Index to Chiropractic Literature
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ID 18789
  Title Short-lasting changes in vertebral position alters lumbar paraspinal muscle spindle sensitivity [platform presentation; the Association of Chiropractic Colleges' Thirteenth Annual Conference, 2006]
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Journal J Chiropr Educ. 2006 Spring;20(1):13-14
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Peer Review Yes
Publication Type Meeting Abstract
Abstract/Notes BACKGROUND: Elucidating the response pattern of paraspinal muscle spindles to biomechanical changes in the spine may be important for understanding neuromuscular control of the spine. Altered information about paraspinal muscle length and velocity might lead to aberrant posture and movement, alter spinal stability, and lead to injury and pain. In arm or limb muscles of humans and cats, a muscle¡¦s length history affects the resting discharge and sensitivity of its muscle spindles. We found similar thixotropic effects in lumbar paraspinal muscles where holding a vertebra in a static position for at least 2 seconds changed resting spindle discharge and its discharge to subsequent movement.

PURPOSE: The aim of this study was to investigate the effects of a static vertebral position less than 2 seconds in duration on static and dynamic responses of lumbar paraspinal muscle spindles.

METHODS: All experiments were performed in anesthetized cats in accordance with the Guiding Principles in the Care and Use of Animals approved by the American Physiological Society. Single unit nerve activity from paraspinal muscle spindle afferents with receptive fields in the low back was recorded from the L6 dorsal root. Paraspinal muscle spindle discharge at an intermediate vertebral position (static test) and during ramp movement of vertebra from that intermediate position (dynamic test) were determined after having held (conditioned) the vertebra in one of 3 positions (hold-long, hold-short or hold-intermediate) for 0, 0.5, 1.0, 1.5 and 2.0 seconds. The hold-short vertebral position unloaded and the hold-long vertebral position loaded the muscle spindles relative to the hold-intermediate vertebral position.

RESULTS: Twelve paraspinal muscle spindle afferents were tested. All were located in either the multifidus or longissimus muscles innervated by the L6 segment spinal cord. For the static test, hold-long relative to hold-intermediate conditioning decreased mean instantaneous frequency of resting discharge by -8.8„b15.9, -13.9„b4.6, -18.9„b9.8, -23.7„b12.4 and -26.8„b14.2 Hz for 0, 0.5, 1.0, 1.5, 2.0 second conditioning durations, respectively. Hold-short conditioning increased the mean frequency by -0.9„b14.3, 3.5„b4.3, 3.8„b5.4, 3.8„b6.7 and 4.3„b6.8 Hz for the same five conditioning durations, respectively. For the dynamic test, hold long relative to hold-intermediate conditioning decreased the mean frequency by -1.1„b0.8, -2.5„b2.3, -4.4„b3.1, -5.6„b3.6 and -7.3„b4.5 Hz for 0, 0.5, 1.0, 1.5, 2.0 second conditioning durations, respectively. On the other hand, hold-short conditioning increased the mean frequency by 1.0„b1.1, 1.1„b1.5, 1.1„b1.7, 2.0„b1.9 and 0.9„b1.9 Hz for the same five conditioning durations, respectively.

DISCUSSION: These results suggest that lumbar paraspinal muscle spindles can experience thixotropic effects when a vertebra is held in a static position for very short durations. Positions that lengthen the paraspinal muscles decrease and positions that shorten the muscles increase spindle responsiveness. However, the effects are most pronounced when the muscles are lengthened. This effect begins within a quarter second and appears to increase with durations up to 2 seconds. That spindle discharge to a given vertebral movement can depend upon the muscle¡¦s previous history would be expected to alter proprioceptive information arriving at the central nervous system. Whether this leads to proprioceptive errors or whether the central nervous system corrects for this instability is not known.

ACKNOWLEDGEMENT: This project is supported by NIH grant R01NS46818. This investigation was conducted in a facility constructed with support from Research Facilities Improvement Program Grant Number C06 RR15433-01 from the National Center for Research Resources, National Institute of Health.

This abstract is reproduced with the permission of the publisher.

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