reported links between hamstrings and lordosis largely stem from correlational studies or multifaceted interventions; future work should incorporate more specific assessments, including imaging, to clarify these relationships.

Conclusion

In summary, this study suggests that hamstring strength and hip mobility have a strong association with pain and disability in young adults with lumbar instability, where hamstring strength, hip mobility, and lumbar instability were operationally defined using clinical screening tests. These findings suggest the potential value of incorporating these measures into clinical assessment and rehabilitation planning, while acknowledging that causal inferences cannot be drawn from this cross-sectional design. Future studies should aim to confirm clinically relevant cut-off values for hamstring strength and hip mobility in larger and more diverse populations, with particular attention to external validation and the exploratory nature of thresholds.

Author contributions

Conceptualization: T.K., Methodology: T.K. and B.C., Validation: A.D-B., Formal analysis: T.K. and A.D-B., Writing - Original Draft: T.K. and B.C., Writing - Review & Editing: A.D-B., Supervision: B.C., Project administration: T.K. All authors reviewed the manuscript.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.