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¹ Palmer Center for Chiropractic Research, Davenport, IA, USA

Proprioceptive information from paraspinal tissues including muscle contributes to neuromuscular control of the vertebral column. We investigated whether the history of a vertebra's position can affect signalling from paraspinal muscle spindles. Single unit recordings were obtained from muscle spindle afferents in the L₆ dorsal roots of 30 Nembutal-anaesthetized cats. Each afferent's receptive field was in the intact muscles of the low back. The L₆ vertebra was controlled using a displacement-controlled feedback motor and was held in each of three different conditioning positions for durations of 0, 2, 4, 6 and 8 s. Conditioning positions (1.0–2.2 mm dorsal and ventral relative to an intermediate position) were based upon the displacement that loaded the L₆ vertebra to 50–60% of the cat's body weight. Following conditioning positions that stretched (hold-long) and shortened (hold-short) the spindle, the vertebra was repositioned identically and muscle spindle discharge at rest and to movement was compared with conditioning at the intermediate position. Hold-short conditioning augmented mean resting spindle discharge by +4.1 to +6.2 impulses s^–1; however, the duration of hold-short did not significantly affect this increase (F_4,145= 0.49, P= 0.74). The increase was maintained at the beginning of vertebral movement but quickly returned to baseline. Conversely, hold-long conditioning significantly diminished mean resting spindle discharge by –2.0 to –16.1 impulses s^–1 (F_4,145= 11.23, P < 0.001). The relationship between conditioning duration and the diminished resting discharge could be described by a quadratic (F_1,145= 9.28, P= 0.003) revealing that the effects of positioning history were fully developed within 2 s of conditioning. In addition, 2 s or greater of hold-long conditioning significantly diminished spindle discharge to vertebral movement by –5.7 to –10.0 impulses s^–1 (F_4,145= 11.0, P < 0.001). These effects of vertebral positioning history may be a mechanism whereby spinal biomechanics interacts with the spine's proprioceptive system to produce acute effects on neuromuscular control of the vertebral column.

(Received 26 July 2005; accepted after revision 4 October 2005; first published online 6 October 2005)
Corresponding author J. G. Pickar: Palmer Center for Chiropractic Research, 741 Brady Street, Davenport, IA 52803, USA. Email: pickar_j{at}palmer.edu

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