Mechanisms of sarcopenia of aging.

Age-related sarcopenia is characterized by decreased muscle mass and muscle strength, and increased muscle fatigability. A decrease in synthesis rates of mixed muscle proteins (average of all muscle proteins), myosin heavy chain (responsible for adenosine triphosphatase action) and mitochondrial proteins (site of adenosine triphosphate production) have been described with aging. Most of these changes start by middle age, thus contributing to the progressive decline in muscle size and function. How closely these changes are related to lifestyle and the decline in several hormones, particularly growth hormone, insulin-like growth factor-I, testosterone and dehydroepiandrosterone, remains to be clearly defined. The ability to measure the specific effects of different types of exercise training on muscle protein metabolism has only recently become available. Thus, future investigations will continue to improve our understanding of protein metabolism in aging skeletal muscles. The development and assessment of successful countermeasures to age-related sarcopenia will hopefully follow these discoveries.