Assessment of Lean Mass and Physical Performance in Sarcopenia.

This review provides a description of the assessment of lean mass and physical performance with particular attention to how these measures are used in the context of sarcopenia, in both research and clinical settings. One of the most common methods to estimate muscle mass is whole-body dual-energy X-ray absorptiometry (DXA). DXA estimates the total amount of lean tissue but does not directly measure muscle mass. Appendicular lean mass (ALM), derived from DXA scans, is the sum of the lean tissue in the arms and legs. ALM alone, or scaled to height squared (ALM/height(2)) or body mass index (ALM/body mass index), is the most common metric used as an approximation of muscle mass in sarcopenia research. Other methods to assess muscle mass include central or peripheral quantitative computed tomography (to determine muscle cross-sectional area and muscle density, a marker of fat i nfiltration into the muscle); magnetic resonance (to assess muscle cross-sectional area and volume); and bioelectrical impedance analysis (to determine fat-free mass). Many approaches to objectively measuring physical performance have been used in sarcopenia research. Muscle strength is often measured: Grip strength is very inexpensive and straightforward to assess, whereas assessment of lower extremity strength is more difficult. However, lower extremity strength may be a more relevant measure than grip strength in the context of mobility outcomes. Dynamic physical performance is also widely measured in research settings and may be emerging as a routine assessment in clinical care. The most widely used measure of physical performance is walking speed over a short distance, usually 3-6 m. Other measures of objective physical performance include the Short Physical Performance Battery that includes gait speed, ability and time to rise from a chair 5 times, and static balance tests; and the Timed Up and Go test that measures the time to rise from a chair and walk a short distance. Finally, longer distance walking tests are also used. "Fast" versions of these tests measure either distance traveled in a set amount of time (as for the 6-minute walk test) or time to walk a set distance (such as the fast long distance corridor walk more than 400 m). The "usual pace" version of the longer distance walking test, the usual-pace 400 m walk, is an objective measure of disability and has been used as an outcome in clinical trials. In summary, there are many methods available to assess muscle mass and physical function, each with advantages and limitations. The choice of what test to use depends on the nature of the research question or the clinical environment and the availability of resources for evaluation.