Effect of different seat heights on lumbar spine flexion during stand-to-sit motion

[Purpose] This study aimed to investigate the movement of the thorax, lumbar spine, and pelvis when healthy participants sit on a chair, and to identify the kinematic characteristics due to changes in the height of the seat. [Participants and Methods] Twenty healthy participants (14 males, 6 females; mean age, 29 ± 5 years) were recruited for this study. They performed stand-to-sit motion using one seat with a height of 100% that of the lower leg length (standard) and another with a height of 60% that of the lower leg length (lower). A three-dimensional motion analysis system and four force plates were used to analyze each joint angle. [Results] The mean lumbar spine flexion angle was significantly increased in the lower versus the standard seat. As a kinematic characteristic, the pelvis tilted posteriorly while the thorax tilted anteriorly, which increased the lumbar spine flexion angle. The pelvis was tilted posteriorly when the hip joint flexed about 60° regardless of the seat height. [Conclusion] The lumbar spine flexion angle increased in the lower seat stand-to-sit motion, which suggested an increase in the load on the lumbar spine. The lumbar spine flexion angle was influenced by the characteristic movements of the thorax and pelvis.