OBJECTIVE: To compare the posteroanterior (PA) stiffness of the lumbar spine when the load is applied in a vertical direction with the stiffness when the load is applied perpendicular to the spinal curve.

DESIGN: The PA stiffness of the lumbar spine was assessed at L1, L3 and L5 on 24 normal subjects using a mechanical spinal mobilization apparatus. The PA stiffness was measured when the load was applied in a vertical direction and when the load was applied perpendicular to the spinal curve at the level being tested.

SETTING: A university biomechanics laboratory.

DESIGN: A repeated-measures design with pseudorandomization of the order of testing.

MAIN OUTCOME MEASURES: Stiffness variable derived from force-displacement curve.

RESULTS: The difference in PA stiffness between the two loading conditions was small at all three levels tested. The stiffness of L5 was significantly lower when the load was applied in the vertical direction compared with the application of the load in the perpendicular direction (p = .0001). Altering the angle of inclination of PA load had no statistically significant effect on PA stiffness at L1 and L3. The mean PA stiffness of the lumbar spine increased in a caudal direction (L1 = 10.4 N/mm, L3 = 11.4 N/mm, L5 = 11.6 N/mm).

CONCLUSION: The orientation of the PA load had only a small effect on the measured PA stiffness and would be unlikely to be detected during manual examination of the spine. However, changes in the orientation of the PA load may be important when considered in relation to the symptom response.

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