The aim of this study was to determine the effect of spinal manipulative therapy (SMT) force magnitude and force duration on the spinal stiffness of a feline preparation. A mechanical device performed simulated SMTs at the L6 spinous process in 22 anesthetised felines. Animals were divided into four groups. Two groups (no preload, preload) received SMT having maximal displacements of 1.0 mm, 2.0 mm and 3.0 mm of total displacement (displacement control). In two other groups (preload, no preload), SMTs were applied with maximal loads of 25%, 55% and 85% body weight (force control). Each of the SMTs were applied in order of increasing displacement or force amplitudes, at increasing durations ranging from 25 to 250 ms. Spinal stiffness was quantified by applying an indentation load to external surface of the back. Linear mixed effects models were fit for post-SMT stiffness variables. When SMT was applied under displacement control with and without a preceding preload, a significant interactive effect occurred between force magnitude and force duration (p ≤ 0.05) for some of the stiffness variables. The findings from this experiment demonstrate that spinal stiffness in a feline model was affected by the interaction of the force amplitude and force duration parameters but the exact nature of this interaction remains unclear. This study provides guidance for further investigation given other SMT parameters not tested here may facilitate the ability of SMT to alter spinal stiffness.