Manipulation: investigation of a proposed mechanism.

The purpose of this study was to investigate whether spinal manipulation alters the posteroanterior stiffness of the manipulated region. Thirty subjects with no history of thoracic pain or contraindication to manipulation participated. The manipulation studied was a posteroanterior thrust applied to the T4-5 spinal level. The effect of the manipulation was compared to a control intervention of supine lying. The posteroanterior stiffness of all subjects was measured at the T4 and T5 levels initially, and remeasured after both the manipulation and control interventions. Change scores relating to the change in posteroanterior stiffness due to manipulation and due to the control intervention were computed. A t-test comparing the change scores between interventions revealed no significant difference. However, the posteroanterior stiffness at T5 was found to be significantly greater than at T4. It was concluded that in the case of asymptomatic subjects these results did not provide support for the hypothesis that posteroanterior stiffness is altered by manipulation.