Declines in skeletal muscle mass occur during normal aging, however, when these declines become pathologically significant as sarcopenia, the loss of muscle strength results in debilitating loss of physical function, independence and ultimately mortality (Hurley, 2000, Rantanen, 1999).
Muscle mass is the major determinant of physical strength, thus the loss of lean mass is thought to be a major contributor to disability in old age. Sarcopenia is also associated with several health problems including falls, fractures, insulin resistance, diabetes, and obesity (Hurley, 2000).
The estimated cost of sarcopenia is $300 billion annually (Booth, 2000) which will only increase with an aging population.
RET interventions may be the only accepted intervention presently to delay the onset or reverse sarcopenia and the associated disabilities (Hurley, 2000). However, the factors contributing to the inter-individual variation in skeletal muscle mass and responses to RET are largely unknown.
Muscle loss occurs at a rate of about 1% per year after the age of 40.
When muscle loss crosses a roughly defined line to pathological significance, the condition, known as sarcopenia, results in declines in overall strength and physical function and the ability to perform tasks of daily living.
The prevalence of sarcopenia has been estimated at 16% in men and 12% in women, aged 70-79, based on appendicular mass adjusted for height squared.
Class II sarcopenia, defined as a skeletal muscle mass/body mass greater than 2 standard deviations below the mean of young adults, has a prevalence of 8% in men and 12% in women resulting in a 2-3 times greater frequency of functional impairment.
Adequate strength is critical for maintaining mobility and avoiding of disability in late life.
Muscular strength is independently associated to and may explain as much as 25% of the variance in overall functional ability (Buchner, 1991).
The prevalence of difficulty with mobility, defined as walking ¼ mile, is 15-20% (Ettinger, 1994) in older adults, limiting participation in community activities and leading to future difficulty with more basic self care (Janssen, 2002; Jagger, 2001; Guralnick, 1993).
Resistance exercise training is a safe and efficacious intervention for sarcopenia by increasing both muscle mass and strength (Fiatarone, 1990, Hurley, 2000, Roth, 2000).
In fact, muscle remains quite responsive to resistance exercise training well into late old age, and the degree of strength gained with resistance exercise training exceeds the gains in mass (Hurley, 1995, Treuth, 1994; Tracy, 1999).
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