Measure of trunk 3D kinematics in sitting posture by stereophotogrammetry. Implications for seat design

This paper describes a method for calculating the vectorial kinematic parameters of 3D human movement by stereophotogrammetry, with special emphasis on an accurate characterization of the Instantaneous Helical Axis. That method is applied to the measure of trunk flexion in sitting posture, the results being interpreted in biomechanical terms, and compared to the mechanical design of articulated work chairs. The axes of pelvic and thoracic movements are found to be located above the seat plane, and to displace vertically as trunk is being flexed, contrary to the usually fixed axes of chairs below the seat. This leads to some discussion about ergonomic shortcomings of chair design for dynamic sitting.