Effects of Thrust Magnitude and Duration on Immediate Postspinal Manipulation Trunk Muscle Spindle Responses

The purpose of this study was to characterize trunk muscle spindle responses immediately after high-velocity, low-amplitude spinal manipulation (HVLA-SM) delivered at various thrust magnitudes and thrust durations.

Secondary analysis from multiple studies involving anesthetized adult cats (N = 70; 2.3-6.0 kg) receiving L6 HVLA-SM. Muscle spindle afferent recordings were obtained from L6 dorsal rootlets before, during, and immediately after HVLA-SM. L6 HVLA-SM was delivered posteriorly-to-anteriorly using a feedback motor with peak thrust magnitudes of 25%, 55%, and 85% of cat body weight (BW) and thrust durations of 25, 50, 75, 100, 150, 200, and 250 ms. Time to the first action potential and muscle spindle discharge frequency at 1 and 2 seconds post-HVLA-SM were determined.

A significant association between HVLA-SM thrust magnitude and immediate (≤2 s) muscle spindle response was found (P < .001). For non-control thrust magnitude, pairwise comparisons (25%, 55%, 85% BW), 55% BW thrust magnitude had the most consistent effect on immediate post-HVLA-SM discharge outcomes (false discovery rate < 0.05). No significant association was found between thrust duration and immediate post-HVLA-SM muscle spindle response (P > .05).

The present study found that HVLA-SM thrust magnitudes delivered at 55% BW were more likely to affect immediate (≤2 s) post-HVLA-SM muscle spindle response.