Core muscle endurance and psychosocial factors affecting functional mobility in chronic low back pain: cross-sectional results

Ethics, setting, and design

This cross-sectional study was performed from April 2023 to March 2024 at the Department of Rehabilitation Medicine in a tertiary care hospital specializing in musculoskeletal disorders in Abha, Saudi Arabia. The Research Ethics Committee of KKU, DSR (REC# 234-2024) gave ethical clearance before the study. All individuals provided their written informed consent before participation. All study procedures were conducted in accordance with the ethical principles outlined in the Declaration of Helsinki.

Participants

Participants were selected using purposive sampling to ensure the sample represented the study’s target population. This sampling method was employed to ensure that all participants met the diagnostic criteria for CLBP and were functionally able to perform the endurance and mobility tasks, which would not have been feasible under a random sampling approach due to expected heterogeneity in eligibility. Participants were recruited via direct provider referrals and informational posters in the outpatient department. The diagnosis of CLBP was made by clinical criteria (pain localized to the region between the lower rib margin and gluteal folds for more than three months without a specific identifiable pathology, such as fracture, infection, or malignancy) (Mohd Isa et al., 2022). Participants were enrolled upon referral from a consulting physician or physiotherapist and were screened using inclusion and exclusion criteria by the research team. Body mass index (BMI) was calculated using weight and height measurements obtained via a calibrated digital scale and stadiometer. Duration of pain was assessed through participant self-report. Pain intensity was measured using a 10-point Visual Analogue Scale (VAS), and disability was assessed using the Oswestry Disability Index (ODI). Inclusion criteria were age 18 to 65 years, clinically diagnosed CLBP, BMI of 18 to 35 kg/m², and ability to perform basic functional mobility tasks, including walking and transitioning from sitting to standing. Participants were eligible if they were medically stable and could provide written informed consent. Basic functional abilities such as walking and sit-to-stand were confirmed during the screening process by direct observation from a licensed physiotherapist. Medically stable was defined as free from acute illness, hemodynamically stable, and not undergoing any ongoing treatment that would contraindicate physical assessments. Exclusion criteria were as follows: neurological disorders (radiculopathy, myelopathy), spine or abdominal surgery in the previous 6 months, significant musculoskeletal injury in the lower limbs, or any other comorbidity that could hinder functional mobility (cardiovascular disease or advanced osteoarthritis). Those who were cognitively impaired or had active psychiatric conditions that could impede their capacity for completing the assessments were also excluded from the study.

Participant use of pain medications was recorded during intake and considered in subgroup sensitivity analyses to account for its potential influence on pain perception and mobility performance.

Anthropometric and clinical outcomes

Anthropometric characteristics included BMI (kg/m²), calculated from self-reported height and weight. Anthropometric characteristics included BMI (kg/m²), calculated from height and weight measurements obtained via a calibrated digital scale and stadiometer. Clinical outcomes assessed were pain intensity using the VAS (0–10 cm), disability using the ODI (%), and pain duration in months. These measures were collected during the initial assessment phase to characterize the sample and explore associations with functional mobility outcomes.

Sample size estimation

The sample size was calculated using G*Power software (version 3.1.9.7) based on a moderate effect size (r = 0.3) derived from prior research (Fallahasady et al., 2022), who examined the relationship between core muscle endurance and functional performance in a musculoskeletal population with similar demographics and methodological design, with a significance level of 0.05 and a power of 0.80. A 10% dropout rate was accounted for, resulting in a target sample size of 136 participants.

Core muscle endurance

Core muscle endurance was assessed using three validated static endurance tests: the plank endurance test (Choi, Kim & Cynn, 2021), the side-bridge endurance test (Hajmirzaei Tafreshi, Minoonejad & Seidi, 2023), and the back extensor endurance test (Clael et al., 2021). All measurements were taken under optimal clinical conditions in a controlled laboratory setting. Trained physiotherapists supervised all assessments, providing detailed verbal instructions to participants and monitoring their movements to ensure proper execution. Participants performed a brief warm-up prior to testing, including light stretching and low-intensity movements for muscle preparation. The warm-up lasted about five minutes and involved dynamic trunk stretches, arm swings, heel raises, and low-intensity walking. The physiotherapist demonstrated each test and allowed participants a moment to briefly confirm their understanding of the protocol to ensure proper performance.

In the plank endurance test (Choi, Kim & Cynn, 2021), participants were instructed to lie prone on a mat with their elbows directly beneath their shoulders and their forearms flat on the ground (Fig. 1). The feet were positioned at hip-width, with the toes flexed to support the body. Participants were then directed to lift their bodies off the mat, maintaining a straight line from head to heels while engaging their core muscles. The physiotherapist emphasized the importance of keeping the hips level and avoiding sagging or arching the lower back. Timing started when the participant assumed the correct plank position and ended when the position was no longer held or the participant voluntarily ended the test. Timing was recorded using a digital stopwatch accurate to 0.01 s.

The side-bridge endurance test (Hajmirzaei Tafreshi, Minoonejad & Seidi, 2023) assessed lateral core muscle endurance. Participants were instructed to assume a side-lying position, with legs extended and aligned, the elbow of the lower arm positioned directly beneath the shoulder, and the forearm perpendicular to the body (Fig. 2). The opposite arm rested alongside the torso. The physiotherapist then directed participants to lift their hips off the mat, create a straight line from head to feet, and engage their oblique muscles while preventing any rotation or twisting of the torso. Timing started when the correct side-bridge position was achieved and ended when the position could no longer be maintained or the participant voluntarily stopped the test. The average of the two times from both sides was calculated for analysis.

The back extensor endurance test was employed to evaluate the endurance of the back extensor muscles (Clael et al., 2021). The participants were prone on a plinth, with their lower bodies secured to an adjustable padded bench (Fig. 3). The iliac crest was aligned with the edge of the support platform, and the lower limbs were bound with adjustable straps or restrained by an attendant. The thumbs-up gesture was designated for participants instructed to cross their arms over their chests. Subsequently, the physiotherapist demonstrated the proper alignment for elevating the trunk from the mat, ensuring the upper body remained parallel to the plinth and perpendicular to the lower body. Participants were given a brief time to assume the correct position, and support was provided as needed to minimize premature fatigue before timing began. Participants were additionally directed to engage their back-extensor muscles while preventing cervical flexion. The timing commenced upon achieving the horizontal posture and concluded when the individual could no longer sustain the position or voluntarily ended the test. Timing was recorded using the same digital stopwatch method as the other endurance tests. The duration was quantified in seconds. Regardless of the tests conducted, standard verbal reinforcement was provided to maintain the position for as long as feasible. Safety protocols were used, including halting participants upon reporting pain or exhibiting indicators of discomfort or perilous postural deviations. Additionally, for each test, aggregated outputs were compiled on a standardized datasheet, ensuring uniformity in the output format utilized by all participants.

Functional mobility

Two widely recognized and standardized assessments were employed to evaluate functional mobility: the TUG and the 5TSTS (Trombini-Souza et al., 2023). The TUG necessitated that participants transition from a seated to a standing position, traverse a distance of three meters, execute a turn, and return to a seated position; the total duration to complete the task was recorded in seconds (Trombini-Souza et al., 2023). The 5TSTS test required participants to rise from a seated posture and thereafter return to sitting for five repeats, with the total duration measured in seconds. The assessments were performed on level, non-slippery surfaces, and participants were provided with verbal instructions and demonstrations before the testing. A standard armless chair, 43 cm in height, was used, positioned against a wall to prevent displacement during the test. To minimize variability, the measures were conducted twice, and the mean score was utilized for analysis. Timing was recorded using a digital stopwatch. To ensure consistency across assessors, two trained physiotherapists independently scored the timing-based tests (TUG and 5TSTS). Inter-rater reliability was evaluated on a random 15% subsample, with intra-class correlation coefficients exceeding 0.90 for all mobility measures, indicating excellent agreement.

Psychological factors

Psychosocial variables were assessed via validated self-report measures. Pain catastrophizing was assessed via the pain catastrophizing scale (PCS), which assesses cognitive and emotional aspects of catastrophizing/risk factors, including rumination, magnification, and helplessness, with higher scores indicative of greater catastrophizing (Franchignoni et al., 2022). The PCS was originally developed and validated by Sullivan, Bishop & Pivik (1995), demonstrating strong internal consistency and predictive validity in chronic pain populations. For subgroup analyses, high-risk participants were defined as those with PCS scores ≥ 24, a threshold supported by prior validation studies as indicating clinically significant catastrophizing (Sullivan, Bishop & Pivik, 1995). The fear-avoidance beliefs questionnaire (FABQ) was employed to assess fear-avoidance beliefs related to physical activity and work activity, with higher scores representing greater avoidance tendencies (Franchignoni et al., 2022). The FABQ was originally introduced by Waddell et al. (1993) as a measure of patients’ beliefs about how physical activity and work may influence their pain, with established reliability and construct validity. High-risk categorization for FABQ followed previously reported thresholds, with scores ≥ 20 indicating elevated fear-avoidant beliefs of clinical importance (Waddell et al., 1993). The PCS and FABQ were administered in English using their validated full-length versions. Participants who required clarification received standardized explanations in their native language, provided by bilingual research staff trained in questionnaire administration. The questionnaires were administered in a standardized, distraction-free environment, with a trained physiotherapist present to clarify instructions when necessary. Participants completed the forms independently to minimize interviewer bias.

Data analysis

Statistical analysis was conducted utilizing SPSS (version 24) and R software (version 4.2). We employed the Shapiro–Wilk test to assess the normality of the data distribution and verified its normality. Consequently, parametric statistical methods may be utilized. Regression assumptions were systematically verified. Linearity and homoscedasticity were assessed through inspection of standardized residual plots, and normality of residuals was confirmed via the Shapiro–Wilk test. Independence of errors was evaluated using the Durbin–Watson statistic. Multicollinearity was assessed with variance inflation factors (VIF), all of which were <2, indicating negligible collinearity even after inclusion of interaction terms. To strengthen model validity, hierarchical regression analyses additionally adjusted for age, sex, BMI, and pain duration as covariates. Participant demographics, core muscle endurance metrics, functional mobility results, and psychosocial factors were reported by descriptive statistics, encompassing means and standard deviations. Categorical variables, including sex, were summarized using frequency counts and percentages. Pearson’s correlation coefficient was employed to analyze the association between core muscular endurance (plank, side-bridge, and back extensor endurance) and functional mobility metrics (TUG test and 5TSST). The independent effects of core muscle endurance and psychosocial factors, assessed by the PCS and FABQ, on functional mobility were analyzed by hierarchical multiple regressions. Given that six correlation tests were performed (three endurance measures × two mobility outcomes), a Bonferroni adjustment was applied to reduce the risk of Type I error. The adjusted significance threshold was set at p < 0.008. In the hierarchical regression analysis, core muscle endurance measures were entered in the first step, followed by psychosocial factors (PCS and FABQ) in the second step, and interaction terms (core muscle endurance × PCS and core muscle endurance × FABQ) in the final step. This approach allowed us to assess the independent and interactive effects of core muscle endurance and psychosocial factors on functional mobility. In the final step, interaction terms were incorporated to investigate the moderating impacts of psychological variables. An independent samples t-test was conducted to assess functional mobility outcomes across high and low-psychosocial risk groups, followed by a one-way ANOVA to verify interaction effects.

Clinical significance

The findings show that core muscle endurance is the main factor influencing functional performance, emphasizing that core strengthening exercises should be prioritized in rehabilitation programs (Clael et al., 2021). Additionally, psychological barriers such as pain catastrophizing and fear-avoidance attitudes play a significant role in the condition (Ho et al., 2022). This study demonstrates that these factors are connected to mobility, regardless of physical traits, and that they interact with physical characteristics to affect outcomes, supporting integrated patient care. Combining physiological training with cognitive-behavioral therapies may improve functional results and boost quality of life for individuals with CLBP. This information can assist physicians in developing comprehensive treatment plans for this group. Since this was a cross-sectional study, causality cannot be established. Future clinical trials are needed to evaluate the causal effects of combined physical and psychological interventions on both functional and psychological outcomes in people with CLBP.

Limitations of the study

The cross-sectional study design inherently limits the ability to explore causal relationships between core muscle endurance and functional mobility. Longitudinal studies are necessary to examine these correlations over time and determine the direction of these associations. Additionally, psychosocial factors were evaluated through self-reported measures, which are vulnerable to response bias. Future research could include objective assessments of psychological states or real-time emotional monitoring. Furthermore, although the sample size was adequate for the studies conducted, the results may not be generalizable to populations with different levels of physical activity, comorbidities, or age groups. Including a more diverse demographic and clinical sample would enhance the external validity of the findings. This study did not consider the potential effects of biomechanical or environmental factors, such as gait patterns or occupational demands, on functional mobility. Future research should explore these factors and assess the effectiveness of combined physical and psychosocial interventions on mobility in patients with CLBP. Additional confounders, like pain medication use and environmental influences (e.g., occupational load), were not fully controlled and should be addressed in future longitudinal studies.