title: Effects of thrust amplitude and duration of high-velocity, low-amplitude spinal manipulation on lumbar muscle spindle responses to vertebral position and movement.
abstract: 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 (∆MIFresting), during ramp movement (∆MIFmovement), and with the vertebra held in the new position (∆MIFposition) were compared.
  
  
  RESULTS
  Thrust duration had a small but statistically significant effect on ∆MIFresting at all 6 thrust amplitudes compared with control (0-millisecond thrust duration). The lowest amplitude thrust displacement (1 mm) increased ∆MIFresting at all thrust durations. For all the other thrust displacements and forces, the direction of change in ∆MIFresting was not consistent, and the pattern of change was not systematically related to thrust duration. Regardless of thrust force, displacement, or duration, ∆MIFmovement and ∆MIFposition were not significantly different from control.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Palmer Center for Chiropractic Research, Davenport, IA 52803, USA.
authors.forename: Dong-Yuan
authors.initials: DY
authors.lastname: Cao
authors.forename: William R
authors.initials: WR
authors.lastname: Reed
authors.forename: Cynthia R
authors.initials: CR
authors.lastname: Long
authors.forename: Gregory N
authors.initials: GN
authors.lastname: Kawchuk
authors.forename: Joel G
authors.initials: JG
authors.lastname: Pickar
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002415:Cats; D005260:Female; D008297:Male; D020393:Manipulation, Spinal / Q000379:methods*; D009068:Movement; D009470:Muscle Spindles / Q000502:physiology*; D055585:Physical Phenomena; D011187:Posture; D013997:Time Factors