Lumbar multifidus muscle size does not differ whether ultrasound imaging is performed in prone or side lying.

This methodological study examined the influence of body position when measuring multifidus muscle size using real-time ultrasound imaging. Previous studies examined multifidus with the subject prone but people with certain conditions cannot be studied in this position, so side lying was investigated as an alternative posture. In 20 normal females (aged 19-45 years), the cross-sectional area (CSA) of lumbar multifidus was measured at the level of the fifth lumbar vertebra (L5) on both the right and left sides, with the subject in prone and in side lying. Multifidus CSA was highly correlated between the two positions on both the right (r=0.90) and left (r=0.91) sides. Paired t-tests found no significant differences between the measurements made in the two scanning postures (right P=0.77; left P=0.16). Bland and Altman plots showed good agreement between the two positions, with no systematic difference. These findings demonstrate that measurements of lumbar multifidus at L5 can be made in either prone or side lying and a valid comparison of the results obtained in both positions can be made.