Effect of spinal manipulation on the development of history-dependent responsiveness of lumbar paraspinal muscle spindles in the cat

Abstract

We determined whether spinal manipulation could prevent and/or reverse the decrease and increase in paraspinal muscle spindle responsiveness caused respectively by lengthening and shortening histories of the lumbar muscles. Single unit spindle activity from multifidus and longissimus muscles was recorded in the L6 dorsal root in anesthetized cats. Muscle history was created and spinal manipulation delivered (thrust amplitude: 1.0mm, duration: 100ms) using a feedback-controlled motor attached to the L6 spinous process. Muscle spindle discharge to a fixed vertebral position (static test) and to vertebral movement (dynamic test) was evaluated following the lengthening and shortening histories. For the static test, changes in muscle spindle responsiveness were significantly less when spinal manipulation followed muscle history (p<0.01), but not when spinal manipulation preceded it (p>0.05). For the dynamic test, spinal manipulation did not significantly affect the history-induced change in muscle spindle responsiveness. Spinal manipulation may partially reverse the effects of muscle history on muscle spindle signaling of vertebral position.

Keywords: Muscle spindle; chiropractic; lumbar spine; muscle history; paraspinal muscle; proprioception; spinal manipulation; thixotropy.

Figure 1

Schematic of the experimental protocol during 3 spinal manipulation conditions and a representative response (inset during the control condition) of one spindle to three the 3 muscle history conditions. Loading protocol shows the change in vertebral position relative to the intermediate position.

Figure 2

Mean discharge frequency of paraspinal muscle spindles for each of the 3 spinal manipulation conditions during the conditioning phase used to create muscle history. Conditioning phase identified graphically in left panel of figure 1. Each symbol represents the mean ± 95% confidence interval of 28 spindles.

Figure 3

Mean change in resting spindle afferent discharge during the static test for the 3 spinal manipulation conditions. Y-axis represents the change in muscle spindle discharge following the hold-long or hold-short compared with the hold-intermediate conditionings (ΔMIF_long or ΔMIF_short). Each symbol represents the mean ± 95% confidence interval of 28 spindles.

Figure 4

Mean change in spindle afferent discharge during the dynamic test for the 3 spinal manipulation conditions. Y-axis represents ΔMF averaged over the entire movement of the dynamic test. Each symbol represents the mean ± 95% confidence interval of 28 spindles.

Figure 5

Time course of changes in muscle spindle discharge during the dynamic test for hold-short compared with hold-intermediate (upper panel) and for hold-long compared with hold-intermediate (middle panel). Bottom panel shows the magnitude of the vertebral movement over which the dynamic test was analyzed. ^* p<0.05 compared with the spinal manipulation control condition. Each symbol represents the average value between its time position and the time position of the previous data point, except for 5% which represents the average value between its time position and time 0 s.