Methods Of Muscle Measurement

Multiple methods of measuring the effects of sarcopenia are used in clinical studies. The use of multiple methods reflects the lack of a standard definition of sarcopenia, the development of new techniques during the last 25 years that improve upon older techniques, and the lack of a technique that is clearly superior to the others. Beyond the purely technical aspects of measuring sarcopenia is the unresolved question of what is the most important level of measurement. Specifically, is motor performance, muscle strength, muscle mass, or muscle cross-sectional area actually the most relevant for study? Though motor performance is likely the most important from a clinical perspective, it is probably the least amenable to clinical study given potential confounding by cognition, medications, comorbid illnesses, and motivation. Fortunately, muscle mass and muscle cross-sectional area show relatively good linkage so measurement of one provides information about the other. This leaves the debate to muscle mass versus strength as the most relevant measure. Studies have shown that declines in muscle cross-sectional area account for much, but clearly not all, of the declines in strength (Goodpaster et al., 2001). This suggests simply measuring muscle mass or cross-sectional area is a valid approach, but misses some of the details that assessing strength instead would provide.

40K measurement Potassium is the predominant intracellular cation, and approximately 60% of total body potassium is concentrated in skeletal muscle (Heymsfield et al., 1995). Consequently, whole-body counting for naturally occurring 40K can be used to quantify total body potassium and hence estimate muscle mass. This method was expanded by the inclusion of total body nitrogen, as measured by prompt-y neutron activation analysis, and equations developed that allowed approximation of skeletal and nonskeletal muscle mass. This technique has been largely supplanted by newer techniques that more directly measure skeletal and more specifically appendicular muscle mass (see Table 81.1).

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