| Summary: | Musculoskeletal disorders (MSDs) have become a major health concern with the industrial revolution and technological advances. The National Institute for Occupational Safety and Health (NIOSH) recently estimated the cost of MSDs at $13 to $20 billion annually. Back, spine, and spinal cord disorders add up to 60% of the total MSD injuries.
Based on pervious studies, it is known that lumbar ligaments play a limited or little role in stabilizing the spine; where as the musculature around the spine is the major stabilizing structure. However, repetitive or prolonged static displacement or load induces creep in passive tissues of the spine including ligaments, disc, and joint capsule. The resulting creep in the ligaments or the laxity developed in the viscoelastic structure causes instability of the spine associated with pain, leaving the spine without protection. This study concentrates on the response of the lumbar spine when subjected to static loading.
Twenty-two in vivo feline preparations were subjected to four different intensities of load, and electrical activity of the multifidus muscles for six lumbar levels and creep behavior of the supraspinous ligament were observed for twenty minutes of loading and seven hours of rest period.
Results show that seven hours of rest period was not enough for the multifidus activity and creep developed in the viscoelastic tissue to return to normal. Larger loads elicited larger initial vertical displacement in the lumbar spine as well as higher creep values. Based on the observed data, four exponential and time dependant models were developed in the mathematical description of the electrical activity and the vertical displacement in the supraspinous ligament for both loading and recovery periods.
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