The comfort of a chair should be assessed based on the alignment between the chair’s features and the user’s behavioral characteristics. Considering modern habits and types of seating, prolonged work often leads to frequent shifts from correct posture (Torso Upright with Parallel Legs) to poor posture (Torso Flexion with Crossed Legs). Thus, exploring how to select an appropriate chair type to accommodate these frequent transitions between TUPLs and TFCLs becomes a topic worthy of thorough investigation. This study collected kinematic and ground reaction force data from 16 healthy males performing the TUPLs-to-TFCLs transition on two types of chairs, Stationary and Swivel, and then processed the data using a whole-body musculoskeletal model. The results revealed the following:​ (1) TFCLs significantly increased the lumbar L4–L5 flexion angle, lateral bending angle, flexion moment, axial rotation moment, pressure, and shear force. Lower back muscle activation increased significantly during TFCLs, whereas abdominal muscle activation decreased (P < 0.05). (2) During the TUPLs-to-TFCLs transition, the SW chair presented a significantly greater lumbar L4–L5 joint lateral bending angle, lateral bending torque, axial rotation torque, and shear force than the ST chair, with lower activation of the lower back and abdominal muscles (P < 0.05). This study revealed that although the comfort derived from reduced muscle activation during TFCLs exists, it actually increases the shear force on the lumbar intervertebral disc. Furthermore, individuals with the habit of TFCL should exercise caution when using SW chairs to reduce lumbar load and potential health risks.