38. Integration of intramuscular pressure with novel muscle activation strategies in a spine model

Abstract

<h3>BACKGROUND CONTEXT</h3> The unstable nature of the spine motivates the use of optimization and electromyography (EMG) methods to estimate its individual muscles force activation. Conventionally, muscles are modelled as force vectors negating enclosed constituents such as intramuscular pressure (IMP), a parameter that has been previously shown to influence muscle performance. <h3>PURPOSE</h3> The purpose of this research was to leverage a novel, validated and fully representative finite elements (FE) model of the spine, inclusive of an accurate representation of IMP defined as a fluid-structure continuum, to explore muscle activation strategies subject to equilibrium static spinal stability constraints. <h3>STUDY DESIGN/SETTING</h3> The FE spine model previously developed by the authors, was validated toward its context of use in exploring activation strategies. The FE model consisted of the thoracic and lumbar vertebral bodies (VBs), intervertebral discs (IVDs), major spine muscles inclusive of their IMP (modelled as fluid-structure continuum), thoracolumbar fascia (TLF) tissue, and a model of the IAP similar to that of IMP, which all added up to a total of 273 tissues. <h3>METHODS</h3> Toward exploring the model's validity, three conventional strategies governing minimizing muscle effort, minimizing IVD compressive forces and maintaining stability at all costs were first investigated. All strategies were subject to an equilibrium static stability constraint whereby the activation strategy aimed to retrieve the spine's initial position after a validated, 30 N.m flexion moment, was applied as an external perturbation. Lastly, two novel IMP-based strategies were devised and explored, that is, minimizing and maximizing IMP. Under physiological muscle force and vertebral displacements constraints, activation strategies were quantified and compared in terms of their ability to maintain an equilibrium static spine position. <h3>RESULTS</h3> Validation efforts proved successful whereby the conventional strategies dictated efficacy in muscular activations whilst maintaining an equilibrium stable spine position, as quantified by vertebral displacements, with a maximum difference of 9.8% from documented data. On the other hand, the minimum IMP novel strategy showed that to maintain a high equilibrium stability, minimum individual muscle forces and IMP of 75N and 50 mmHg, respectively, were sufficient. Lastly, excessive increase in IMP, as dictated by the maximum novel IMP strategy, resulted with a compartmental pressure of up to 93 mmHg between muscles, as well as high tensional TLF forces reaching 56.9N. <h3>CONCLUSIONS</h3> This research concluded on the capabilities of FE models to investigate IMP-based activation strategies governed by stability constraints, as shown by the validation results of the conventional strategies investigated. In addition, the requirements of the minimum IMP strategy to achieve stability supported the role of muscles in achieving efficient mobility and stable joints—in other words, maximizing muscular endurance while maintaining a stable spine position. Lastly, the excessive activation of IMP showed a key role of muscles sharing radial loads with surrounding tissues. This led to a build-up of compartmental pressure between muscles, putting the TLF under higher tensional forces, and thus alluding to another role of the TLF which is to passively store loads. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.