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

AUTHORS: Joel Pickar, D.C., Ph.D., Paul Sung, Ph.D., Yu-Ming Kang, Ph.D., and Weiqing Ge, Ph.D.


Spinal manipulation has long been used to treat patients with low back and neck pain. The most common form of this manipulation has been termed high-velocity low-amplitude, meaning the thrust is performed at a high-velocity but with little depth at the skin contact on the patient. Clinical skills in applying such a manipulation lies in the chiropractor’s ability to control speed, length and force of the load as well as the direction in which the load is applied, and the contact point on which the load is applied. Control over its mechanical delivery is therefore then assumed to be related to its clinical effects. Biomechanical changes evoked by a manipulation are thought to have physiological consequences caused, at least in part, by changes in neural output from paraspinal tissues.

If the activation of neural pathways does contribute to the effects of a manipulation, it seems reasonable to anticipate that neural discharge might increase or decrease as the thrust duration approaches a threshold value. We hypothesized that the relationship between the duration of an HVLA thrust to a vertebrae and paraspinal muscle spindle discharge would be nonlinear. In addition, we anticipated that muscle spindle discharge would be more sensitive to larger amplitude thrusts.

Thus, we conducted a neurophysiological study of spinal manipulation using the lumbar spine of an animal model. We gave impulse thrusts to the spinous process of the L6 vertebra of deeply anesthetized animal models while recording nerve activity from the dorsal roots of muscle spindle afferent nerves innervating the lumbar paraspinal muscles. We used a feedback motor to deliver the impulse thrusts. The motor’s drive arm was securely attached to the L6 spinous process via a forceps.

We found that as thrust duration became shorter, the discharge of the lumbar paraspinal muscle spindles increased in a particular fashion (curvilinear). A concave-up inflection occurred near the 100-ms duration, eliciting both a higher frequency discharge compared with the longer durations and a faster rate of change as thrust duration was shortened. This pattern was seen quite clearly in paraspinal afferent nerves with receptive fields both close and far from the midline. Paradoxically, spindle afferents were almost twice as sensitive to the 1-mm thrust compared with the 2-mm-amplitude thrust. This may be related to the small versus large signal range properties of muscle spindles.

We concluded that 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, this may be an important factor in the therapeutics benefit of manipulation.

The full abstract of this study was published in The Spine Journal, Volume 7, Issue 5, September-October 2007, pages 583-595.

The Palmer Center for Chiropractic Research is located on Palmer’s Davenport Campus in the William and Jo Harris Building.
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