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Chronic low back pain (cLBP) is a multifactorial condition, including paraspinal muscle asymmetry. Understanding the relationship between muscle asymmetry and cLBP and how this varies by sex is crucial for targeted interventions. From January 2022 to December 2023, 250 participants were enrolled: 117 without back pain (no-BP) (61 females, 56 males) and 133 with cLBP (69 females, 64 males). MRI assessed the cross-sectional area (CSA), functional CSA (FCSA), and fat infiltration (FI) of the paraspinal muscles, including the psoas major (PM), quadratus lumborum (QL), erector spinae (ES), and multifidus (MF), at all lumbar levels. Asymmetry indices were calculated for CSA (CAI), FCSA (FCAI), and FI (FIAI). Data were analyzed using multiple linear and logistic regression. Muscle asymmetries were observed in both sexes and groups, with significant differences in the CSA and FCSA of the QL in males and notable FI asymmetries in the cLBP group. In women with cLBP, higher ES CAI and FCAI at L5/S1 (p < 0.05) and lower PM FCAI at L4/5 (OR 0.869, 95% CI 1.008–1.150, p = 0.03) were observed. FIAI was significantly higher in cLBP for ES at L2/3 and PM, MF, and ES at L4/5 (p < 0.05). Men with cLBP had lower PM CAI and FCAI at L1/2 and L4/5 (p < 0.05), with varying FIAI levels. FIAI correlated with cLBP duration and intensity in both sexes (p < 0.05). Paraspinal muscle asymmetry, particularly in fat infiltration, is associated with cLBP and varies by sex. These findings support sex-specific approaches to managing cLBP. [ABSTRACT FROM AUTHOR]

Background: The flexion relaxation phenomenon (FRP) is characterized by suddenly reduced paraspinal muscle activity during full flexion. Previous studies showed significant differences in FRP and flexion angles in chronic low back pain (cLBP) patients compared to individuals without back pain (no-BP). However, the relationship between FRP and flexion angles remains insufficiently understood in older populations. Thus, this study investigated the relationship between FRP and flexion angles concerning to the age and presence of cLBP., Methods: Forty no-BP subjects (20m/20f; mean age 41.5 years) and thirty-eight cLBP patients (19m/19f; mean age 43.52 years) performed maximum full upper body flexion task. Electromyographic (EMG) measurements were conducted to assess the activity of lumbar erector spinae (ESL), thoracic erector spinae (EST), and multifidus (MF). Lumbar, thoracic, and pelvic angles at the onset (OnsetL/T/P) and offset of the FRP (OffsetL/T/P) and maximum trunk inclination (MaxL/T/P) were calculated. The FRP was evaluated using a flexion relaxation ratio (FRR)., Results: cLBP patients showed smaller FRR in MF and right ESL compared to no-BP individuals ( p < 0.05), while no differences were found in flexion angles between two groups. Subjects over 40 showed smaller FRR in MF and ESL, and smaller flexion angles on OffsetL and MaxL ( p < 0.05). Age-related analysis in the cLBP group revealed that patients over 40, compared to younger ones, had smaller FRR in MF and ESL, and smaller values in all thoracic and lumbar flexion angles ( p < 0.05). While in no-BP group, significant larger flexion angles in OnsetL and OffsetT ( p < 0.05) were observed in participants over 40. Pain-related analysis in the older group revealed that the cLBP patients, compared to no-BP individuals, had smaller FRR in right MF and right ESL, and smaller values in all lumbar and thoracic flexion angles ( p < 0.05), while in younger group, there were no significant pain-related differences in FRR, with larger values in all lumbar flexion angles ( p < 0.05)., Conclusion: Our findings indicate a reduction or absence of FRP in cLBP patients compared to no-BP individuals, with age being a significant factor as those over 40 showed smaller FRP and flexion angles compared to younger individuals., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Zhang, Firouzabadi, Yang, Liu and Schmidt.)