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FIGURE 12-14 Muscle Mass: TBN-TBK Model. Main features of the neutron activation-whole body counting method of estimating muscle and nonmuscle lean tissue mass. TBK and TBN are total body potassium and nitrogen, respectively. The applied model assumes stable TBK/TBN ratios of muscle and nonmuscle lean mass components.

body skeletal muscle derived by the Burkinshaw-Cohn model and total body skeletal muscle by CT in healthy men and men with acquired immunodeficiency syndrome (AIDS) [SMBurkinshaw = 0.990 ×SMcr -6.4; r = 0.83, p = 0.0001, SEE = 4.4 kg, n = 25]. Compared with CT, the Burkinshaw method underestimated skeletal muscle by a mean of 6.9 kg (20.1%, p = 0.0001) and 6.3 kg (23.2%, p = 0.01) in healthy men and men with AIDS, respectively. The hypothesized cause of large between-individual differences in estimated versus measured skeletal muscle (R2 of 0.65 and SEE of 4.4 kg) and observed bias is model errors (Wang et al., 1996b). Second, the method involves radiation exposure, and alternative methods are available with little or no radiation exposure and with equivalent or superior accuracy. Lastly, very few centers have the necessary facilities for measuring TBN and TBK.

The potential exists to develop improved skeletal muscle mass formulas based on TBN and TBK, and studies such as these should be carried out in the future.

Monitoring Changes in Skeletal Muscle Mass Over Time

An important and inadequately studied topic is selection of a measurement method for monitoring changes in skeletal muscle mass and composition over time. The following is an overview of the potential role of each method in evaluating skeletal muscle changes over time.

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