Muscle Mass, Quality, and Composition Changes During Atrophy and Sarcopenia.

Skeletal muscle mass (SMM) and muscle strengh reach their peak in 20s to 40s of age in human life and then decrease with advancing age. The decrease rate of muscle strength or power was twice to four times as large as that of the SMM. Thus, the normalized muscle force (muscle strength divided by SMM) also decreases in aging. It depends on the number of factors in skeletal muscle tissues and neuromuscular system. In human study, SMM cannot be measured directly without dissection so that all of the methodologies are indirect methods to assess SMM, even computing tomography or magnetic resonance imaging. Dual-energy X-ray absorptiometry, ultrasonography, anthropometry, and bioelectrical impedance analysis (BIA) are used as secondary indirect methods to estimate SMM. Recent researches show muscle composition changes in aging, and in particular, the ratio of muscle cell mass (MCM) against SMM decrease and relative expansion of extracellular water (ECW) and extracellular space is observed with advancing age and/or decrease of physical function. The intracellular water (ICW) and ECW estimated by segmental bioelectrical impedance spectroscopy or multifrequency BIA are good biomarkers of the ratio of MCM against SMM in limbs. The BIS and other state-of-the-art technology for assessment of muscle mass, quality, and composition are useful to fully understand the muscle atrophy in a living organism.