Plinth padding confounds measures of posteroanterior spinal stiffness

This study investigated whether the presence of plinth padding influences measured posteroanterior spinal stiffness. Two measures of posteroanterior stiffness, the slope of the loading curve (K) and the displacement at 30 N (D30) were made at three vertebral levels: L3, T12 and T6, on a rigid and a padded plinth in 24 asymptomatic subjects. Analysis of variance demonstrated a significant reduction in K and increase in D30 when measured on the padded compared to the rigid surface and an interaction effect for both the K and D30 measures, indicating that the effect of the padding depends upon the vertebral level tested. The correlations between the padded and unpadded stiffness measures ranged from 0.70 to 0.87. The data from this study suggest that the type of plinth surface needs to be standardised when evaluating PA stiffness.