OBJECTIVE: Characterization of the responses to spinal posteroanterior loading has involved measuring the gradient of the linear region of the force-displacement relationship between 30 N and 100 N of applied force. However, forces applied during manual assessment of stiffness may reach 300 N. This study investigated whether the force-displacement relation remains linear above 100 N.

DESIGN: A single-case-study design with replication was used.

SUBJECTS: Two male subjects, with no history of low back pain requiring treatment over the preceding twelve months, participated in this study.

METHODS: Posteroanterior loads of 275 N were applied to the 1.3 vertebra of both subjects using a mechanical device. The force-displacement responses in these subjects were measured and analyzed to examine the linearity of the data.

RESULTS: The responses from the two subjects were similar. In both cases, the responses were approximately linear at all levels of load up to 275 N. Because of the small degree of nonlinearity, there was a tendency for the stiffness to show moderate increases as the load increased. Stiffness was approximately 25% greater when calculated between 30 N and 275 N than when calculated between 30 N and 100 N.

CONCLUSION: In these two subjects, the force-displacement relation was approximately linear for all three force intervals considered, although there was some increasing stiffness with increasing force.

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