The Meaning of Muscle Mass for Health, Disease, and Strength Exercises

Abstract

Skeletal muscle is the largest organ of the mature body having major contributions to endogenous amino acid supply, insulin-dependent glucose removal and total body energy expenditure. Muscle mass is a determinant of metabolic homeostasis, physical strength and daily living activities. Higher lean mass equates to higher nutritional reserve and strength whereas lower muscle mass (sarcopenia) is a major contributor to disability and increased mortality. The definition of ­sarcopenia is arbitrary and its diagnosis is dependent on the methodology of muscle mass assessment, reference groups and cut off points. Methods used to assess muscle mass such as anthropometry, BIA or DXA are unable to distinguish aqueous and non aqueous components of the muscle mass whereas this is accomplished with the more expensive CT or MRI. Data are referred as kg of muscle mass, % of the body mass or correcting either appendicular or total muscle mass for height (kg/m²). The most used data comes from either DXA or BIA assessment in male and female populations of Rosseta, New Mexico (NM Elder Health Survey) and NHANES III studies with cut off points at –  2 SD for both genders. There is no cut off for hypertrophic-muscle mass. There are also no consensual criteria for defining sarcopenic-obesity or the fat frailty, the worse condition with regard to the difficulties of performing physical tasks. For this purpose muscle mass must be associated with some type of functional test (strength and power) to assess muscle quality. This chapter highlights the roles of muscle mass, its measurements and variations in health and diseases, and the lack of data for correctly interpreting muscle mass and function in the presence of obesity and strength exercises.