Relationship between body segment movements and center of pressure shifts during trunk lean movements while sitting in healthy adults.

Introduction: Sitting activities involve trunk leaning, which requires precise coordination of the head, trunk, and pelvis to maintain the center of pressure (COP) within limits of stability. However, how the movement in each segment is associated with COP displacement remains unclear. Methods: Twenty-eight healthy volunteers performed seated trunk leaning twice in left, right, forward, and backward directions. COP displacements and angular changes were measured in head, trunk, and pelvic tilt in all directions, and additionally in the hip during forward-backward leaning. Within-session reliability of measured indices was assessed using intraclass correlation coefficients (ICCs) and Bland-Altman analyses, and relationships between segment angle changes and COP displacements were examined using mixed-effects models. Results: ICCs between two trials showed substantial to very high correlations (ICC = 0.84-0.95), and Bland-Altman analyses showed neither fixed nor proportional bias, suggesting sufficient reliability. Pelvic tilt angle changes were significantly associated with COP displacements during left-right and forward-backward leaning ( Conclusions: Pelvic tilt was associated with COP displacement during leaning, regardless of direction, indicating the importance of pelvic shifts in strategies for maintaining sitting balance in healthy adults.