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.
Highlights
Skeletal muscles produce the forces required to maintain body posture and complete the myriad of activities associated with daily living
The decline in skeletal muscle strength has been related to the quality of force per unit area of the muscle tissue related to factors such as: fat infiltration (Rahemi et al 2015), lateral force transmission between fibers through the endomysium
A group of muscle fibers each wrapped by an endomysium sheet and joined together in a bundle A continuation of the fascicle, but with tissue properties that reflects the stiffness of the muscular–tendon junction The muscle tissue that surround the fascicle and that will be compressed as the fascicle contracts/expands
Summary
Skeletal muscles produce the forces required to maintain body posture and complete the myriad of activities associated with daily living. The ability of muscles to produce force relies on the activation level and on properties related to their structure (e.g., fiber length, pennation angle, number of fibers, physiological cross-sectional area) and the connective materials into which the fibers are embedded to form the muscle macrostructure. Intrinsic factors refer to alterations in the mechanical properties of the fibers and the matrix tissue (Gao et al 2008; Ward et al 2009; Plate et al 2013; Wood et al 1985; Wang et al 2014). Extrinsic factors can refer to alterations in the proportion of the matrix relative to the fibers (Smith and Barton 2014), length of the fibers and the spatial distribution of muscle fibers from the same motor unit (Faulkner et al 2007; Kadhiresan et al 1996)
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