This study investigated the relationships among the lateral deviation of the thorax, crosssectional area of the psoas major muscle, and left-right asymmetry of the pelvic rotation angle. [Participants and Methods] The participants were 17 healthy males. The cross-sectional area of the psoas major muscle and change in pelvic rotation angle during isometric hip flexion were measured. [Results] The cross-sectional area of the psoas major muscle was significantly larger on the right and the change in the pelvic rotation angle to the ipsilateral side was significantly larger on the left. Furthermore, there was a significant negative correlation of the amount of lateral deviation of the thorax with the ratio of the left and right psoas major muscle cross-sectional areas, and a significant positive correlation with the ratio of the left and right pelvic rotation angles. [Conclusion] The results suggest that it is important to assess lateral deviation of the thorax in the upright position to evaluate changes in the cross-sectional area of the psoas major muscle during isometric hip flexion.