Plinth padding and measures of posteroanterior lumbar stiffness.

OBJECTIVE: To investigate whether measurement of posteroanterior (PA) lumbar stiffness is affected by the presence of padding on the testing plinth.
DESIGN: Within a repeated-measures design, measurements were made of lumbar PA stiffness in subjects without low back pain on both a rigid and a padded plinth surface.
SUBJECTS: Nineteen subjects with no history of any low back pain requiring treatment over the preceding 12 months participated in this study.
METHODS: PA stiffness was measured at L3 on two occasions under two different conditions. The first condition involved measurement of lumbar stiffness on a rigid plinth surface; the second involved measurement on a padded plinth surface. A reliable mechanical device was used to obtain the PA stiffness measures.
RESULTS: Mean lumbar PA stiffness values obtained when testing on a padded plinth were 2.86 N/mm less than those values obtained when testing the same lumbar spines on a rigid plinth. A paired t test showed a significant difference between the PA stiffness measures at L3 obtained on the padded plinth and those obtained on the rigid plinth (t = 6.66, df = 18, p < or = .0001).
CONCLUSIONS: These findings suggest that to improve the reliability of lumbar PA stiffness assessment, testing should be performed under the same plinth surface conditions.