| A relationship exists between seating posture and back pain. The normal position of the lumbar spine during sitting has long been debated. 52 years ago, Keegan hypothesised a relationship between lordosis and trunk-thigh angle using plain radiography of only 4 individuals. No one has replicated his original study. The aim of this study was to define the optimal sitting posture by investigating the alterations in lumbar curve and intervertebral disc (IVD) morphology in variable sitting positions in 22 individuals using a 0.6T upright positional MRI scanner. Measurements of lumbar lordosis angles, disc height and translation of nucleus pulposus (NP) were made on subjects with no history of back pain. We found IVD height showed a tendency to decrease as lordosis decreased. The NP showed movement within a limited range in normal IVDs without change of its area. The optimal sitting position was with a trunk-thigh angle of 135 degrees. This position caused least strain on the lumbar spine, most significantly when compared with an upright sitting posture. We recommend positional MRI in evaluation of future seating design.
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