Evidence shows that not only changes in skeletal muscle mass but changes in strength and other factors underpinning muscle quality play a role in muscle function decline and impaired mobility associated with aging. Changes in both strength and quality may precede loss of muscle mass. Skeletal muscle function deficit (SMFD) is a terminology that embraces this evolving conceptualization of sarcopenia and age-related muscle dysfunctions. This chapter provides a discussion on sarcopenia in the

more »

... xt of SMFD, including operational definitions and methodological challenges associated with their establishment; integration of muscle quality into SMFD; efforts to identify diagnostic cutoff values for low muscle mass and weakness and their predictive validity to mobility disability; need for standardized muscle quality assessment; clinical and public health relevance and research opportunities. Changes in muscle composition, based on excessive levels of inter-and intramuscular or intramyocellular fat are striking features increasingly addressed in the literature, found to affect muscle metabolism and peak force generation. Methods to easily and rapidly assess muscle composition in multiple clinical settings and with minimal patient burden are needed. Further characterization of SMFD should emphasize integration of muscle quality and factors behind changes in quality, as well as associated clinical and research implications.