On the molecular level the body can be divided into four main components: lipid, water, protein, and minerals (Wang et al., 1992). In addition to fat, the remaining molecular components of the body, namely, water, proteins, and minerals, are found in relatively stable proportions to one another and are grouped under FFM (Wang et al., 1992).

At the anatomical level, the body includes adipose tissue, skeletal muscle, bone, visceral organs, and other related components (Wang et al., 1992). Adiposity and fatness are closely related, and the two, for military purposes, can be considered equivalent components. An individual's fat mass is related to overall energy balance, health, fitness, and appearance. Increases in fat mass and percent body fat are associated with greater morbidity and mortality due to such factors as lipid-mediated cardiovascular risk (Seidell et al., 1987). Increased fatness is associated with a decrease in some aspects of fitness, as will be discussed briefly in this chapter and in greater detail in Chapter 3. Percent body fat and fat distribution also contribute to the subjective assessment of appearance.

The difference between body weight and fat mass is FFM, a component associated with both strength and endurance. Under usual circumstances, skeletal muscle constitutes about one half of FFM (Wang et al., 1996). Hence, FFM and closely related adipose tissue-free mass are usually considered surrogate measures of skeletal muscle mass. For example Hodgdon and colleagues (1990) observed a positive association among Navy women between FFM (but not fat mass) and maximal box lifting capacity and other strength measures. Gender differences in strength disappeared when adjusted for FFM, presumably reflecting greater skeletal muscle mass in men than in women (Conway et al., 1989).

Early studies of strength and endurance relied on FFM as a surrogate for skeletal muscle mass, but the recent introduction of whole-body multislice magnetic resonance imaging (Heymsfield et al., 1997) and dual-energy x-ray absorptiometry (DXA) provides two important opportunities: the possibility of expanding body composition-functional studies, and the potential of developing anthropometric skeletal muscle prediction models. As an example of skeletal muscle functional studies, Mello and colleagues (1995) evaluated appendicular skeletal muscle mass by DXA and manual material handling in males and females. They observed a significant correlation between muscle mass and lift/carry tasks. Development of anthropometric skeletal muscle mass prediction models has been proposed as a goal for future military research efforts (Friedl, 1997).

Skeletal muscles are all anchored to bones, and the skeleton is an integral component of the anatomical body composition level. The skeleton consists of structural proteins enmeshed in a calcium hydroxyapatite mineral matrix. At the molecular level bone mineral mass and density are evaluated for whole-body or regions using DXA methods. Imaging methods such as magnetic resonance imaging can quantify skeletal mass and dimensions or bone-related components at the anatomical level. The element calcium, found almost entirely in bone, was evaluated in classical skeletal studies using a method referred to as neutron activation analysis. Genetic, dietary, and exercise/training factors all influence the risk of skeletal injuries in the military.

An important body composition concept is that stable relationships exist between some components at the same or different levels. For example, a stable relationship exists in healthy adults between two components at the molecular level, total body water (TBW) and FFM. In populations, the ratio of TBW to FFM is approximately 0.73 (Wang et al., 1992). Stable body

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