Emerging properties of a micromechanical muscle contraction model

Abstract

The structural response of active skeletal muscle is derived from a combination of sarcomere contraction properties, collagen fibres in the connective tissue, and incompressibility. The role of collagen tissues could be substantial but is still not completely understood. Only few studies have looked into the functionality of collagen reorientation within connective tissues during contraction. The objective of this paper is to propose a discrete static micro-mechanical model to study the sensitivity of kinematic properties of skeletal muscle depending on initial braiding angle of the endomysium lattice and the stiffness of collagen fibers. Two models are developed, first a hexagonally shaped single muscle fibre surrounded by connective tissue, and second, a fascicle model encompassing a single fiber surrounded by neighbouring fibres on all sides with their portion of connective tissue. Connective tissue is modelled in a crossed-helical setup of tension-only elements. Contraction is simulated by pretensioned 'sarcomere' spring elements that follow the muscle force-length relationship. the model calculates displacements, forces, pressures and work developed as a result of contraction.The most important conclusion of the model is that the contribution of braiding angle and collagen stiffness can be considered very important. For a sufficiently small angle, the observed model behaviour can solely be attributed to sarcomere properties. When the orientation angle is increased, the passive properties of the myofascia start playing a larger role. The model response then becomes very much influenced by a pressure term that is induced by stretch in the collagen acting on the incompressible muscle. Although validation methods are limited, the model results are promising and further research is encouraged.