OBJECTIVE: One commonly used clinical procedure involves the assessment of stiffness during the application of a posteroanterior force to the spinous process of a vertebra. The clinician commonly applies a force normal to the sagittal spinal curve and attempts to evaluate whether the stiffness of the response is also normal. The purpose of this study was to determine whether the stiffness of posteroanterior movements in normal subjects significantly differed when the force was applied to L3, L4 or L5 and when the rate of application changed from quasistatic to cyclical loading.

DESIGN: Experimental investigation of characteristics of normal subjects.

SETTING: University biomechanics laboratory.

INTERVENTIONS: None.

MAIN OUTCOME MEASURES: Posteroanterior stiffness at L3, L4 and L5 during quasistatic and cyclical loading.

RESULTS: The posteroanterior response was found to be stiffest at L5, least at L3 and intermediate at L4. Quasistatic loading produced lower stiffness than cyclical loading at .5 Hz.

CONCLUSIONS: The clinical assessment of the low lumbar spine through manual loading requires a knowledge of the likely variations of posteroanterior stiffness. This study has established that in normal subjects there are systematic effects due to the location of load and the rate of loading.

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