OBJECTIVE: One explanation for the poor reliability of manual judgments of posteroanterior (PA) stiffness may be that if manual therapists use different forces when testing, different stiffness is perceived. This study was conducted to examine measurements of lumbar PA stiffness obtained using a device programmed to generate different loading forces.

SUBJECTS: Twenty-five subjects with no history of low back pain and a mean age of 23.5 yr were measured.

METHODS: Measures of lumbar PA stiffness were obtained using a mechanical device that applied a testing force of 200 N to the skin overlying the L3 spinous process. Six stiffness coefficients were determined from the force/displacement curve obtained from each subject by performing linear regressions from 30-80 N, 30-150 N, 30-200 N, and from 30-83.3 N, 83.3-136.7 N, and 136.7-200 N. Intraclass correlation coefficients and repeated measures analysis of variance were used to analyze the data.

RESULTS: Although moderate reliability [ICC 2,1 = 0.67] was found for stiffness measures arising from increasingly wide force-interval regressions (30-80 N, 30-150 N, 300-200 N), poor reliability [ICC (2,1) = 0.39] was found for stiffness measures arising from same-width, higher force regressions (30-83.3 N, 83-137 N, 137-200 N). In both cases there were significant differences between the obtained K stiffness values corresponding to different force intervals.

CONCLUSION: These results show that if therapists push harder, different stiffnesses will be felt. Studies using instrumental measurement of spinal stiffness to obtain 'K' values should report the force intervals used. Also, revised protocols for manually judging PA stiffness should ensure that stiffness is assessed by sampling specified force intervals rather than the raters determining their own force limits.

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