Influence of Intervertebral Fixation and Segmental Thrust Level on Immediate Post-Spinal Manipulation Trunk Muscle Spindle Response in an Animal Model

Objective: To characterize the effect of unilateral (single and two-level) lumbar facet/zygapophysial joint fixation on paraspinal muscle spindle activity immediately following L4 or L6 high velocity low amplitude spinal manipulation (HVLA-SM) delivered at various thrust durations. Methods: Secondary analysis of immediate (≤2 s) post-HVLA-SM trunk muscle spindle response from two studies involving anesthetized adult cats (n = 39; 2.3–6.0 kg) with either a unilateral single (L5/6) or two-level (L5/6 and L6/7) facet joint fixation. All facet fixations were contralateral to L6 dorsal root recordings. HVLA-SM was delivered to the spinous process in a posterior-to-anterior direction using a feedback motor with a peak thrust magnitude of 55% of average cat body weight and thrust durations of 75, 100, 150, and 250 ms. Time to 1st action potential and spindle activity during 1 and 2 s post-HVLA-SM comparisons were made between facet joint fixation conditions and HVLA-SM segmental thrust levels. Results: Neither two-level facet joint fixation, nor HVLA-SM segmental level significantly altered immediate post-HVLA-SM spindle discharge at tested thrust durations (FDR > 0.05). Conclusions: Two-level facet joint fixation failed to alter immediate (≤2 s) post-HVLA-SM spindle discharge when compared to single-level facet joint fixation at any thrust duration. Segmental thrust level did not alter immediate post-HVLA-SM spindle response in two-level facet joint fixation preparations.