Relationship between responsiveness of paraspinal muscle spindles during a 50ms spinal manipulative thrust and the distance of their receptive fields from the point of thurst (denoted by “SM”). The incrementing letters refer to the relative location of the receptive fields grouped by spinal region (inset): “A”, multifidus muscle;, “B”, multifidus muscle near the L6-7 or L7-S1 facet joint; “C”, longissumus muscle near the L6-7 or L7-S1 facet joint; “D”, medial portion of longissimus: “E”, lateral portion of longissimus; and “F”, iliocostalis muscle.