Effect of direction of applied mobilization force on the posteroanterior response in the lumbar spine

Objective: The purpose of this study was to investigate whether changing the direction of applied force affects measured posteroanterior stiffness and associated pelvic (sacral) and lower thoracic rotations. Design: A repeated measures design was used. Setting: University biomechanical laboratory. Participants: Twenty-four subjects' (14 male, 10 female) with no history of recent low back pain or contraindications to mobilization volunteered for testing. Main Outcome Measure: Posteroanterior stiffness was assessed at vertebral levels L3 and L5 through use of 3 sagittal plane directions of applied force; the directions differed by 10 degrees. The amount of sacral and lower thoracic rotation that occurred during loading between 30 and 100 N was also recorded. Results: A small but significant variation of stiffness with direction of applied force was found. At L3, mean stiffness was greatest when the posteroanterior force was applied in a base direction; it was 11% less when the force was applied 10 degrees more caudad than the base direction and 14% less when the force was applied 10 degrees more cephalad than the base direction. There was no significant effect of direction when the force was applied at L5. Both sacral and thoracic rotations displayed significant variation with direction of force when load was applied at L5, with decreasing rotation as the force was applied in a more caudal direction. Conclusion: Posteroanterior stiffness in individuals without back pain is affected by the sagittal plane direction in which the posteroanterior force is applied to the lumbar spine. Remote (thoracic and sacral) movements are also affected by the direction of posteroanterior force. Direction of applied force should therefore be controlled, particularly in the research setting.