When we sit for a long time, we don’t maintain a completely fixed posture; instead, we unconsciously adjust our posture. In this study, we investigated postural change strategies during prolonged seated work, considering both musculoskeletal and contact loads. Utilizing a musculoskeletal model for sitting posture analysis, muscle forces, spinal loads, and chair reaction forces were estimated, and postural change strategies were analyzed. The results showed that for some participants, seat reaction force decreased in the latter half of the sitting time, and pelvic angle and musculoskeletal load distribution changed. It is suggested that postural change strategies, although limited, were influenced by discomfort due to soft tissue compression in the buttocks, and that postural changes aimed at reducing contact loads.