A finite element approach to study skeletal muscle tissue

Teklemariam, Aron (2016) A finite element approach to study skeletal muscle tissue. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

This dissertation investigates force generation in muscle using a finite element (FE) approach to model electrical activity and mechanical force production within skeletal muscle. The work proposes new FE models design/formulations to answer specific research questions related to skeletal muscle properties. The focus is on two specific determinants of skeletal muscle force: the activation and the connective tissue. A FE model was created and designed to study the impact of the dielectric and geometric (pennation) properties of the muscle tissues on the electric activation signal detected on the skin surface by bipolar electrodes (surface electromyography, sEMG). The model shows that when considering parallel muscle fibres the tissue, attenuated mainly frequencies in the physiological range (92-542 Hz). This study revealed a strong impact of the muscle fibres pennation angle, on the detected signal (low pass filtering effect); suggesting that the low pass filtering behaviour observed in experimental data is due to the geometry (curvature or pennation) rather than the dielectric properties. The model informed recommendations for sEMG experimental protocol to increase the inter-electrodes distance when measuring sEMG of pennated muscles. A micromechanical model of the muscle tissue was created to explore the influence of the connective tissue properties (endomysium) on the total muscle force production. The constitutive model was used to study the mechanical consequence of clustering of fibres due to the remodelling of the motor units, which occurs with ageing. An FE model with a bundle of 19 fibres was designed and simulated activating 21% and 37% of the fibres in a distributed and clustered pattern. Results showed for both activation levels that the pattern of the strain distribution changed with an increased deformation toward the centre of the bundle. This could lead to excessive unbalanced stresses if higher deformations are involved. The micromechanical model can be used to study muscle force determinants at a fascicle level. It showed the importance of the fibre distribution during the muscle activation and the consequences of age related alterations on force production.