Sitting biomechanics part I: Review of the literature

Objective: To develop a new sitting spinal model and an optimal driver's se at by using review of the literature of seated positions of the head, spine , pelvis, and lower extremities. Data Selection: Searches included MEDLINE for scientific journals, engineer ing standards, and textbooks. Key terms included sitting ergonomics, sittin g posture, spine model, seat design, sitting lordosis, sitting electromyogr aphy, seated vibration, and sitting and biomechanics. Data Synthesis: In part I, papers were selected if (1) they contained a fir st occurrence of a sitting topic, (2) were reviews of the literature, (3) c orrected errors in previous studies, or (4) had improved study designs comp ared with previous papers. In part II, we separated information pertaining to sitting dynamics and drivers of automobiles from part I. Results: Sitting causes the pelvis to rotate backward and causes reduction in lumbar lordosis, trunk-thigh angle, and knee angle and an increase in mu scle effort and disc pressure. Seated posture is affected by seat-back angl e, seat-bottom angle and foam density, height above floor, and presence of armrests. Conclusion: The configuration of the spine, postural position, and weight t ransfer is different in the 3 types of sitting: anterior, middle, and poste rior. Lumbar lordosis is affected by the trunk-thigh angle and the knee ang le. Subjects in seats with backrest inclinations of 110 to 130 degrees,with concomitant lumbar support, have the lowest disc pressures and lowest elec tromyography recordings from spinal muscles. A seat-bottom posterior inclin ation of 5 degrees and crests can further reduce lumbar disc pressures and electromyography readings while seated. To reduce forward translated head p ostures, a seat-back inclination of 110 degrees is preferable over higher i nclinations. Work objects, such as video monitors, are optimum at eye level . Forward-tilting, seat-bottom inclines can increase lordosis, but subjects give high comfort ratings to adjustable chairs, which allow changes in pos ition.