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. 2019 Oct;18(5):1401-1413.
doi: 10.1007/s10237-019-01152-2. Epub 2019 May 2.

A micromechanical muscle model for determining the impact of motor unit fiber clustering on force transmission in aging skeletal muscle

Aron Teklemariam  1 , Emma Hodson-Tole  2 , Neil D Reeves  2 , Glen Cooper  3
Affiliations

A micromechanical muscle model for determining the impact of motor unit fiber clustering on force transmission in aging skeletal muscle

Aron Teklemariam et al. Biomech Model Mechanobiol. 2019 Oct.

Abstract

This study used a micromechanical finite element muscle model to investigate the effects of the redistribution of spatial activation patterns in young and old muscle. The geometry consisted of a bundle of 19 active muscle fibers encased in endomysium sheets, surrounded by passive tissue to model a fascicle. Force was induced by activating combinations of the 19 active muscle fibers. The spacial clustering of muscle fibers modeled in this study showed unbalanced strains suggesting tissue damage at higher strain levels may occur during higher levels of activation and/or during dynamic conditions. These patterns of motor unit remodeling are one of the consequences of motor unit loss and reinnervation associated with aging. The results did not reveal evident quantitative changes in force transmission between old and young adults, but the patterns of stress and strain distribution were affected, suggesting an uneven distribution of the forces may occur within the fascicle that could provide a mechanism for muscle injury in older muscle.

Keywords: Fiber clustering; Finite element modeling; Force; Motor unit; Muscle; Spacial activation patterns.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Fig. 1
Fig. 1
Geometry and finite element (FE) model of the muscle tissue. a Geometry of the tissue containing three distinct regions (defined in Table 1) the fascicle, the sounding tissue and the extended tissue. b The fascicle is inside the surrounding tissue and is comprised of 19 muscle fibers each of which is wrapped in an endomysium sheet. c Finite element model of the tissue showing dashed line where the model was cut to obtain axial stress values at this section for analysis
Fig. 2
Fig. 2
Boundary conditions of the FE model. The axial displacement is not permitted at the far ends, but other directions are permitted to allow the expansion of the fascicle as it is contracting. The outer layer of the surrounding tissue is fixed in all directions
Fig. 3
Fig. 3
Distribution of strain and stress values predicted through the fascicle cross section, for simulations of young (more compliant extended tissue) and middle age (stiffer extended tissue) tissue for the low activation cases (20% activation). Distribution of the active fibers are shown on the left
Fig. 4
Fig. 4
Distribution of strain and stress values predicted through the fascicle cross section, for simulations of young (more compliant extended tissue) and middle age (stiffer extended tissue) tissue for the higher activation cases (40% activation). Distribution of the active fibers are shown one the left
See this image and copyright information in PMC

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