Changes of Muscle Thicknesses of the Back Muscles in Lateral and Prone Position Using Ultrasonography

[Purpose] The purpose of this study was to compare the anterior-posterior (AP) diameter of back muscles measured in the lateral position to that measured in the prone position using ultrasonography. [Participants and Methods] The participants were 20 healthy elderly men over 65 years old (mean age, 71.4 ± 5.4 years). The AP diameter of the right multifidus muscle (L2) (LM (L2)), a multifidus muscle (L5) (LM (L5)), and the erector spinae muscles (ES) were measured on longitudinal images. The changes in the AP diameter in the lateral and prone positions were measured. [Results] The AP diameter of LM (L2) and ES increased significantly in the lateral position. There was no significant difference in the AP diameter of LM (L5) between the lateral and prone positions. [Conclusion] Care should be taken when measuring back muscles in the lateral position with ultrasonography, as increased muscle activity may alter the AP diameter.