Investigation of intrinsic spine muscle properties to improve musculoskeletal spine modelling

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Zwambag, Derek P

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University of Guelph


Spine muscles are known to generate large compressive loads and play a vital role in spine stabilization. Spine loads and stability are often estimated using computational models; yet, models cannot account for inherent differences in intrinsic muscle properties, as these data are unavailable. This dissertation was borne out of this need to further understand the characteristics of spine muscles. Part A of this dissertation consisted of three experiments each designed to address a specific research question. Each experiment also generated normative data, which were combined in Part B to create a custom musculoskeletal spine model capable of predicting dynamic active and passive muscle moments. Generic muscle models do not accurately predict whole muscle passive stresses. Experiment I investigated passive muscle stress differences following facet joint injury. Passive muscle stresses were not altered 28 days following injury. Data from control animals were used to model passive muscle stresses throughout physiological sarcomere lengths. Experiment II was designed to determine the sarcomere lengths of spine muscles based on posture. Physiological sarcomere lengths were measured from human cadavers in a neutral posture using laser diffraction; sarcomeres of muscles posterior to the spine were shorter than muscles anterior to the spine. During modelled flexion, posterior muscles became strained and sarcomeres approached the optimal length for active force production. These data were incorporated into the model to estimate sarcomere lengths of spine muscles based on spine posture. Spine muscles display a unique phenomenon known as 'flexion relaxation', which occurs when extensor muscles 'turn off' despite substantial demand near full trunk flexion. Passive tissues are believed to support the weight of the upper body. Experiment III further tested this proposed mechanism using a pulley system to manipulate the weight of the upper body. These data were used to validate the active and passive muscle moments predicted by the musculoskeletal model. The model predicted: a) the occurrence of flexion relaxation; b) that decreasing the external moment caused flexion relaxation to occur earlier; and c) the requirement for abdominal muscle activity at full flexion. The model also suggested that muscle generates greater passive moments than ligaments in full flexion.



Spine, Muscle, Mechanics, Flexion Relaxation, Sarcomere Length, Modelling, Elastic Modulus, Biomechanics