In classical ballet, dancers must maintain a rigid, erect body posture (pull-up posture), which reduces the physiological curvature of the spine. Previous studies have revealed the critical role of the trunk muscles in the control of such posture. We studied 2 female professional ballet dancers (A,B), 2 female ordinary ballet practitioners (C,D), and 1 female who had no experience in ballet (E) (age 30.2±1.26 yr). Their spinal curvature and trunk muscles were evaluated using X-ray and computed tomography (CT) scans. From the CT scans, the cross-sectional areas of the trunk muscles were measured using computer software. X-ray images of the spinal column were taken in a relaxed posture (RP) and the required ballet posture (BP). Spinal alignment was evaluated by measuring the sacral inclination angle (SIA) and the lumbar lordosis angle (LLA). Changes in the SIA and LLA were measured when the subjects moved from the RP to the BP. Center of gravity (COG) agitation was measured using a Wii Balance Board for 15 s in the 6th and 5th ballet foot positions. Sway.velocity (cm/s), Sway. area (cm2) and RMS.area (cm2) were measured as parameters of the COG agitation. Subjects A, B, C and D, who had experience in ballet, tended to have larger trunk muscle cross-sectional areas, especially for the psoas major (PSA) and lateral abdominal (LAT) muscles, and showed a decrease of both SIA and LLA in the ballet posture. There was a tendency for the change in the LLA to be correlated with the cross-sectional areas of the PSA and the LAT. The cross-sectional areas of the LAT, PSA, and erector spinae muscles showed a marked negative correlation with COG agitation parameters in the 6th and 5th ballet foot positions. These findings suggest that these muscles contribute to the classical ballet-specific posture.