Characteristics of Paraspinal Muscle Spindle Response to Mechanically Assisted Spinal Manipulation: A Preliminary Report

Abstract

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.

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.

Keywords: Afferent Neurons; Muscle Spindles; Spinal Manipulation; Spine.

Figure 1

Photographs of the Activator V (A), and Pulstar (B) mechanical-assisted manipulation devices.

Figure 2

Example of a paraspinal muscle spindle response to an Activator V mechanically-assisted manipulation (MAM) at magnitude setting of 2. Note the increase in discharge during preload and thrust delivery followed by a period of discharge cessation and subsequent return to resting discharge frequency. Time required for spindle discharge to return to 95% of baseline MIF/MF for a minimum period of 1s was 4.5 and 10.5s respectively.

Figure 3

Three recordings from the same muscle spindle afferent in response to mechanically-assisted manipulation (MAM) using the Pulstar at the 22, 44 and 67N device settings (A–C respectively). Mean discharge frequency was the greatest during the 22N thrust (A). Peak mean discharge frequencies were similar for the 44N (B) and 67N (C) MAM thrust. The 67N thrust exhibited a more pronounced decrease in post-MAM discharge. The secondary peak in mean frequency discharge with the 67N thrust may have been the result of the increased forces and/or recoil contact due to the device being hand-held.