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Paraspinal 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.

The goal of this study was to identify behavioral, environmental, and constitutional factors associated with paraspinal muscle asymmetry.

A cross-sectional study of 202 adult male twins was conducted.

Data 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.