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Heymsfield's laboratory has estimated that 75 percent of total body muscle mass is found in the arms and legs. Other regions of interest such as the lower leg also can be identified and isolated using the software. This approach has been validated by comparing total body muscle mass determined by CT to appendicular skeletal muscle estimated by DXA. The correlation coefficient between the two is 0.95, which indicates a strong association and the feasibility of using DXA for the quantitation of muscle mass in humans in vive.

The primary limitations on the accuracy of using imaging techniques to estimate muscle mass are those imposed by the technology employed. As mentioned above, even the most accurate methods of CT and MRI have a minimal error of measurement of 2 to 3 percent. Thus, for estimating changes in muscle mass, the expected change would have to be greater than 6 percent to make the measurement feasible. Such a change in muscle mass could be seen only under drastic circumstances, such as a change in total body weight of more than 10 percent. Current research includes the further development of BIA and imaging techniques, improvement of the urinary creatinine method, and perhaps most importantly, developments in the measurement of dynamic in vive changes in muscle metabolism, composition, and subsequent muscle function.

Control of Muscle Mass and Function

The function and regulation of skeletal muscle mass have been reviewed by Nair in Chapter 6. Muscle mass constitutes 40 to 45 percent of body weight and accounts for approximately 70 percent of body cell mass. Nair presents a model that links muscle mass and function (in the form of contractility) with metabolic processes. Skeletal muscle has important locomotive and metabolic functions. Many of the metabolic functions that occur in skeletal muscle depend on muscle mass as well as interrelated factors such as circulating hormone levels, training status, and age. Thus, indicators of muscle function such as strength and endurance are influenced by these factors as well.

Strength. Although muscle mass and strength are significantly correlated, muscle strength may be regulated independently from muscle mass. Strength is a functional indicator of overall muscle "quality." Sarcopenia syndrome, which refers to the age-related decline in muscle mass, is characterized by loss of strength, power, speed, and endurance, as well as by poor balance, resulting in an increased potential for bone injury due to falls. According to a model presented by Nair, adjustment for the age-related decline in muscle mass reveals an apparent functional impairment, which points to a disturbance in the quality of muscle with age and inactivity.

Endurance. Evaluation of endurance with measures such as maximal oxygen consumption reveals gender differences; however these differences



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