BackgroundParaspinal muscle asymmetry in cross-sectional area (CSA) and composition have been associated with low back pain and pathology. However, substantial multifidus muscle asymmetry also has been reported in men who were asymptomatic, and little is known about other factors influencing asymmetry.ObjectiveThe goal of this study was to identify behavioral, environmental, and constitutional factors associated with paraspinal muscle asymmetry.DesignA cross-sectional study of 202 adult male twins was conducted.MethodsData were collected through a structured interview, physical examination, and magnetic resonance imaging. Measurements of multifidus and erector spinae muscle CSA and the ratio of fat-free CSA to total CSA were obtained from T2-weighted axial images at L3–L4 and L5–S1.ResultsIn multivariable analyses, greater asymmetry in multifidus CSA at L3–L4 was associated with lower occupational physical demands and less disk height narrowing. Handedness was the only factor associated with multifidus muscle CSA asymmetry at L5–S1. For the erector spinae muscle, greater age, handedness, and disk height narrowing were associated with CSA asymmetry at L3–L4, and sports activity, handedness, disk height narrowing, and familial aggregation were associated with CSA asymmetry at L5–S1. In multivariable analyses of asymmetry in muscle composition, familial aggregation explained 7% to 20% of the variance in multifidus and erector spinae muscle side-to-side differences at both levels measured. In addition, handedness and pain severity entered the model for erector spinae muscle asymmetry at L5–S1, and disability, handedness, and disk height narrowing entered the model for multifidus muscle asymmetry at L5–S1.LimitationsReliance on participants' recall for low back pain history, occupation, and physical activity levels was a limitation of this study.ConclusionsFew of the factors investigated were associated with paraspinal muscle asymmetry, and associations were inconsistent and modest, explaining little of the variance in paraspinal muscle asymmetry.