OBJECTIVE: The purpose of this investigation was to study the spinal deformations produced by applying a force to a lumbar vertebra and to compare the deformation responses at three different loading rates.

SETTING: The study was conducted in a laboratory setting using a mechanical device to simulate the manipulative procedure being studied.

SUBJECTS: Twelve volunteers (mean age 28.1 yr) formed a convenience sample for testing. All subjects had no significant history of low back pain.

INTERVENTIONS: During loading the displacements of the skin were measured at the point of loading, as well as over the spinous processes of L1, T11, T8, and S1 and 30 mm caudad to S1. The loading rates were 1.0 Hz, 0.5 Hz and a very slow "quasistatic" rate. The order of presentation of the loading rates was systematically varied between subjects to minimize the effect of order on the responses. Between 20 and 100 N, the force-displacement relations were highly linear and were characterized by the coefficients of the equation of the line of best fit.

MAIN RESULTS: At all three loading rates there were displacements throughout the entire region studied. Changing from quasistatic loading to cyclical loading produced a significant reduction in displacements.

CONCLUSIONS: This study found that when a force was applied over L3 there was generalized movement of the whole lumbar and lower thoracic spine. Therefore, a diversity of tissues would be expected to contribute to the resistance to lumbar posteroanterior movements.

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