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Spinal stiffness changes throughout the respiratory cycle

¹School of Physiotherapy, University of Sydney, Lidcombe, Sydney, New South Wales 1825; ²Prince of Wales Medical Research Institute and University of New South Wales, Sydney, New South Wales 2052; and ³Department of Physiotherapy, The University of Queensland, Brisbane, Queensland 4072, Australia; and ⁴Department of Physiotherapy, Umeå University, SE-901 87 Umeå, Sweden

Submitted 10 October 2002 ; accepted in final form 28 April 2003

Posteroanterior stiffness of the lumbar spine is influenced by factors, including trunk muscle activity and intra-abdominal pressure (IAP). Because these factors vary with breathing, this study investigated whether stiffness is modulated in a cyclical manner with respiration. A further aim was to investigate the relationship between stiffness and IAP or abdominal and paraspinal muscle activity. Stiffness was measured from force-displacement responses of a posteroanterior force applied over the spinous process of L₂ and L₄. Recordings were made of IAP and electromyographic activity from L₄/L₂ erector spinae, abdominal muscles, and chest wall. Stiffness was measured with the lung volume held at the extremes of tidal volume and at greater and lesser volumes. Stiffness at L₄ and L₂ increased above base-level values at functional residual capacity (L₂ 14.9 N/mm and L₄ 15.3 N/mm) with both inspiratory and expiratory efforts. The increase was related to the respiratory effort and was greatest during maximum expiration (L₂ 24.9 N/mm and L₄ 23.9 N/mm). The results indicate that changes in trunk muscle activity and IAP with respiratory efforts modulate spinal stiffness. In addition, the diaphragm may augment spinal stiffness via attachment of its crural fibers to the lumbar vertebrae.

lumbar posteroanterior stiffness; muscle activity; diaphragm; intra-abdominal pressure; manual therapy