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STUDY DESIGN A cross-sectional comparative study. OBJECTIVE The present study aimed to investigate the relationship between the flexion-relaxation phenomenon asymmetry of lumbar muscles and trunk lateral Range of Motion (ROM) asymmetry in non-specific chronic low back pain (NSCLBP) patients. SUMMARY OF BACKGROUND DATA Imbalance in trunk muscle activation between right and left sides can induce pain by loading the spine incorrectly, especially in NSCLBP patients. A previous study reported a greater asymmetry in the flexion-relaxation phenomenon of the erector spinae in NSCLBP patients than in asymptomatic participants (AP). Imbalance of muscle properties, such as trunk ROM, has been suggested as a possible cause of this observed asymmetry. METHODS Twenty-eight NSCLBP patients and twenty-two AP performed three standing maximal trunk flexions. Surface electromyography were recorded bilaterally for erector spinae longissimus and lumbar multifidus. A flexion-relaxation ratio was calculated for each muscle. The fingertip-to-thigh test was performed to assess trunk lateral ROM. Each parameter's asymmetry was calculated as the absolute difference between right and left sides. RESULTS NSCLBP patients present a significantly lower trunk lateral ROM than AP. Flexion-relaxation ratio asymmetry of the erector spinae was significantly greater in NSCLBP patients than in AP (p < 0.05). Flexion-relaxation ratio asymmetry of the multifidus and trunk lateral ROM asymmetry were not significantly different between groups. Significant correlation (r = 0.49) between flexion-relaxation ratio asymmetry of Erector spinae and trunk lateral ROM asymmetry was observed only for NSCLBP patients. CONCLUSIONS The present findings showed that flexion-relaxation ratio asymmetry of erector spinae longissimus is moderately correlated with trunk lateral ROM asymmetry. In addition, the results confirmed that NSCLBP patients present a reduced trunk lateral ROM, flexion-relaxation ratio asymmetry of the erector spinae which is correlated with trunk rotation. These findings suggested an imbalance spine loading which can contribute to the persistence of pain. LEVEL OF EVIDENCE 3.