Skeletal muscle myofilament adaptations to aging, disease, and disuse and their effects on whole muscle performance in older adult humans

Abstract

Skeletal muscle contractile function declines with aging, disease, and disuse. In vivo muscle contractile function depends on a variety of factors, but force, contractile velocity and power generating capacity ultimately derive from the summed contribution of single muscle fibers. The contractile performance of these fibers are, in turn, dependent upon the isoform and function of myofilament proteins they express, with myosin protein expression and its mechanical and kinetic characteristics playing a predominant role. Alterations in myofilament protein biology, therefore, may contribute to the development of functional limitations and disability in these conditions. Recent studies suggest that these conditions are associated with altered single fiber performance due to decreased expression of myofilament proteins and/or changes in myosin-actin cross-bridge interactions. Furthermore, cellular and myofilament-level adaptations are related to diminished whole muscle and whole body performance. Notably, the effect of these various conditions on myofilament and single fiber function tends to be larger in older women compared to older men, which may partially contribute to their higher rates of disability. To maintain functionality and provide the most appropriate and effective countermeasures to aging, disease, and disuse in both sexes, a more thorough understanding is needed of the contribution of myofilament adaptations to functional disability in older men and women and their contribution to tissue level function and mobility impairment.

Stuck in gear: age-related loss of variable gearing in skeletal muscle

“…The changes to muscle gearing observed here may also contribute to a reduction in maximum muscle shortening velocity. Decreases in muscle shortening velocity with age have previously been attributed to decreases in fiber length, changes in fiber type composition, and reduced shortening velocity of individual fibers (Raj et al, 2010;Miller et al, 2014). However, the present results suggest that age-related changes to gearing may have contributed to the decrease in maximum muscle shortening velocity.…”

“…These findings support the hypothesis that variable gearing results from the interaction of contractile and connective tissues, potentially due to its ability to effect force-dependent variation in muscle shape change. They also suggest that age-related changes in connective tissues stiffness impact muscle performance, and may therefore explain why large, consistent changes are observed in muscle performance with age (Brooks and Faulkner, 1994;Morse et al, 2005;Kubo et al, 2007;Raj et al, 2010), in the absence of such consistent changes at the cellular (fiber) level (Miller et al, 2014). This may provide novel explanations for the loss of functionality associated with aged and fibrotic muscles (Lieber and Ward, 2013).…”

Building strength, endurance, and mobility using an astaxanthin formulation with functional training in elderly

“…Many factors have been implicated as contributors to a rise in specific force with training, including neuromuscular,35 muscle architecture,36 and cellular mechanisms 37. Our testing approach and the observed changes with training and treatment minimize the impact of several of the factors and highlight one factor that could raise strength.…”

Age, human performance, and physical employment standards

“…The major contributor to age-related sarcopenia is the simultaneous neuromuscular alterations such that there is selective age-related denervation of motor units, particularly type II fibres (Callahan and Kent-Braun 2011;Miller et al 2013). As a result, there is a proportional shift towards type I muscle fibres and an increase in the size of the remaining motor units, which subsequently increases the activation threshold of the muscle and decreases the speed of contraction (Miller et al 2014). Functionally, sarcopenia and denervation can lead to marked reductions in walking speed and endurance (Ostchega et al 2004;Kenny et al 2008) as well as stability (Brill et al 2000;Kenny et al 2008), all of which are crucial requirements for adequate work ability in many industries.…”

“…Functionally, sarcopenia and denervation can lead to marked reductions in walking speed and endurance (Ostchega et al 2004;Kenny et al 2008) as well as stability (Brill et al 2000;Kenny et al 2008), all of which are crucial requirements for adequate work ability in many industries. Fortunately, regular resistance training has been shown to delay and even reverse the progression of sarcopenia and denervation in aging adults (Parente et al 2008;Harber et al 2009;Toth et al 2012;Miller et al 2014).…”