Objective. Intramuscular adipose tissue (IMAT) and sarcopenia may adversely impact mobility function and physical activity. This study determined the association of locomotor muscle structure and function with mobility function in older adults. Method. 109 older adults with a variety of comorbid disease conditions were examined for thigh muscle composition via MRI, knee extensor strength via isometric dynamometry, and mobility function. The contribution of strength, quadriceps lean tissue, and IMAT to explaining the variability in mobility function was examined using multivariate linear regression models. Results. The predictors as a group contributed 27–45% of the variance in all outcome measures; however, IMAT contributed between 8–15% of the variance in all four mobility variables, while lean explained only 5% variance in only one mobility measure. Conclusions. Thigh IMAT, a newly identified muscle impairment appears to be a potent muscle variable related to the ability of older adults to move about in their community. 1. Introduction The European Working Group on Sarcopenia in Older People [1] has recently suggested that the definition of sarcopenia should include not only low muscle mass but also low muscle function (strength or performance) because sarcopenia is a syndrome that is characterized by both. The loss of muscle mass and strength are problems that plague many older adults, though the causal and correlative link between diminishing lean mass and strength is not well supported in aging humans [2]. Aging is also characterized by increasing depots of fat both between and within skeletal muscles [3]. This increase in intramuscular adipose tissue (IMAT), and its lipotoxic effect, has been identified as a potential contributor to declining strength and muscle quality; with some also associating it with mobility limitations in older adults [4–8]. The recent structural focus on IMAT content rather than lean muscle mass is important as it could provide a plausible explanation for age-associated strength and mobility deficits. Fatty infiltration into skeletal muscle may alter muscle fiber orientation and hence the force producing capabilities of the whole muscle. IMAT may also be a metabolically active component of muscle that secretes inflammatory cytokines leading to systemic inflammation, much like visceral adipose tissue, that can inhibit muscle force production even in the absence of muscle atrophy [9]. Further, IMAT may be an appropriate therapeutic rehabilitation target as some suggest its presence can blunt the strength response to
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