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Sarcopenia (from the Greek meaning "poverty of flesh") is the degenerative loss of skeletal muscle mass (0.5-1% loss per year after the age of 25), quality, and strength associated with aging. Sarcopenia is a component of the frailty syndrome. It can be differentiated from cachexia in that cachexia includes malaise and is secondary to an underlying pathosis (such as cancer), whereas sarcopenia may occur in healthy people and does not necessarily include malaise.
Sarcopenia is not a disease or a syndrome, and it is not always even a medical sign, because the degree to which it is normal (physiologic) versus abnormal (pathologic) is not determined. As of 2009[update] there is no generally accepted definition of sarcopenia in the medical literature.
The European Working Group on Sarcopenia in Older People (EWGSOP) developed a clinical definition and consensus diagnostic criteria for age-related sarcopenia, using the presence of both low muscle mass and low muscle function (strength or performance).
Sarcopenia is characterized first by a muscle atrophy (a decrease in the size of the muscle), along with a reduction in muscle tissue quality, characterized by such factors as replacement of muscle fibres with fat, an increase in fibrosis, changes in muscle metabolism, oxidative stress, and degeneration of the neuromuscular junction and leading to progressive loss of muscle function and frailty. Sarcopenia is determined by two factors: initial amount of muscle mass and rate at which aging decreases muscle mass. Due to the loss of independence associated with loss of muscle strength, the threshold at which muscle wasting becomes a disease is different pathologically from person to person.
Simple circumference measurement does not provide enough data to determine whether or not an individual is suffering from severe sarcopenia. Sarcopenia is also marked by a decrease in the circumference of distinct types of muscle fibers. Skeletal muscle has different fiber-types, which are characterized by expression of distinct myosin variants. During sarcopenia, there is a decrease in "type 2" fiber circumference (Type II), with little to no decrease in "type I" fiber circumference (Type I), and deinervated type 2 fibers are often converted to type 1 fibers by reinnervation by slow type 1 fiber motor nerves.
Satellite cells are small mononuclear cells that abut the muscle fiber. Satellite cells are normally activated upon injury or exercise. These cells then differentiate and fuse into the muscle fiber, helping to maintain its function. One theory is that sarcopenia is in part caused by a failure in satellite cell activation.
In evolutionary history, there was a selection advantage over larger muscle mass in the late Paleolithic, resulting in an environmental mismatch between the human genome and moden sedentary lifestyle.
A working definition for diagnosis was proposed in 1998 by Baumgartner et al which uses a measure of lean body mass as determined by dual energy X-ray absorptiometry (DEXA) compared to a normal reference population. His working definition uses a cut point of 2 standard deviations below the mean of lean mass for gender specific healthy young adults. 
This methodology shows it is predictive of negative outcomes and it is also a method familiar to most clinicians. This latter point is especially true for those that treat the geriatric population, given its similarity to the 1996 World Health Organization (WHO) methodology for definitive diagnosis of osteoporosis, which also uses DEXA, but uses a measure of lean mass rather than bone mineral density (BMD). DEXA is widely used already in clinical settings in developed countries to identify and monitor severity of osteoporosis. And the degree of sarcopenia can be measured using DEXA in patients being evaluated for osteoporosis, at the same time with the same scan, with no added cost or radiation exposure to the patient.[medical citation needed]
Since Baumgartner’s working definition first appeared, some consensus groups have refined the definition, including the recent joint effort of the European Society on Clinician Nutrition and Metabolism (ESPEN) Special Interest Groups (SIG) on geriatric nutrition and on cachexia-anorexia in chronic wasting diseases. Their consensus definition is:
Lack of exercise is thought to be a significant risk factor for sarcopenia. Even highly trained athletes experience the effects of sarcopenia. Even Master class athletes who continue to train and compete throughout their adult life, exhibit a progressive loss of muscle mass and strength, and records in speed and strength events decline progressively after age 30.
Exercise is of interest in treatment of sarcopenia; there is evidence of increased ability and capacity of skeletal muscle to synthesize proteins in response to short term resistance exercise.
There may be agents approved for the treatment of sarcopenia such as growth hormone. Growth hormone has been shown to heighten muscle growth and prevent sarcopenia at all human stages of life by both acting as a hypertrophic and antioxidant.12
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Due to the lessened physical activity and increased longevity of industrialized populations, sarcopenia is emerging as a major health concern. Sarcopenia may progress to the extent that an older person may lose his or her ability to live independently. Furthermore, sarcopenia is an important independent predictor of disability in population-based studies, linked to poor balance, gait speed, falls, and fractures. Sarcopenia can be thought of as a muscular analog of osteoporosis, which is loss of bone, also caused by inactivity and counteracted by exercise. The combination of osteoporosis and sarcopenia results in the significant frailty often seen in the elderly population.
There remain many opportunities for further refinement of reference populations by ethnic groups, and to further correlate the degrees of severity of sarcopenia to overt declines in functional performance (preferably using verified functional tests), as well as incidence of hospitalization admissions, morbidity and mortality. The body of research points toward severe sarcopenia being predictive of negative outcomes, similar to those already shown to exist with the Frailty syndrome, as defined by the criteria set forth in 2001 by Fried et al.