[ 12 , 13 ] originally defined sarcopenia as being two standard deviations below the normal appendicular muscle mass divided by height squared , numerous groups have examined the prevalence of low muscle mass .
multiple techniques have been used to measure muscle mass , e.g. , dual - energy x - ray absorptiometry ( dexa ) , computed tomography , mri , ultrasound , bioelectrical impedance , and anthropometry . on the average ,
513  % of older persons over 60  years of age have low muscle mass , with the prevalence increasing to as high as 50  % in persons over the age of 80  years [ 11 , 14 ] .
normal aging is associated with approximately a 1  % loss of muscle from 30  years of age , and this loss tends to accelerate after the age of 70  years . in a recent study ,
.   found that about 20  % of community - dwelling persons in italy had low muscle mass .
in korea , the prevalence of sarcopenia using the baumgartner criteria was 0.8  % in women and 1.3  % in men over 60  years of age . in barcelona ,
low lean mass was present in 33  % of elderly women and 10  % of males .
in taiwan , low muscle mass was present in 2.5  % of community - dwelling women and 5.4  % of males .
two medical conditions , stroke and hip fracture , rapidly lead to an increase in muscle loss [ 22 , 23 ] .
this appears to be due to disuse and inflammation , and in the case of stroke , denervation .
in addition , persons with diabetes mellitus have accelerated muscle loss [ 24 , 25 ] .
persons with muscle loss in combination with excess fat are considered to have obese sarcopenia [ 2628 ] .
using the european working group on sarcopenia in older people ( ewgsop ) definition , 4.6  % of males and 7.9  % of women in hertfordshire had sarcopenia . in japan ,
21.8  % of men and 22.9  % of women aged 65 to 89  years had sarcopenia . in a population of persons
80  years and older , 12.5  % had sarcopenia by the ewgsop definition . in nursing home residents ,
sarcopenic individuals had a greater than threefold increase in falls [ 21 , 37 ] .
sarcopenia has also been shown to prospectively predict mobility and instrumental activities of daily living ( iadl ) disability .
the prevalence of sarcopenia was slightly less common using the international working group on sarcopenia ( iwgs ) criteria compared to the ewgsop .
the sarcopenia , cachexia and wasting disorders ( scwd ) definition was found to predict adl and iadl difficulties , frailty , and mortality in a longitudinal study .
the foundation for the national institutes of health ( fnih ) criteria were based on developing cutoffs using a variety of large epidemiological studies .
these criteria are more restrictive with only 1.3  % of men and 2.3  % of women being defined as having sarcopenia .
while there was a strong negative percent agreement with the other definition , the positive percent agreements were low ranging from 5 to 32  % .
there is a need to compare the fnih criteria to other definitions to determine which has the best predictive ability .
recently , it has been determined that fracture risk can be determined almost as accurately by the frax questions as by measuring bone mineral density .
this raises the question of whether or not a simple questionnaire can be used to identify persons with sarcopenia .
the sarc - f questionnaire has been validated in two published studies ( table  3 ) [ 44 , 45 ] .
found that sarc - f had comparable predictive activity to ewgsop , iwgs , and the asian working group for sarcopenia .
like the others in this asian population , it had modest predictive value for 4-year physical limitation .
cao et al .   have also provided evidence for validity of the sarc - f .
the physical frailty phenotype as originally defined by fried et al .   has been demonstrated to be highly predictive of poor outcomes [ 47 , 48 ] . a simple questionnaire 
the frail ( table  2)has been developed and shown to be equally predictive of poor outcomes [ 4953 ] .
while sarcopenia is clearly a major component of frailty , they are not identical conditions .
overlap between sarcopenia and frailty ranges from 50 to 70  % .table 2comparison of the brief screens for sarcopenia and frailty(a ) sarc - f(b ) frailcomponentquestionscoringstrengthhow much difficulty do you have in lifting and carrying 10  lb?none  =  0fatiguesome  =  1resistance ( can you climb a flight of stairs?)a lot or unable  =  2aerobic ( can you walk a block?)assistance in walkinghow much difficulty do you have walking across a room?none  =  0illness ( > 5)some  =  1loss of weight ( 5% in 6  months)(three or more positive answers = frail ; one or two positive answers = prefrail)a lot , use aids , or unable  =  2rise from a chairhow much difficulty do you have transferring from a chair or bed?none  =  0some  =  1a lot or unable without help  =  2climb stairshow much difficulty do you have climbing a flight of 10 stairs?none  =  0some  =  1a lot or unable  =  2fallshow many times have you fallen in the past year?none  =  013 falls  =  14 falls  =  2 comparison of the brief screens for sarcopenia and frailty
these are listed in table  3 . to these needs to be added the measurement of motor unit number index , which can be used to assess the number and the size of the motor units .
this is important as loss of motor unit input to muscle is an important cause of sarcopenia in at least half of older persons.table 3biomarkers for sarcopeniabiomarkeragingexercisecommentscreatine kinasealdolase ( a)correlates with walking speed in oldcoenzyme qmore strongly correlated with ffmmlc1troponin t?creatine ( u)correlates with muscle massmyoglobincreatinine ( u ) ( ?
/cystatin c)correlates with muscle j - shaped curve with functionn - terminal propeptide iii collagenincreased with testosteronec - terminal agrin fragmentcorrelates with degeneration of the neuromuscular junction biomarkers for sarcopenia
2 ) [ 57 , 58 ] . disuse coupled with aging is the major underlying cause : poor blood flow to muscle , especially the muscle capillaries due to a decline in nitric oxide production , is another important age - related cause of sarcopenia . aging is associated with an increase in mitochondrial abnormalities leading to damage to the mitochondrial membrane permeability pore and apoptosis.fig .
2the causes of sarcopenia the causes of sarcopenia aging is associated with a physiological anorexia of aging that leads to weight loss [ 59 , 60 ] .
weight loss results in a 75  % loss of fat and a 25  % loss of muscle and bone .
the increase of fat during weight regain is one of the major causes of sarcopenic obesity .
the age - related loss of motor neuron end plates is a major component of sarcopenia .
it leads not only to muscle wasting but also to a decrease in muscle function .
loss of anabolic hormones , such as testosterone , dhea , growth hormone , and insulin - growth factor 1 , occurs with aging . of these
, testosterone has been shown to be the hormone that most closely determines the decline in muscle mass and strength .
testosterone is important not only for protein synthesis but also for the maintenance of satellite cells [ 62 , 63 ] .
insulin resistance , which occurs with aging and obesity , plays an important role in decreasing available glucose and protein for muscle anabolism .
obesity and disease lead to an increase in proinflammatory cytokines ( e.g. , interleukin-6 , interleukin-1 , and tumor necrosis factor alpha ) .
it is now clear that resistance exercise is the primary therapeutic strategy to prevent and reverse sarcopenia [ 6670 ] .
there is evidence that leucine - enriched essential amino acid supplementation will increase muscle mass and probably function [ 7275 ] .
vitamin d has been shown to enhance muscle function in persons with low muscle function ( < 50  nmol ) [ 76 , 77 ] .
there is a small amount of data suggesting that testosterone will increase muscle mass and strength and may improve function in older frail persons with hypogonadism [ 7881 ] .
selective androgen receptor modulators ( sarms ) have shown some promise in increasing muscle mass and stair climb .
a number of antibodies that modulate myostatin and the activin ii receptor are in clinical trials .
ghrelin agonists , which increase food intake and release growth hormone , are also under evaluation .
loss of muscle mass is commonly associated with loss of bone , making these individuals at high risk for hip fractures .
persons with muscle loss develop accelerated loss of muscle mass and strength when they develop a variety of diseases such as heart failure , chronic obstructive pulmonary disease , and renal failure [ 8689 ] . like osteoporosis
it is time for physicians to screen for sarcopenia and provide treatment for it  at a minimum resistance exercise and protein and vitamin d supplementation .
 sarcopenia is now defined as a decline in walking speed or grip strength associated with low muscle mass . 
 sarcopenia leads to loss of mobility and function , falls , and mortality . 
 sarcopenia is a major cause of frailty , but either condition can occur without the other being present . 
 sarcopenia is present in about 5 to 10  % of persons over 65  years of age . 
 it has multiple causes including disease , decreased caloric intake , poor blood flow to muscle , mitochondrial dysfunction , a decline in anabolic hormones , and an increase in proinflammatory cytokines . 
 basic therapy includes resistance exercise and protein and vitamin d supplementation . 
 there is now a simple screening test available for sarcopenia  sarc - f . 
 all persons 60  years and older should be screened for sarcopenia and treated when appropriate . 
Prevalence of sarcopenia
Validation of the new sarcopenic definitions
Can sarcopenia be predicted by a questionnaire without measurements?
Differentiating between frailty and sarcopenia
Sarcopenia and biomarkers
Pathophysiology of sarcopenia
Therapeutic approaches
Conclusion