Sarcopenia is a term utilized to define the loss of muscle mass and strength that occurs with aging. Progressive muscle wasting occurs with aging. The prevalence of clinically significant sarcopenia is estimated to range from 8.8% in young old women to 17.5% in old old men. Sarcopenia is determined by two factors: the initial amount of muscle mass and the rate at which it declines with age. The rate of muscle loss with age appears to be fairly consistent, approximately 1%–2% per year past the age of 50 years. A loss of 30% of reserve capacity limits normal function, whereas a decrease of 70% results in failure of that system. After reaching a peak in early adult years, skeletal muscle mass gradually declines beginning at about age 45. Loss of skeletal muscle mass below a critical threshold may lead to functional impairment and physical disability. Advanced skeletal muscle loss may affect quality of life, the need for supportive services, and ultimately the need for long-term care in older persons.
Two key observations associated with sarcopenia include a loss of skeletal muscle fiber number and a change in the cross-sectional area (CSA) of the remaining fibers. Various mechanisms have been put forth to explain the change in total muscle mass observed including: (a) a lack of regular physical activity (‘‘use it or lose it’’); (b) a change in protein metabolism (a deficit between protein synthesis versus degradation); (c) alterations in the endocrine milieu (decrease in growth hormone (GH) and testosterone and an increase in cortisol and cytokines); (d) a loss of neuromuscular function); (e) altered gene expression; (f) apoptosis; other factors may also contribute in part to sarcopenia. Approximately 65%–80% of the amino acids are resynthesized into proteins during muscle protein turnover, whereas about 20%–35% of amino acids have to be supplied through the diet, resulting in the average turnover of muscle of 8%–12% per day. For muscle to maintain its mass, the rate of protein synthesis must be in balance with the rates of degradation to amino acids in combination with dietary absorption maintaining the difference in amino acid utilization. For sarcopenia to occur, only small imbalances between synthesis and degradation over many years are necessary to eventually result in a significant loss of muscle mass.
Age-related reductions in muscle mass and strength are also accompanied by a reduction in motor unit (MU) number and histological changes (angulated fibers, fiber-type clumping), which are suggestive of neuronal remodeling in elderly people. The available data suggest that the pathogenesis of sarcopenia is multifactorial. Intrinsic aging changes in the muscle and nerve represent one set of causes, but poor nutritional status, a decline in anabolic hormones and cytokines, and atherosclerosis all appear to accelerate the process.