synthesis. The tissue concentration of a specific protein is determined by the balance between protein breakdown and protein synthesis. Nutrient supply to the muscle tissue (for ATP production) and the removal of metabolic by-products (e.g., carbon dioxide) are dependent on uninterrupted and dynamic circulatory systems. All of these individual processes are controlled by regulatory mechanisms, which include circulating and local levels of hormones and substrates, which in turn are influenced by the physiological state of the individual in terms of age, gender, nutritional status, exercise, and chronic or acute illness. Using aging as an example, this brief review outlines some of the control mechanisms and other biological factors involved in the regulation of muscle mass and function.

Aging can be described as a model in which many of the regulatory mechanisms are disrupted, resulting in functional disabilities involving both locomotive and metabolic aspects.

In spite of vigorous attempts by individuals to avert the physical impact of age, all population-based studies show a relentless loss of muscle mass and strength with aging. In addition, increased muscle fatigability and decline in endurance capacity substantially retard the functional capabilities of the elderly population. Some evidence indicates that this phenomenon is not solely due to a loss of muscle quantity, but also to an impairment of muscle quality (Reed et al., 1991; Rooyackers et al., 1996) (Figures 6-2 and 6-3). The combination of loss

FIGURE 6-2 Muscle strength is not solely determined by muscle mass. With advancing age, a continuous loss of muscle efficiency occurs, indicating that muscle quality is declining. This study normalized muscle strength, quadriceps isokinetic strength, and leg extension for regional muscle mass (measured by Dual Photon X-ray) and showed a progressive decline with aging (P < 0.05-0.01). * Significant difference (P< 0.01) from young age group; § Significant difference (P< 0.05) from middle age group. Source: Adapted from Balagopal et el. (1997).