J Cachexia Sarcopenia Muscle (2014) 5:9–18
`DOI 10.1007/s13539-014-0130-5
`
`REVIEW
`
`Assessing skeletal muscle mass: historical overview
`and state of the art
`
`Steven B. Heymsfield & Michael Adamek &
`M. Cristina Gonzalez & Guang Jia & Diana M. Thomas
`
`Received: 14 January 2014 /Accepted: 22 January 2014 / Published online: 15 February 2014
`# Springer-Verlag Berlin Heidelberg 2014
`
`Abstract
`Background Even though skeletal muscle (SM) is the largest
`body compartment in most adults and a key phenotypic mark-
`er of sarcopenia and cachexia, SM mass was until recently
`difficult and often impractical to quantify in vivo. This review
`traces the historical development of SM mass measurement
`methods and their evolution to advances that now promise to
`provide in-depth noninvasive measures of SM composition.
`Methods Key steps in the advancement of SM measurement
`methods and their application were obtained from historical
`records and widely cited publications over the past two cen-
`turies. Recent advances were established by collecting infor-
`mation on notable studies presented at scientific meetings and
`their related publications.
`Results The year 1835 marks the discovery of creatine in meat
`by Chevreul, a finding that still resonates today in the D3-
`creatine method of measuring SM mass. Matiegka introduced
`an anthropometric approach for estimating SM mass in 1921
`with the vision of creating a human “capacity” marker. The
`1940s saw technological advances eventually leading up to
`the development of ultrasound and bioimpedance analysis
`methods of quantifying SM mass in vivo. Continuing to seek
`an elusive SM mass “reference” method, Burkinshaw and
`
`S. B. Heymsfield (*) : M. Adamek : G. Jia
`Pennington Biomedical Research Center, Louisiana State University,
`6400 Perkins Rd, Baton Rouge, LA 70808, USA
`e-mail: Steven.Heymsfield@pbrc.edu
`
`Cohn introduced the whole-body counting-neutron activation
`analysis method and provided some of the first detailed re-
`ports of cancer cachexia in the late 1970s. Three transforma-
`tive breakthroughs leading to the current SM mass reference
`methods appeared in the 1970s and early 1980s as follows: the
`introduction of computed tomography (CT), photon absorpti-
`ometry, and magnetic resonance (MR) imaging. Each is ad-
`vanced as an accurate and/or practical approach to quantifying
`whole-body and regional SM mass across the lifespan. These
`advances have led to a new understanding of fundamental
`body size-SM mass relationships that are now widely applied
`in the evaluation and monitoring of patients with sarcopenia
`and cachexia. An intermediate link between SM mass and
`function is SM composition. Advances in water-fat MR im-
`aging, diffusion tensor imaging, MR elastography, imaging of
`connective tissue structures by ultra-short echo time MR, and
`other new MR approaches promise to close the gap that now
`exists between SM anatomy and function.
`Conclusions The global efforts of scientists over the past two
`centuries provides us with highly accurate means by which to
`measure SM mass across the lifespan with new advances
`promising to extend these efforts to noninvasive methods for
`quantifying SM composition.
`
`Keywords Body composition . Nutritional assessment .
`Sarcopenia . Cachexia
`
`M. C. Gonzalez
`Post-graduation Program in Health and Behavior, Catholic
`University of Pelotas, Pelotas, Rio Grande do Sul, Brazil
`
`1 Introduction
`
`G. Jia
`Department of Physics and Astronomy, Louisiana State University,
`Baton Rouge, LA, USA
`
`D. M. Thomas
`Department of Mathematics, Montclair State University, Montclair,
`NJ, USA
`
`Sarcopenia, sarcopenic obesity, and cachexia all share a com-
`mon phenotype: relative reductions in the amount, composi-
`tion, and function of skeletal muscle [1]. Although often the
`largest compartment at the tissue-organ level of body compo-
`sition [2], quantifying the total mass and composition of
`skeletal muscle in living subjects proved remarkably difficult
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`J Cachexia Sarcopenia Muscle (2014) 5:9–18
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`over the past century compared to fat, bone mineral, body
`fluids, and other relatively large compartments of clinical
`interest.
`The complexity of measuring skeletal muscle mass seems
`hard to imagine today when we have several easily applied
`and widely available measurement techniques that provide
`accurate regional and whole-body estimates from birth on-
`ward. We review here the history of this struggle and the
`recent advances that provide us with powerful tools to study
`the disorders that involve loss of skeletal muscle mass and
`related function. This remarkable journey unfolds over almost
`two centuries, covers several continents, and represents the
`imaginative contributions of scientists across the globe.
`
`2 Foundation discoveries
`
`2.1 A muscle-urine connection
`
`While it is hard to precisely identify the first historical step in
`quantifying skeletal muscle mass, the likely initial discovery
`of a measurement approach belongs to the renowned French
`chemist and polymath Michel-Éugène Chevreul, who in 1835
`first identified a chemical compound in meat extracts that he
`named creatine, the Greek word for flesh [3]. Otto Folin,
`working almost two decades later at Harvard, transformed
`the field by developing a highly sensitive measurement meth-
`od in 1905 and then going on to establish the “creatinine
`coefficient” as a focus of research that persists today, more
`than one-century later [4]. Folin astutely recognized that indi-
`viduals excrete a relatively stable amount of creatinine in the
`urine over time, and he calculated his coefficient as grams per
`24 h/kg body weight reflecting a link between creatine pro-
`duction and body mass [4]. Max Bürger, working with muscle
`disease patients at the University of Kiel, first estimated a
`creatinine equivalence in 1919 reported as 1 g/day urinary
`creatinine=22.9 kg of whole skeletal muscle [5]. By 1928,
`Hahn and Meyer closed the loop by establishing the source of
`urinary creatinine as the stable nonenzymatic breakdown of
`creatine, 98 % of which is in skeletal muscle at fairly stable
`concentrations [6]. Nathan Talbot at Harvard, in a widely cited
`1938 paper [7], provided support for a creatinine equivalence
`of 17.9 kg/g with ingestion of a general diet. By the late 1960s,
`investigators such as Joan Graystone working at Johns Hop-
`kins University were further refining the creatinine equiva-
`lence (20 kg/g) and used this method to describe aspects of
`childhood skeletal muscle growth and development [8, 9].
`Urine is difficult to collect over several days as is ideally
`required for the creatinine-skeletal muscle mass method. This
`limitation led investigators to seek an alternative strategy, and
`one based on isotopic measurement of the creatine pool size
`emerged in the early 1970s. With this approach, investigators
`apply the general formula, skeletal muscle mass=creatine
`
`pool/skeletal muscle creatine concentration. Kreisberg et al.,
`working at
`the University of Alabama in Birmingham,
`used this approach in 1970 with 14C-labeled creatine
`[10], and Picou et al.
`in 1976 working at
`the Tropical
`Metabolism Unit
`in Jamaica made similar measurements
`with 15N-creatine [11].
`A consistent theme through all of this research was the lack
`of an in vivo reference method for quantifying skeletal muscle
`mass in humans. Reliance was largely based on data from
`animal studies, a few available human cadaver dissections, or
`skeletal muscle biopsies. Remarkably, it was not until 1996
`that “official” proof of concept for the creatine-creatinine
`approach for estimating skeletal muscle was obtained in
`humans. Wang et al. [12], working at Columbia University,
`collected urine over several days from 12 healthy adult men
`who had total body skeletal muscle mass measured by the
`emerging whole-body computed tomography (CT) method.
`Wang confirmed the strong link between urinary creatinine
`and total body skeletal muscle mass in healthy men with an R
`of 0.92 and an SEE of 1.89 kg (p<0.0001). The power of
`modern three-dimensional whole-body imaging brings us to
`the present when Stimson et al. [13] used magnetic resonance
`imaging (MRI) to estimate skeletal muscle mass and the stable
`isotope deuterated creatine (DCR) to quantify the creatine
`pool size in adults. The isotope, enclosed in a gel capsule, is
`ingested, and a urine sample collected several days later is
`used to estimate DCR by mass spectroscopy. The measured
`dilution space is strongly correlated (r=0.87) with total body
`skeletal muscle mass as measured with MRI [13].
`
`2.2 Muscle as a human capacity marker
`
`Just after the collapse of the Habsburg monarchy, Jindřich
`Matiegka, working at the University of Prague a few years
`after World War I, communicated his body composition vision
`in a May 7, 1920 letter to the prominent US physical anthro-
`pologist Aleš Hrdlička: “I shall send my proposal for estab-
`lishing a commission that would work out a method for
`assessing work efficiency of the human body. I justify my
`proposal by pointing out that it is a duty of anthropology to
`develop a method for testing human physical capacity, similar
`to the methods worked out by psychologists to test mental
`capacity. Mental and physical capacities together constitute
`the working capacity and determine the working efficiency of
`the person” [14]. A year later, Matiegka published what is
`likely the seminal paper describing anthropometric measure-
`ment of skeletal muscle mass and three other functional body
`compartments [15].
`Matiegka’s system considered body weight (W) the sum of
`four components,
`
`O þ D þ M þ R;
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`where O is skeletal (osseous) weight, D is skin plus subcuta-
`neous adipose tissue weight, M is skeletal muscle weight, and
`R is remaining weight. Using measuring devices that would be
`considered crude by modern standards, Matiegka developed
`his model based on height, bone and extremity breadths,
`skinfolds, and calculated body surface area.
`Sarcopenia, sarcopenic obesity, and cachexia are all
`circumscribed by adverse outcomes captured by Matiegka’s
`vision of a connection between musculoskeletal mass and
`function as it relates to human “working efficiency.” Another
`seven decades were needed before anthropometric measure-
`ments and models as pioneered by Matiegka could be directly
`linked to actual measurements of whole-body skeletal muscle
`mass, either by dissection [16] or with in vivo CT and MRI
`measurements [17].
`Matiegka’s anthropometric method provides an opportuni-
`ty to ask the question, what exactly do we mean by skeletal
`muscle mass? The five-level model reported by Wang and
`colleagues at Columbia University in 1992 [2] provides a
`framework for responding to this question. Wang’s model
`describes the human body as five interconnected areas starting
`with an elemental level followed by molecular, cellular, tissue-
`organ, and whole-body levels. With body surface measure-
`ments, anthropometry provides a skeletal muscle mass esti-
`mate at the tissue-organ level that includes skeletal muscle
`fibers, nerves, blood vessels, progenitor cells, connective tis-
`sue, tendons, intermuscular adipose tissue, and even in some
`cases bone. By contrast, the creatine-creatinine method largely
`maps the creatine pool located within skeletal muscle cells
`[18]. Wang’s model provides an important framework for
`linking the mechanistic basis of a method with specific mea-
`sured skeletal muscle structures. Each method provides a
`skeletal muscle mass estimate that can be categorized and
`modeled as one of Wang’s five levels. Wang’s model also
`provides a context for discussing the emerging topic of muscle
`“quality” that can be framed as ranging from whole-muscle to
`cells, intracellular organelles, and individual molecular
`substrates.
`
`2.3 Piecing elements together
`
`The opening of the nuclear age following World War II led to
`the introduction of new body composition techniques such as
`whole-body 40K counting and prompt-γ neutron activation
`analysis for measuring total body potassium (TBK) and total
`body nitrogen (TBN), respectively [19]. Burkinshaw et al. at
`the University of Leeds [20] and Cohn et al. at Brookhaven
`National Laboratory [21] introduced the historically important
`TBK-TBN elemental method of estimating total body skeletal
`muscle mass between 1978 and 1980. The Burkinshaw-Cohn
`model assumes that the K to N ratios of skeletal muscle and
`nonskeletal muscle lean mass are constant at 3.03 and
`1.33 mmol/g, respectively. Skeletal muscle mass (in
`
`kilograms) can then be calculated as=[TBK (in millimoles)
`− 1.33 × TBN (in grams)]/51.0. This innovate method was
`applied to early studies of cancer cachexia by Cohn et al. [21,
`22] and Burkinshaw [23]. The lack of other suitable skeletal
`muscle mass reference methods at the time led Lukaski et al. at
`the United States Department of Agriculture in Grand Forks
`[24] to provide the first validation of the urinary 3-
`methylhistidine method for estimating skeletal muscle mass
`against skeletal muscle mass values derived from
`Burkinshaw-Cohn’s method in 1981. The 3-methylhistidine
`method for measuring skeletal muscle mass is not in use today,
`being replaced by contemporary advanced and more practical
`methods such as CT and MRI.
`An early conceptual precursor to the models derived by
`Burkinshaw and Cohn was reported by Chinn in 1967 who
`was working at the United States Army Research and Nutri-
`tion Laboratory in Denver [25]. Chinn developed a mathemat-
`ical model for estimating skeletal and nonskeletal muscle
`protein from measured TBK and 24-h urinary creatinine.
`While imaginative in their design, this group of methods is
`of little practical value today due to the complexity of instru-
`mentation required, the potential for radiation exposure, and
`the need to collect serial burdensome 24-h urine samples.
`What these methods do reveal are the lengths to which inves-
`tigators went in trying to quantify the remarkably large but
`elusive skeletal muscle compartment in vivo. These efforts
`were motivated by the need to quantify skeletal muscle mass,
`not only body fat, the two main tissue targets of wasting
`diseases such as cancer.
`
`3 Modern era
`
`3.1 Growth of biomedical imaging
`
`X-rays Few would have predicted that Röentgens’ discovery
`of X-rays in 1895 at the University of Würzburg [26] would
`ultimately be the basis of a whole family of biomedical
`imaging methods that today serve as the main tool for quan-
`tifying skeletal muscle mass and composition. More than half
`a century later, Harold C. Stuart and his colleagues at Harvard
`first applied an X-ray method for estimating extremity fat and
`muscle “widths” in growing children [27]. While Stuart’s
`method gained acceptance among the research community at
`the time, it was to be three more decades before major ad-
`vances transformed the field. Within a very short time span in
`the early 1970s, all three of our main contemporary clinical
`and reference methods, CT, MRI, and dual-energy X-ray
`absorptiometry (DXA), came into existence.
`The first of these approaches to gain acceptance in clinical
`medicine was CT, initially applied in 1971 to a clinical patient
`by Godfrey Hounsfield working at EMI near London [28].
`Hounsfield’s approach was formulated on theoretical
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`underpinnings provided earlier by Allan M. Cormack at the
`University of Cape Town and later at Tufts University [29].
`Unlike Stuart’s two-dimensional plain X-rays [27], CT pro-
`vided high-contrast cross-sectional images with pixel attenu-
`ation related to tissue physical density [30]. Pixels, or picture
`elements, could now be easily separated into those traversing
`adipose tissue (density, ∼0.92 g/cm3) and those passing
`through whole skeletal muscle tissue (∼1.04 g/cm3). By
`1975, Hounsfield and his colleagues built a whole-body CT
`scanner, and within a few months of each other, three papers
`appeared in 1978–1979 that used this revolutionary imaging
`approach for quantifying regional skeletal muscle areas
`[31–33]. Several years later, Tokunaga et al. at Osaka Univer-
`sity in 1983 [34] and Sjöström et al. at the University of
`Gothenburg in 1984 [35] reported the first evaluations of total
`body composition by CT using a multi-slice head-toe ap-
`proach. The emphasis in these early studies was on the eval-
`uation of regional and total body adipose tissue.
`Each CT pixel or volume element (voxel) represents tissue
`attenuation and is defined in Hounsfield units (HU). A value
`of 0 HU represents water, −1,000 HU represents air, with
`negative and positive values characteristic of fat and lean
`tissues, respectively. An early observation following the in-
`troduction of CT was the impact of conditions such as hepatic
`steatosis and hemosiderosis to quantitatively influence the
`measured CT numbers [36]. Investigators today continue to
`use and expand on the use of CT numbers to provide qualita-
`tive and functional information on skeletal muscle tissue
`beyond measures of size and shape [37].
`
`Magnetic resonance Isidor Rabi, working at Columbia Uni-
`versity in 1938, led pioneering investigations into the mag-
`netic resonance properties of atomic nuclei [38].
`Rabi’s research was advanced independently in 1946 by
`Felix Bloch at Stanford [39] and Edward Purcell at Harvard
`[40] with their discovery of nuclear magnetic resonance
`(NMR). The Bloch-Purcell research provided basic informa-
`tion about a molecule’s chemical and structural properties in
`liquids and solids. Paul Lauterbur, working at the State Uni-
`versity of New York (SUNY) at Stony Brook, wrote a classic
`paper in 1973 on an imaging approach he referred to as
`zeugmatography that took unidimensional NMR spectrosco-
`py to a second dimension of spatial orientation [41]. Peter
`Mansfield, at the University of Nottingham, showed during
`the early 1970s that a useful imaging technique could be
`created by advancing magnetic field gradients with appropri-
`ate mathematical analysis [42]. Raymond Damadian, working
`at SUNY Downstate, completed construction of the first
`whole-body MRI scanner in 1977 [43] and equipment for
`hospitals became available during the early 1980s. Foster
`and colleagues, working with an early prototype scanner at
`the University of Aberdeen, reported sequences in 1984 that
`discriminated well between adipose and skeletal muscle
`
`tissues [44]. Cadaver and regional human validation studies
`followed and by 1991, Robert Ross and colleagues at Queens
`University in Kingston showed good agreement between
`whole-body adipose tissue estimates by multi-slice MRI and
`CT in the rat [45]. Ross et al. in the mid-1990s advanced their
`MRI studies to humans, quantifying both whole-body adipose
`tissue and skeletal muscle mass [46, 47]. Selberg et al. report-
`ed whole-body anatomical skeletal muscle mass estimates by
`nuclear magnetic resonance in 1993 [48].
`Two Swiss scientists, Kurt Wüthrich and Richard Ernst,
`contributed further to the field with their pioneering NMR
`studies [49, 50] that contribute to in vivo chemical analysis of
`skeletal muscle and other tissues. Magnetic resonance spec-
`troscopy is widely used today as a means of assessing skeletal
`muscle quality, particularly with the examination of such
`components as intramyocellar lipid.
`The profound contribution to medical science by the de-
`velopment of imaging methods, notably CT and MRI, have
`led to ten Nobel Prizes as follows: Röentgen (1901), Rabi
`(1944), Bloch and Purcell (1952), Hounsfield and Cormack
`(1979), Ernst (1991), Lauterbur and Mansfield (2003), and
`Wüthrich (2002).
`
`Differential absorptiometry While CT and MRI are not wide-
`ly available or affordable whole-body methods for conducting
`sarcopenia and cachexia trials, DXA has now largely met that
`unmet need by providing surrogate estimates of regional and
`whole-body skeletal muscle mass at relatively low cost and
`with minimal radiation exposure. Systems are available
`throughout the world and are well-calibrated for monitoring
`patients over time or for conducting between-center research.
`Often linked with sarcopenia and frailty, osteoporosis with
`pathological fractures is an important medical condition re-
`quiring clinical evaluation and treatment monitoring. Bone
`quality was typically evaluated with plain X-rays until the
`introduction of single-photon absorptiometry (SPA) by Cam-
`eron and Sorenson at the University of Wisconsin in 1963
`[51]. Bone mineral density was typically evaluated with SPA
`at sites such as the wrist that consist mainly of skeleton with a
`photon source such as 125I. In addition to bone, skin, overlying
`adipose tissue, and skeletal muscle also lead to photon atten-
`uation, and thus axial skeletal sites such as the spine and hip
`could not be reliably evaluated for osteoporosis with SPA.
`This problem was solved in 1970 by Mazess et al. at the
`University of Wisconsin [52] who proposed a dual-photon
`method of separating soft tissue from bone using isotope
`sources such as 241Am or 153Gd. The novel approach included
`partitioning soft tissue into lean and fat components using the
`differential photon characteristics of traversed tissue elements
`such as carbon, hydrogen, oxygen, and electrolytes [53]. The
`first whole-body dual-photon absorptiometry systems ap-
`peared at medical facilities in the early 1980s and were later
`replaced in 1987 with DXA systems. The low radiation
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`exposure, widespread availability, and relatively low per pa-
`tient scan cost provided a new practical opportunity for inves-
`tigators to quantify body composition in children and adults.
`An important feature of DPA and DXA is an ability to isolate
`body regions during the analysis procedure and thus investiga-
`tors for the first time could evaluate fat, lean soft tissue, and bone
`mineral separately for the extremities, trunk, and other selected
`body regions. Heymsfield and colleagues at Columbia Univer-
`sity reported a method of estimating appendicular skeletal mus-
`cle mass from DPA in 1990 [54], and two years later, Fuller et al.
`at Cambridge [55] reported a similar approach using DXA. The
`DPA and DXA approaches relied on the observation that a large
`proportion of measured appendicular lean soft tissue is skeletal
`muscle. Additionally, a large percentage (∼50 %) of total body
`skeletal muscle mass is in the extremities. These developments
`opened an important new window to the study of conditions
`such as frailty and sarcopenia. The coevolution of MRI provided
`an opportunity to relate measured appendicular lean soft tissue
`to total body skeletal muscle mass as first reported by Kim et al.
`at Columbia University in 2002 [56].
`The parallel development tracks and major milestones for
`development of X-ray-based and magnetic resonance-based
`imaging methods are presented in Figs. 1 and 2, respectively.
`
`Defining muscle structural relations with emerging imaging
`methods The new flow of information on human skeletal
`muscle mass stimulated ideas on how to integrate whole body
`and regional measurements into existing body composition
`paradigms. It was Quetelet at the Brussels Observatory in
`1835 who is credited with the concept that body weight scales
`in humans as height2 [57]. This observation gave rise to body
`mass index (weight/height2), a measure of body shape and
`
`adiposity that is independent of height. Theodore VanItallie,
`working at Columbia University, extended Quetelet’s concept
`in 1990 by suggesting that body composition also be expressed
`as height-normalized indices [58]. Seeking a means of “diag-
`nosing” sarcopenia as part of an epidemiology study,
`Baumgartner et al. at the University of New Mexico [59]
`developed the appendicular skeletal muscle mass index (appen-
`dicular lean mass/height2) in 1998 based on DXA extremity
`lean soft tissue estimates. Heymsfield et al. at Columbia Uni-
`versity confirmed in 2007 [60] and later in 2011 [61] that total
`body skeletal muscle mass as measured by MRI and appendic-
`ular lean soft tissue as measured by DXA scale similar to height
`as does body weight, approximately as height2.
`A related concept was advanced by Webster et al. at the
`Harrow MRC Clinical Research Center in 1983 who showed
`that fat mass adjusted for height2, and by inference fat-free
`mass index, is highly correlated with body mass index [62].
`This observation led Webster and his colleagues to propose a
`stable composition of “excess weight” as about one fourth fat-
`free mass. In other words, as a person’s adiposity increases so
`does their lean mass, including skeletal muscle. Forbes, work-
`ing at the University of Rochester, advanced this concept by
`introducing the “companionship” rule describing a curvilinear
`relationship between fat-free mass and total body fat. Forbes
`and others since have confirmed the longitudinal validity of
`“Forbes’ Rule” by showing relatively large gains or loss of fat-
`free mass with changes in energy balance at low levels of
`subject adiposity [63]. Since a large fraction of fat-free mass is
`skeletal muscle, we can infer that with alterations in energy
`and protein balance, relatively large changes in skeletal mus-
`cle will occur in sarcopenic or cachectic patients who may
`have low baseline levels of adiposity.
`
`Fig. 1 Brief chronology of X-ray
`research highlights on the path to
`developing methods of measuring
`human skeletal muscle mass
`in vivo. Nobel Prize awardees are
`noted in italics. ASM
`appendicular skeletal muscle, CT
`computed tomography, DPA dual-
`photon absorptiometry, DXA
`dual-energy X-ray
`absorptiometry, MRI magnetic
`resonance imaging, SM skeletal
`muscle, SPA single photon
`absorptiometry
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`the United States Naval Research Institute, described the use
`of ultrasound to diagnose gallstones [71]. Ultrasound technol-
`ogy has since evolved to include many different technologies
`and is in widespread clinical use. The value of ultrasound in
`the clinical setting comes in part from portability and lack of
`ionizing radiation exposure as with many other clinical
`methods. Bullen et al. at Copenhagen’s Bispebjerg Hospital
`[72] and Booth et al. at Birmingham’s Dudley Road Hospital
`[73] used A-mode ultrasound in the early 1960s as an alter-
`native to skinfold calipers in measuring subcutaneous fat layer
`thickness. Ikai and Fukunaga, at the University of Tokyo,
`reported the use of ultrasound for measuring skeletal muscle
`cross-sectional areas in 1968 [74]. Ultrasound is gaining in-
`creasing acceptance today as a clinical and research tool for
`evaluating skeletal muscle mass at baseline and over time with
`various interventions. Technologies such as ultrasound
`elastography provide additional information about skeletal
`muscle quality [75], beyond that of simply muscle shape or
`size.
`
`4 Recent advances and future potential
`
`A remarkable range of technologies is now available for
`quantifying regional and total body skeletal muscle mass in
`almost any setting and at any age, even in utero. An important
`observation undergoing intense discussion in the sarcopenia
`field is the observation that skeletal muscle mass and function
`decline at different rates during the aging process [76, 77].
`What are the mechanisms of these effects? Should “function-
`al” measures replace or complement “structural” measures as
`diagnostic components of conditions associated with skeletal
`muscle changes over time? Important focuses in this debate
`are the linkages between skeletal muscle structure, composi-
`tion, and function. At the center of this chain, and one that
`holds future promise, is the assessment of skeletal muscle
`composition.
`
`Separating adipose and muscle tissues Matiegka’s a nthropo-
`metric skeletal muscle compartment would by necessity in-
`clude some adipose tissue interspersed between muscle fibers
`and compartments [15]. This marbling effect of intramuscular
`adipose tissue (IMAT) has been recognized in the animal
`science literature for decades, and ultrasound is the traditional
`measurement method of choice. The IMAT compartment,
`weighing several kilograms in adults [78] and a feature of
`sarcopenia, can now be accurately quantified my CT, MRI, or
`ultrasound. Anatomic skeletal muscle can be separated into
`two parts, IMAT and IMAT-free skeletal muscle.
`Another MRI approach is to separate skeletal muscle tissue
`into two molecular level components, fat and fat-free muscle.
`A range of methods has been developed to accomplish this
`end that collectively is referred to as fat-suppression
`
`Fig. 2 Brief chronology of magnetic resonance imaging research high-
`lights on the path to developing methods of measuring human skeletal
`muscle mass in vivo. Nobel Prize awardees are noted in italics. AT
`adipose tissue, MRI magnetic resonance imaging, NMR nuclear magnetic
`resonance, SM skeletal muscle
`
`Bioimpedance analysis Nyboer (1959, Wayne State Univer-
`sity, Detroit [64]), Thomasset (1962, University Claude Ber-
`nard, Lyonn [65]), and Hoffer (1969, University of Alabama,
`Birmingham [66]) all made substantial contributions to
`bioimpedance analysis concepts and technology in the late
`1950s and 1960s. The path to current applications for mea-
`suring skeletal muscle mass with bioimpedance technology
`was paved largely by the seminal contribution of Leslie Or-
`gan’s group at the Medical University of South Carolina in
`1994 [67]. Organ introduced a practical six-electrode tech-
`nique for segmental bioimpedance analysis that provided
`separate resistance and reactance measurements for each ex-
`tremity and the trunk using only peripheral electrode sites.
`This approach has evolved to the widely used eight contact-
`electrode method now in use today. As with DPA and DXA,
`Organ’s method allows for separate measurements of the
`extremities and trunk. A few years later, Tan et al. at Columbia
`University in 1997 reported the first contact electrode system
`based on Organ’s concept devoted specifically to measuring
`appendicular impedance that was then used to develop pre-
`diction equations for total body skeletal muscle mass [68].
`Methods such as multifrequency bioimpedance spectroscopy
`and derived measures such as phase angle [69] are increasing-
`ly being used to go beyond estimation of skeletal muscle mass
`to evaluation of muscle quality.
`
`Ultrasound Karl Dussik, a neurologist at the University of
`Vienna, first used ultrasound technology in 1942 for diagnos-
`ing brain tumors [70]. Several years later, George Ludwig, at
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`Harvest Trading Group - Ex. 1119
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`J Cachexia Sarcopenia Muscle (2014) 5:9–18
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`15
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`techniques [79]. These approaches all rely on the observation
`that hydrogen nuclei in the chemical environments of water
`and fat have different MRI relevant parameters, primarily
`relaxation time, and resonance frequency or so-called chemi-
`cal shift. These differences can be exploited to suppress the
`fat-bound proton signal, and two main groups of techniques
`are available, relaxation-dependent (e.g., short inversion-time
`inversion recovery) and chemical shift-dependent (e.g.,
`Dixon-based fat suppression) [80]. Scanning protocols now
`in development will soon provide estimates of regional and
`total body fat-free skeletal muscle volume. Possibilities exist
`to either fully or partially automate the analysis protocol, thus
`removing a major current task of expert hand image segmen-
`tation as is required for quantitative CT and MRI muscle
`analysis.
`
`Separating muscle extracellular and intracellular
`compartments With aging and development of sarcopenia or
`with cachexia-induced skeletal muscle atrophy, there is a
`relative expansion of the extracellular space that includes fluid
`and connective tissue with loss of muscle fibers [81, 82].
`Several new magnetic resonance techniques hold promise
`for providing information related to the extracellular compart-
`ment [83].
`The first of these methods is referred to as T1 mapping
`equilibrium contrast magnetic resonance that gives estimates
`of myocardial and potentially skeletal muscle extracellular
`volume fraction [84]. The protocol requires the use of an
`imaging contrast agent and so far has shown promise in
`detecting pathological changes in human myocardial tissue,
`representing a relative increase in extracellular volume and
`fibrosis [85]. Noncontrast T1-mapping also provides mea-
`sures of cardiac fibrosis [86].
`A second developing approach, magnetic resonance
`elastography [87, 88], provides information on skeletal muscle
`viscoelastic and mechanical properties that may represent in-
`creased connective tissue with functional consequences. The
`method has been used to measure skeletal muscle stiffness, a
`property that can change significantly depending upon the
`