Understanding the biomechanics of spinal manipulative therapy requires knowing how loads are transmitted to deeper structures. This investigation monitored displacement at sequential depths in thoracic paraspinal tissues parallel with surface load directions. Participants were prone and a typical preload maneuver was applied to thoracic tissues. Ultrasound speckle tracking synchronously monitored displacement and shear deformation of tissue layers in a region of interest adjacent to load application to a depth of 4cm. Cumulative and shearing displacements along with myoelectric activity were quantitatively estimated adjacent to loading site. The cephalocaudal cumulative displacement in layers parallel to the surface were, in order of depth, 1.27 (SD=0.03), 1.18 (SD=0.02), and 1.06 (SD=0.01) mm (P<0.000), respectively. The superficial/intermediate shear was 2.1±2.3% whereas the intermediate/deep shear was 4.4% (SE=3.7, P=0.014). Correlation of tissue layers was stronger with application site displacement at the surface (0.87<r<0.89) than with muscle activation (0.65<r<0.67).
Surface loading of the torso in combined posteroanterior and caudocephalic directions result in both displacement of tissues anteriorly and in shearing between tissue layers in the plane of the tissues strata to depths that could plausibly affect spinal tissues. Displacements of tissues more likely arise passively, consistent with load transmitted by the retinacula cutis and epimuscular force pathways. Displacements are similar in magnitude to those known to evoke biologically relevant responses in both animal and human studies.