• Cao D-Y
• Reed WR
• Long CR
• Kawchuk GN
• Pickar JG

• Lumbar Vertebrae / physiology
• Manipulation, Spinal
• Movement
• Muscle Spindles / physiology
• Paravertebral Muscles / physiology

Objective: Mechanical characteristics of high-velocity, low-amplitude spinal manipulations (HVLA-SMs) can vary. Sustained changes in peripheral neuronal signaling due to altered load transmission to a sensory receptor's local mechanical environment are often considered a mechanism contributing to the therapeutic effects of spinal manipulation. The purpose of this study was to determine whether variation in an HVLA-SM's thrust amplitude and duration alters the neural responsiveness of lumbar muscle spindles to either vertebral movement or position.

Methods: Anesthetized cats (n = 112) received L6 HVLA-SMs delivered to the spinous process. Cats were divided into 6 cohorts depending upon the peak thrust force (25%, 55%, 85% body weight) or thrust displacement (1, 2, 3 mm) they received. Cats in each cohort received 8 thrust durations (0-250 milliseconds). Afferent discharge from 112 spindles was recorded in response to ramp and hold vertebral movement before and after the manipulation. Changes in mean instantaneous frequency (∆MIF) during the baseline period preceding the ramps (∆MIF_resting), during ramp movement (∆MIF_movement), and with the vertebra held in the new position (∆MIF_position) were compared.

Results: Thrust duration had a small but statistically significant effect on ∆MIF_resting at all 6 thrust amplitudes compared with control (0-millisecond thrust duration). The lowest amplitude thrust displacement (1 mm) increased ∆MIF_resting at all thrust durations. For all the other thrust displacements and forces, the direction of change in ∆MIF_resting was not consistent, and the pattern of change was not systematically related to thrust duration. Regardless of thrust force, displacement, or duration, ∆MIF_movement and ∆MIF_position were not significantly different from control.

Conclusion: Relatively low-amplitude thrust displacements applied during an HVLA-SM produced sustained increases in the resting discharge of paraspinal muscle spindles regardless of the duration over which the thrust was applied. However, regardless of the HVLA-SM's thrust amplitude or duration, the responsiveness of paraspinal muscle spindles to vertebral movement and to a new vertebral position was not affected.

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