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STUDY DESIGN A retrospective cohort study. OBJECTIVES To investigate the relationship between skeletal muscle status of the trunk and the progression of adolescent idiopathic scoliosis (AIS) during adulthood. SUMMARY OF BACKGROUND DATA To date, studies evaluating the risk factors for progression of AIS have principally focused on radiographic parameters. METHODS Eighty-two women with AIS managed conservatively, who had a major curve ≥30° at skeletal maturity (Risser grade ≥4), were enrolled. Patients had been followed-up, on average, over 23.5 years (range, 12-37 years) after skeletal maturity, and were ≥30-years-old at the time of the survey (mean, 40.1 years). The ratio of the cross-sectional area (CSA) and signal intensity (SI) of muscles (multifidus [MF], erector spinae [ES], and psoas major [PM]) were evaluated using axial T2-weighted magnetic resonance images at the level of L4. RESULTS The mean progression of the main thoracic and thoracolumbar/lumbar (TL/L) curves after skeletal maturity was 8.2° (mean annual rate of 0.4°) and 7.2° (mean annual rate of 0.3°), respectively. The mean CSA ratio was as follows: MF, 23.3%; ES, 79.6%; and PM, 40.5%. The mean SI ratio was as follows: MF, 34.9%; ES, 31.7%; and PM, 20.9%. On multivariate logistic regression, a higher SI ratio of the MF was predictive of a progression of the Cobb angle and translation of the apical vertebra, with a lower CSA ratio of the ES contributing to the progression of the Cobb angle for the TL/L curve. CONCLUSIONS In patients with AIS who have a major curve ≥30° at skeletal maturity, patients with greater TL/L curve progression have lower skeletal muscle volume and higher fatty degeneration of the lumbar extensor muscles in adulthood. However, further longitudinal or prospective studies are necessary to clarify the causal relationship between scoliosis progression and trunk muscular status. LEVEL OF EVIDENCE Level III, retrospective cohort study.