<p>Sitting posture affects driver comfort and health. To investigate driver posture under autonomous vehicle conditions, this study measured the three-dimensional coordinates of external marker points on the driver, considering different backrest tilt angles and the presence or absence of a headrest. The posture Angle corresponds to the human body segment and represents the human body posture. Using the anatomical relationship between the external marker points and the endpoints of the posture angle, the posture angle was calculated and analysed to obtain the pattern of variation of the posture angle. The results show that increasing the backrest angle generally increases the angles of the head, neck, thoracic, abdomen, and elbows, while the knee angle remains unaffected. The pelvic angle is influenced by the headrest, showing consistent behavior when a headrest is present but not without it. At a 70° backrest angle, the pelvis tilts backward, indicating insufficient lumbar support in the current seat design. These findings provide an important reference for future seat design.</p> Graphical Abstract <p>The study measured the body marking points using a coordinate measuring machine. Calculations were made using the points to analyse the pattern of changes in sitting posture and it was concluded from the idiosyncratic changes that adjustable lumbar support as well as headrests are important for sitting posture comfort.</p> <p>fx1</p> <p></p>