one of the dramatic changes associated with human aging is the progressive decline of skeletal muscle mass . several prospective studies have suggested that muscle mass decreases by approximately 6% per decade after mid - life . interestingly , results from the health aging , and body composition ( abc ) study showed that the decreased muscle strength is predominantly due to a lower muscle mass . there are significant differences among individuals in peak muscle mass , the age at which muscle loss begins , and the amount of muscle that is lost over time . sarcopenia ,  which originated from the greek words sarx ( flesh ) and penia ( loss ) , to refer to the age - related loss of skeletal muscle mass . at the cellular level , sarcopenia is accompanied by a loss of innervation and adaptive changes in the proportions of slow and fast motor units , as well as in the cross - sectional area of muscle fibers . primary sarcopenia is the term used to define sarcopenia that is caused by aging itself , whereas secondary sarcopenia describes sarcopenia that is caused by disuse ( immobility or physical inactivity ) , disease ( advanced organ failure , malignancy , neurodegenerative , or endocrine diseases ) , and inadequate nutrition . sarcopenia is closely associated with frailty , physical disability , hospitalization , osteoporosis , osteoarthritis , and even mortality . several different definitions of sarcopenia have been suggested , although no consensus definition has been adopted . in 1998 , baumgartner et al . first defined sarcopenia as an appendicular skeletal muscle mass ( asm ) ( kg)/height ( m ) of less than two standard deviations ( sds ) below the mean of a young reference group . adopted the skeletal muscle mass index ( smi = skeletal muscle mass / body mass  100 ) to establish the prevalence of sarcopenia in older americans . they considered subjects with an smi within one to two sds of young adult values to have class i sarcopenia , and those with an smi below two sds of young adult values to have class ii sarcopenia . in the health abc study , newman et al .   proposed an alternative definition of sarcopenia using appendicular lean mass ( alm ) adjusted for height and body fat mass ( residuals ) . they indicated that baumgartner s definition ( asm / height ) is strongly correlated with body mass index ( bmi ) , and therefore identifies fewer obese individuals as sarcopenic . to examine the prevalence of sarcopenia in korean adults and explore its impact on health outcomes , we previously established a cohort study called the korean sarcopenic obesity study ( ksos ) . the prevalence of sarcopenia differed according to the definition applied , as well as age and sex . since then , the definition of sarcopenia has evolved to highlight muscle strength and physical performance . the european working group on sarcopenia in older people ( ewgsop ) defined sarcopenia as the presence of both low muscle mass and low muscle strength or performance . recently , the foundation for the national institutes of health ( fnih ) sarcopenia project suggested using alm with adjustment for bmi to define low muscle mass . using a dataset from nine large observational studies containing more than 25,000 participants , the fnih sarcopenia project established alm / bmi ratio cutoff values of < 0.789 for men and recently , kim et al .   compared skeletal muscle mass indices and described their clinical implications . further studies might be needed to compare the definitions of sarcopenia with regard to their impact on disability , cardiometabolic risk profiles , and mortality . various methods can be used to assess muscle mass and strength ( table 1 ) . anthropometric measurements , such as mid - upper arm circumference , calf circumference , and skin fold thickness , are not recommended for diagnosing sarcopenia since they are prone to error . computed tomography ( ct ) and magnetic resonance imaging are able to effectively distinguish fat from other soft tissues , which makes these the standard techniques for evaluating muscle mass in research . however , limited access , the high cost , and the risk of radiation ( with ct ) preclude the wider use of these techniques in clinical practice . therefore , dual energy x - ray absorptiometry ( dxa ) is the preferred method for correctly evaluating body composition ; it is widely used to assess muscle mass in research studies because of good precision and safety with relatively low radiation . therefore , measuring alm using dxa has become a standard criterion in most current sarcopenia definitions . bioimpedance analysis ( bia ) is an inexpensive , easy to use , and reproducible method that is considered a portable alternative to dxa . however , neither dxa nor bia can distinguish between extracellular and intracellular water , making these techniques prone to error depending on the hydration status of the patient . muscle strength is commonly evaluated using handgrip strength , which is an easy , reliable , and inexpensive method for identifying elderly adults at risk for disability . based on the ewgsop definition , cutoffs for grip strength are < 20 kg for women and < 30 kg for men . in contrast , the fnih sarcopenia project established cutoffs of < 16 kg for women and < 26 kg for men . knee flexion techniques are appropriate for research purposes ; however , their use in clinical practice is limited by the need for specific machines . the short physical performance battery ( sppb ) is used in both clinical and research settings to measure physical performance . it combines gait speed , chair - rise time , and balance assessment to generate a standard measurement . usual gait speed , which is part of the sppb , might be adopted as a single parameter to provide a predictive value for disability . previous studies have demonstrated that a walking speed of less than either 1.0 or 0.8 m / sec is associated with adverse outcomes . timed get - upand - go examines the time needed to accomplish a series of functionally critical tasks ; it can also serve as a performance measurement . several underlying mechanisms have been linked to the development of sarcopenia , although not all have been fully elucidated . the relative contribution of the different mechanisms may vary over time in an individual with sarcopenia . most mechanisms of sarcopenia are also associated with visceral obesity , which may lead to a vicious cycle of intricate interactions among risk factors . .   demonstrated that insulin resistance is related distinctly to the different diagnostic criteria for sarcopenia ; it is better reflected by relative muscle mass than by absolute muscle mass or muscle strength . aging is related to changes in a variety of hormones , including testosterone , estrogen , growth hormone , insulin - like growth factor 1 , and corticosteroids . , previous studies have suggested that sarcopenia is an inflammatory state that is driven by proinflammatory cytokines and oxidative stress . oxidative stress modulates the expression of transcription factors , such as nuclear factor-b , which enhances proteolytic pathways and increases the production of proinflammatory cytokines . tumor necrosis factor  impairs protein synthesis in skeletal muscle by altering translation initiation , which may contribute to sarcopenia . in a prospective , population - based study , higher levels of interleukin 6 and c - reactive protein were associated with a greater decline in muscle strength . another pivotal factor in the regulation of skeletal muscle mass is myostatin , also known as growth / differentiation factor 8 , which inhibits muscle cell growth and differentiation . sarcopenia is independently associated with insulin resistance , and in the ksos study , type 2 diabetes was independently associated with an increased risk of sarcopenia . diabetes has been suggested to be an intermediate step in the development of frailty in individuals with sarcopenia . furthermore , we previously found that individuals with a lower muscle mass have an increased risk of nonalcoholic fatty liver disease , which is now recognized as a feature of metabolic syndrome . the coexistence of sarcopenia and metabolic syndrome further aggravates the risk of cardiovascular risk factors such as type 2 diabetes , hypertension , and hyperlipidemia in adult japanese women . previous prospective studies have consistently shown a relationship between low muscle mass or muscle strength and an increased risk of mortality [ 39 - 41 ] . in the british regional heart study , atkins et al .   reported that sarcopenia is associated with greater cardiovascular mortality and allcause mortality . hirani et al .   showed that sarcopenia , as defined by the fnih criteria , is associated with an increased risk of mortality , disability , and institutionalization in community - dwelling older men . several studies have suggested that deteriorated muscle strength is a more important risk factor for mortality than a decline in muscle mass . obesity is an important health threat that is a major risk factor for metabolic and cardiovascular morbidity and mortality . the prevalence of obesity in middle - aged and older adults has doubled since 1980 , and it continues to increase worldwide . the major age - related changes in body composition include an increase in body fat and a decline in skeletal muscle , although bmi may remain relatively unchanged . sarcopenic obesity was first defined by baumgartner   as the co - presence of sarcopenia and obesity , as measured using dxa . we subsequently introduced the ratio of visceral fat to thigh muscle area ( vmr ) , as measured using ct , as a single indicator of sarcopenic obesity . the complex interplay of common pathophysiological mechanisms , such as increased proinflammatory cytokines , oxidative stress , insulin resistance , and hormonal changes and decreased physical activity , underlie the close relationship between sarcopenia and obesity ( fig . we reported that the homoeostasis model assessment of insulin resistance ( homa - ir ) and vitamin d levels are independently associated with sarcopenic obesity in men , whereas homa - ir and high - sensitivity c - reactive protein are associated with sarcopenic obesity in women . a vicious cycle may exist between the accumulation of ectopic fat and the loss of skeletal muscle mass since they have a reciprocal influence on each other . sarcopenia reduces physical activity , which leads to decreased energy expenditure and increases the risk of obesity . in contrast , an increase in visceral fat induces inflammation , which contributes to the development of sarcopenia . in our longitudinal study , visceral obesity was independently associated with the future loss of skeletal muscle mass after adjusting for confounding factors . thus , it has been hypothesized that sarcopenic obesity may have a greater impact on metabolic diseases and cardiovascular morbidity and mortality than either sarcopenia or obesity alone . recent studies have emphasized the influence of sarcopenic obesity on cardiometabolic risk and health outcomes . several cross - sectional studies in elderly koreans have demonstrated that individuals with sarcopenic obesity have worse cardiovascular risk profiles , including hyperglycemia , hypertension , dyslipidemia , insulin resistance , and lower cardiorespiratory fitness [ 55 - 57 ] . similarly , a taiwanese study showed that sarcopenic obesity is associated with the highest risk of metabolic syndrome . furthermore , sarcopenia exacerbated obesity - associated dysglycemia and insulin resistance in a cross - sectional study from the national health and nutrition examination survey iii ( nhanes iii ) . however , there have been conflicting results regarding whether patients with sarcopenic obesity have the worst risk profiles . several cross - sectional studies have reported that obese individuals have more cardiovascular risk factors than those with sarcopenic obesity . a limited number of studies have investigated the effects of sarcopenia and obesity on cardiovascular disease ( cvd ) and mortality . stephen and janssen   showed that sarcopenic obesity is associated with increased cvd risk based on muscle strength but not muscle mass . in the british regional heart study , subjects with sarcopenic obesity had a significantly higher risk of mortality compared to nonsarcopenic , nonobese subjects . in contrast , in a study using data from the nhanes iii study , older women with sarcopenia had a higher risk of all - cause mortality , independent of obesity . a recent meta - analysis demonstrated that sarcopenic obesity is associated with a 24% increase in the risk of all - cause mortality compared to patients without sarcopenic obesity , particularly in men . although bmi is a simple estimator of obesity , it can not fully reflect muscle mass and body fat . accumulating evidence underscores the need to consider muscle function and mass when evaluating the risk of obesity in elderly people . visceral fat and muscle mass seem to have opposing influences on cardiometabolic morbidity and mortality . sarcopenic obesity is a relatively novel concept that has become increasingly important in the aging population . there has been some evidence that sarcopenic obesity may be associated with an increased risk of mortality and cardiovascular risk factors compared to sarcopenia or obesity alone . however , several different definitions of sarcopenia limit the clinical application of sarcopenia and sarcopenic obesity with regard to metabolic disorders and cvd . a consensus definition of sarcopenia is needed to promote the standardized diagnosis and management of sarcopenic obesity . furthermore , in addition to reducing body fat , increasing muscle mass and strength is required to promote healthy aging .
sarcopenia is an age - associated loss of muscle mass and decline in muscle strength ; it is common in older adults and is associated with significant morbidity and mortality . despite its prevalence , there is currently no universally adopted definition of sarcopenia . 
 in addition to low muscle mass measurements , recent research has recognized the importance of muscle strength and physical performance . 
 aging induces changes in body composition , such as an increase in visceral fat and reduced muscle mass . 
 recently , the new concept of sarcopenic obesity has emerged , reflecting a combination of sarcopenia and obesity . 
 the rapidly increasing prevalence and serious consequences of sarcopenic obesity are recognized as a critical public health risk in the aging society . 
 sarcopenia and obesity share several pathophysiological mechanisms , and they may potentiate each other . the present paper reviews the definitions and techniques used to measure sarcopenia , as well as the health outcomes of sarcopenic obesity . 
 it also highlights the role of diminished muscle mass and strength in cardiometabolic disease mortality . 
 additional research may be needed to promote the identification and management of sarcopenia and sarcopenic obesity in the elderly population .