A study from Iowa, Canada examined in 20 cats how preload during HVLA manipulations (High Velocity, Low Amplitude) affects muscle spindle input in lumbar paraspinal muscles. Primary afferent activity of muscle spindles in lumbar paraspinal muscles (m. longissimus lumbalis and Mm. multifidii) was recorded from the L6 dorsal root in anesthetized cats. It was found that a longer compared to a shorter preload duration produced a significantly greater increase in paraspinal muscle spindle discharge during the manipulative thrust. Additionally, a smaller compared to a larger preload resulted in a greater increase in paraspinal muscle spindle discharge. Preload characteristics should be considered when assessing the therapeutic benefit of this intervention both in clinical efficacy studies and in clinical practice.
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Methods:Primary afferent activity of muscle spindles in lumbar paraspinal muscles was recorded from the L6 dorsal root in anesthetized cats. HVLA spinal manipulation at the L6 vertebra was preceded by either no preload or systematic variations in the magnitude of preload, duration, and the presence or absence of a downward notch point. The immediate effects of preload on muscle spindle responses to HVLA manipulation were determined by comparing mean instantaneous discharge frequencies (MIF) during the thrust phase of HVLA manipulation with baseline values. Longer-lasting effects of preload on spindle responses through the HVLA technique were determined by comparing MIF during a slow ramp-and-hold movement of the L6 vertebra before and after HVLA manipulation. Results: The smaller compared to the larger preload magnitude and the longer compared to the shorter preload duration significantly increased (P = .02 and P = .04, respectively) muscle spindle responses during the HVLA thrust. The absence of preload had the greatest effect on changing MIF. Interactions between preload magnitude, duration, and the downward notch point often produced statistically significant but likely physiologically small changes in the passive signaling properties of the muscle spindle after manipulation. References: Reed W, Long C et al . Neural responses to the mechanical parameters of a high-velocity, low-amplitude spinal manipulation: effect of preload parameters. J Manipulative Physiol Ther. 2014 Feb;37(2):68-78
