Response of lumbar paraspinal muscles spindles is greater to spinal manipulative loading compared with slower loading under length control

Abstract

Background context: Spinal manipulation (SM) is a form of manual therapy used clinically to treat patients with low back and neck pain. The most common form of this maneuver is characterized as a high-velocity (duration <150 ms), low-amplitude (segmental translation <2 mm, rotation <4 degrees , and applied force 220-889 N) impulse thrust (high-velocity, low-amplitude spinal manipulation [HVLA-SM]). Clinical skill in applying an HVLA-SM lies in the practitioner's ability to control the duration and magnitude of the load (ie, the rate of loading), the direction in which the load is applied, and the contact point at which the load is applied. Control over its mechanical delivery is presumably related to its clinical effects. Biomechanical changes evoked by an HVLA-SM are thought to have physiological consequences caused, at least in part, by changes in sensory signaling from paraspinal tissues.

Purpose: If activation of afferent pathways does contribute to the effects of an HVLA-SM, it seems reasonable to anticipate that neural discharge might increase or decrease in a nonlinear fashion as the thrust duration approaches a threshold value. We hypothesized that the relationship between the duration of an impulsive thrust to a vertebra and paraspinal muscle spindle discharge would be nonlinear with an inflection near the duration of an HVLA-SM delivered clinically (<150 ms). In addition, we anticipated that muscle spindle discharge would be more sensitive to larger amplitude thrusts.

Study design/setting: A neurophysiological study of spinal manipulation using the lumbar spine of a feline model.

Methods: Impulse thrusts (duration: 12.5, 25, 50, 100, 200, and 400 ms; amplitude 1 or 2 mm posterior to anterior) were applied to the spinous process of the L6 vertebra of deeply anesthetized cats while recording single unit activity from dorsal root filaments of muscle spindle afferents innervating the lumbar paraspinal muscles. A feedback motor was used in displacement control mode to deliver the impulse thrusts. The motor's drive arm was securely attached to the L6 spinous process via a forceps.

Results: As thrust duration became shorter, the discharge of the lumbar paraspinal muscle spindles increased in a curvilinear fashion. A concave-up inflection occurred near the 100-ms duration eliciting both a higher frequency discharge compared with the longer durations and a substantially faster rate of change as thrust duration was shortened. This pattern was evident in paraspinal afferents with receptive fields both close and far from the midline. Paradoxically, spindle afferents were almost twice as sensitive to the 1-mm compared with the 2-mm amplitude thrust (6.2 vs. 3.3 spikes/s/mm/s). This latter finding may be related to the small versus large signal range properties of muscle spindles.

Conclusions: The results indicate that the duration and amplitude of a spinal manipulation elicit a pattern of discharge from paraspinal muscle spindles different from slower mechanical inputs. Clinically, these parameters may be important determinants of an HVLA-SM's therapeutic benefit.

Figures

FIGURE 1

Comparison of programmed and actual thrust A) durations and B) displacements. In A symbols denote means of all cats and horizontal bars denote the minimum and maximum actual thrust duration. Solid line is the line of identity between actual and programmed thrust duration. In B symbols denote thrust displacements for each cat and “X” denotes their means. Overlapping data result in thicker symbols.

FIGURE 2

Relationship between conduction velocity and the type of lumbar paraspinal muscle spindle afferent based upon classification of its receptive ending to a ramp and hold stretch (see Methods).

FIGURE 3

Examples of responses from primary (left column) and secondary (right column) lumbar paraspinal muscle spindle afferents for the 400 (top row), 200 (middle row) and 100ms (bottom row) manipulative thrust durations. Each panel shows the displacement caused by the thrust (top trace) and instantaneous discharge plots obtained from original recordings of action potentials (bottom plot). The starting time of the x-axis is arbitrary; all recordings were preceded by a 10 second control period. Note, for the 25ms thrust the x-axis is expanded.

FIGURE 4

Effect of thrust duration on paraspinal muscle spindle discharge during 1mm (top row) and 2mm (bottom row) displacements of the L6 vertebra. Y-axis represents the difference between mean instantaneous discharge frequency during the thrust compared to the prior 10 second control period. Grayed symbols represent the response of those afferents (n=13) whose discharge did not consistently increase as thrust duration shortened (see text). See last paragraph of Results for description of inset. Inset x-axis scale identical to larger plots.

FIGURE 5

Relationship between responsiveness of paraspinal muscle spindles during a 50ms spinal manipulative thrust and the distance of their receptive fields from the point of thurst (denoted by “SM”). The incrementing letters refer to the relative location of the receptive fields grouped by spinal region (inset): “A”, multifidus muscle;, “B”, multifidus muscle near the L6-7 or L7-S1 facet joint; “C”, longissumus muscle near the L6-7 or L7-S1 facet joint; “D”, medial portion of longissimus: “E”, lateral portion of longissimus; and “F”, iliocostalis muscle.

FIGURE 6

Effect of the magnitude of thrust displacement on the sensitivity of lumbar paraspinal muscle spindles to the thrust velocity of an impulse load. Shown as mean ±1 SEM.

FIGURE 7

Effect of the surgical preparation on the average spinal stiffness in response to the manipulative thrust velocity. Shown as mean ±1 SE.