Characteristics of Paraspinal Muscle Spindle Response to Mechanically Assisted Spinal Manipulation: A Preliminary Report
SOURCE: J Manipulative Physiol Ther. 2017 (Jun 17) [Epub]
William R. Reed, DC, PhD,
Joel G. Pickar, DC, PhD,
Randall S. Sozio, BS, LATG,
Michael A.K. Liebschner, PhD,
Joshua W. Little, DC, PhD,
Maruti R. Gudavalli, PhD
Palmer Center for Chiropractic Research,
Palmer College of Chiropractic,
OBJECTIVES: The purpose of this preliminary study is to determine muscle spindle response characteristics related to the use of 2 solenoid powered clinical mechanically assisted manipulation (MAM) devices.
METHODS: L6 muscle spindle afferents with receptive fields in paraspinal muscles were isolated in 6 cats. Neural recordings were made during L7 MAM thrusts using the Activator V (Activator Methods Int. Ltd., Phoenix, AZ) and/or Pulstar (Sense Technology Inc., Pittsburgh, PA) devices at their 3 lowest force settings. Mechanically assisted manipulation response measures included (a) the time required post-thrust until the first action potential, (b) differences in mean frequency (MF) and mean instantaneous frequency (MIF) 2 seconds before and after MAM, and (c) the time required for muscle spindle discharge (MF and MIF) to return to 95% of baseline after MAM.
RESULTS: Depending on device setting, between 44% to 80% (Pulstar) and 11% to 63% (Activator V) of spindle afferents required >6 seconds to return to within 95% of baseline MF values; whereas 66% to 89% (Pulstar) and 75% to 100% (Activator V) of spindle responses returned to within 95% of baseline MIF in <6 seconds after MAM. Nonparametric comparisons between the 22 N and 44 N settings of the Pulstar yielded significant differences for the time required to return to baseline MF and MIF.
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CONCLUSION: Short duration (<10 ms) MAM thrusts decrease muscle spindle discharge with a majority of afferents requiring prolonged periods (>6 seconds) to return to baseline MF activity. Physiological consequences and clinical relevance of described MAM mechanoreceptor responses will require additional investigation.
From the Full-Text Article:
Low back pain (LBP) continues to be a major societal health problem, and prevalence rates are only expected to increase with the aging of the American population. [1, 2] More than 80% of the population will experience an episode of LBP during their lives, with 20% to 40% experiencing a reoccurrence of an LBP episode within 1 year after the initial onset in working populations. [3, 4] Low back pain can be treated with multiple approaches including nonpharmacologic, pharmacologic, and surgical interventions. The efficacy, favorable side effect profile, cost effectiveness, and high patient satisfaction associated with the nonpharmacologic approach of spinal manipulation have led to multiple clinical practice guidelines and evidence reports recommending spinal manipulation for certain types of acute and chronic LBP. [5–11] However, the appropriate use and clinical efficacy of spinal manipulation could be greatly enhanced if underlying peripheral and central neurophysiological mechanisms were clearly established. Accomplishing these objectives will require combined contributions from both basic and clinical investigations.
Spinal manipulation traditionally involves applying a single short-lever, high-velocity, low-amplitude (HVLA) thrust of short duration to a targeted dysfunctional vertebral motion segment and/or joint. High-velocity, low-amplitude thrusts can be delivered manually with direct hand contact (≤150 ms thrust duration) or using commercially available mechanically assisted devices (≤10 ms thrust duration). [12–18] Peak forces achieved during a manipulative thrust can vary dramatically depending on anatomic location of treatment, localized tissue compliance, clinician and/or patient’s physical traits, spinal manipulative technique, mechanically assisted device used, and/or device setting. [15–25] A distinct advantage mechanically assisted manipulation (MAM) offers over manual spinal manipulation is that thrust velocity and thrust magnitude can be standardized. Mechanically assisted manipulation is thought to be an effective alternative to manual thrust manipulation based on both comparable measures of vertebrae movements [12, 26–30] and similar clinical outcomes reported in a few smaller studies. [31–34] A recent randomized clinical trial comparing manually delivered spinal manipulation and MAM indicated differences in short-term clinical efficacy,  thereby suggesting that distinct physiological mechanisms of action might be responsible for the underlying therapeutic effects of these 2 types of HVLA spinal manipulation.