Pitfalls in the measurement of muscle mass: a need for a reference standard

itions of sarcopenia include the measurement of muscle mass, but the techniques and threshold values used vary. Indeed, the literature does not establish consensus on the best technique for measuring lean body mass. Thus, the objective measurement of sarcopenia is hampered by limitations intrinsic to assessment tools. The aim of this study was to review the methods to assess muscle mass and to reach consensus on the development of a reference standard. MethodsLiterature reviews were performed by members of the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis working group on frailty and sarcopenia. Face-to-face meetings were organized for the whole

to assessment tools. The aim of this study was to review the methods to assess muscle mass and to reach consensus on the development of a reference standard. MethodsLiterature reviews were performed by members of the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis working group on frailty and sarcopenia. Face-to-face meetings were organized for the whole group to make amendments and discuss further recommendations. ResultsA wide range of techniques can be used to assess muscle mass. Cost, availability, and ease of use can determine whether the techniques are better suited to clinical practice or are more useful for research. No one technique subserves all requirements but dual energy X-ray absorptiometry could be considered as a reference standard (but not a gold standard) for measuring muscle lean body mass. ConclusionsBased on the feasibility, accuracy, safety, and low cost, dual energy X-ray absorptiometry can be considered as the reference standard for measuring muscle mass. KeywordsLean mass; Muscle mass; Lean body mass; Reference standard Received: 4 May 2017; Revised: 5 September 2017; Accepted: 12 October 2017 *Correspondence to: Fanny Buckinx, M.Sc., PhD candidate, University of Liège, Department of Public Health, Epidemiology and Health Economics, CHU-Sart-Tilman, B23, Quartier Hôpital, Avenue Hippocrate, 13, 4000 Liège, Belgium. Tel.: +32 4366 49 33, Email: fanny.buckinx@ulg.ac.be ORIGINAL ARTICLE © 2018 The Authors. Journal of Cachexia, Sarcopenia and Muscle published by John Wiley & Sons Ltd on behalf of the Society on Sarcopenia, Cachexia and Wasting Disorders Journal of Cachexia, Sarcopenia and Muscle2018;9: 269–278 Published online 19 January 2018 in Wiley Online Library (wileyonlinelibrary.com)DOI:10.1002/jcsm.12268 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any me- dium, provided the original work is properly cited and is not used for commercial purposes.

Background The term sarcopenia wasfirst used by Rosenberg et al. in 1989 1 to refer to a progressive loss of skeletal muscle mass with advancing age. Baumgartner, defined sarcopenia as ap- pendicular skeletal muscle mass (kilogram)/height 2 (metre 2 ) being less than two standard deviations below the mean of a reference group. 2 Since then, the conceptual definition of sarcopenia has expanded to include impaired muscle strength and/or physical performance. In turning a concep- tual definition to an operational definition, several have been proposed, 2–10 but no consensus has yet been reached. The multidimensional nature of sarcopenia implies that its do- mains should be objectively assessed. 11,12 Therefore, valid, standardized, reliable, accurate, and cost-effective tools are necessary for the identification of sarcopenia. 13,14 Currently, all the proposed definitions include the measurement of muscle mass but the techniques used to assess it vary. In re- cent years, four main techniques have been commonly used to estimate muscle mass: bioelectric impedance (BIA), dual energy X-ray absorptiometry (DXA), computed tomography (CT), and magnetic resonance imaging (MRI) to replace an- thropometry. 15–17 In addition to these, several emerging techniques for the assessment of muscle mass are now avail- able. Each rely on different technologies and assess different aspects of muscle mass (e.g. total body muscle mass, appen- dicular muscle mass, or mid-thigh muscle cross-sectional area) (Figure 1). At the organizational level, the body can be separated into chemical or anatomical distinct compart- ments. The 2-compartment model divides the body weight into fat mass and fat free mass or FFM. 18 Body composition techniques are based on these organizational levels. There- fore, the objective measurement of sarcopenia is hampered by limitations intrinsic to assessment tools. 11,19 From a clini- cal and epidemiological point of view, it is important to have a consensual technique. The use of different diagnostic methods may lead to different prevalence of sarcopenia and may therefore have significant consequences on preven- tive or therapeutic strategies. Matiegka reported in 1921 what was to become a classic anthropometric approach to quantifying skeletal muscle mass. 20 Matiegka’s method divided body weight into four parts:

view, it is important to have a consensual technique. The use of different diagnostic methods may lead to different prevalence of sarcopenia and may therefore have significant consequences on preven- tive or therapeutic strategies. Matiegka reported in 1921 what was to become a classic anthropometric approach to quantifying skeletal muscle mass. 20 Matiegka’s method divided body weight into four parts: skeleton, skeletal muscle, skin plus subcutaneous adi- pose tissue, and the remainder. Others that followed Matiegka were limited by a lack of reference standards for skeletal muscle mass measurement until the introduction of CT by Hounsfield. 21 After this phase, CT, MRI, and DXA were being used to measure muscle mass (so a more specific mus- cle assessment compared with the general FFM). Subse- quently, BIA equations were developed to predict muscle mass (instead of FFM). The availability of DXA systems, with modest scan cost, low radiation exposure, short scan time, and extensive information provided from a whole body scan makes this approach the most widely used in sarcopenia re- search at the present time. 17,22 Indeed, imaging methods such as MRI and CT are expensive methods and are not ac- cessible to the majority of clinicians and researchers. 23 Never- theless, the literature has not established consensus on the ‘best’technique to measure muscle mass. Because of the need for consensus and standardization for both clinicians and researchers, the widely used techniques measuring mus- cle mass are reviewed in the succeeding text and recommen- dations derived therefrom. Methods The European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis working group on frailty and sarcopenia consists of clinical scientists and experts in thefield of musculoskeletal diseases. Different members of Figure 1Body compartments based on reference man. 270 F. Buckinxet al. Journal of Cachexia, Sarcopenia and Muscle2018;9: 269–278 DOI: 10.1002/jcsm.12268

the working group were asked to prepare a literature review on the role of lean mass measurement in the assessment of sarcopenia (M.C.), the measurement of Lean Body Mass with DXA (M.V. and K.E.), with bioimpedance (S.M.), and with emerging techniques (R.F.). The topic‘how to produce refer- ence standards for the assessment of Lean Body Mass’was also discussed (E.D.). Each member prepared a list of the most important papers based on their literature search and made a set of preliminary recommendations. For each item, a complete literature search was performed to identify new or additional randomized controlled trials and systematic reviews/meta-analysis, if any, not used in the existing guide- lines. The MEDLINE (pubmed) database was searched using the name of each technique for measuring body composition as a search term, together‘with lean body mass’, limiting re- sults to‘humans’,‘randomized controlled trials’,‘meta- analysis’,‘systematic reviews’,and‘guidelines’. A similar search was adapted for the Embase database, and each item was also searched in the Cochrane Database of systematic re- views. The reference list of relevant retrieved articles was hand-searched for additional resources when member of the working group were interrogated for their knowledge on articles or congress abstract in press. A free web search was also performed and considered. Searches were per- formed from the year 2000 and updated until September 2016, with the additional evidence constantly provided to the working group members for selection of the best evi- dence according to the panel. The subsequent step was a face-to-face meeting for the whole group to make amendments and discuss further rec- ommendations. The plan of the manuscript was also discussed and shared conclusions were reached. Results First, it seems important to clarify several terms. Skeletal muscle mass is the largest component of adipose tissue–free body mass in humans. 24 Lean mass also known as lean body mass is a fat-free and bone mineral–free component that in- cludes muscle and other components such as skin, tendons, and connective tissues (Figure 2). Appendicular lean soft tis- sue is the sum of lean soft tissue from both arms and legs. 26 A large proportion of total-body skeletal

body mass in humans. 24 Lean mass also known as lean body mass is a fat-free and bone mineral–free component that in- cludes muscle and other components such as skin, tendons, and connective tissues (Figure 2). Appendicular lean soft tis- sue is the sum of lean soft tissue from both arms and legs. 26 A large proportion of total-body skeletal muscle is found in the extremities, and a large proportion of appendicular lean soft tissue is skeletal muscle (Figure 1). 25 Measurement of lean body mass and muscle mass with imaging techniques Dual energy X-ray absorptiometry Dual energy X-ray absorptiometry is the most widespread technique for measuring body composition. 27 DXA uses two different energy spectra to differentiate two materials: either bone or soft tissue, which is the basis for the measurement of bone mineral density (BMD) and content or lean soft tissue mass and fat mass in locations where bone is absent. Taken together, DXA provides an estimate of three body compart- ments, that is, lean, bone, and fat. At bone locations, lean and soft tissue are interpolated from the surroundings. These measurements can be performed for the whole body and for several regions (e.g. trunk, arms, and legs). 28,29 The principle of using DXA for measurement of body composition is based on the notion that when a beam of X-rays is passed through a complex material, the beam is attenuated in proportion to the composition and thickness of the material. The use of two different energy spectra is the basis to separately quan- tify the amount of bone mineral and soft tissue or of fat and lean mass. Lean soft tissue and adipose tissue are mostly comprised by water and organic compounds, which restrict theflux of X-rays less than bone. 15,30 DXA is able to assess to- tal body lean soft tissue mass (which includes skeletal muscle mass as well as the mass of all other organs) and appendicu- lar lean soft tissue mass (i.e. an estimate of the muscle mass contained in the limbs, which represents about 75% total body skeletal muscle mass). 27 Appendicular lean soft

than bone. 15,30 DXA is able to assess to- tal body lean soft tissue mass (which includes skeletal muscle mass as well as the mass of all other organs) and appendicu- lar lean soft tissue mass (i.e. an estimate of the muscle mass contained in the limbs, which represents about 75% total body skeletal muscle mass). 27 Appendicular lean soft tissue mass measured by DXA is highly correlated with both MRI (r= 0.88;P<0.001) and CT (r= 0.77–0.95,P<0.0001) measures of skeletal muscle volume. 25,31–39 In vivoprecision errors depend on DXA equip- ment, population, local versus whole body measurements, age, and degree of obesity. Recently published values for ap- pendicular lean soft tissue mass range from below 1–3.0%. Higher errors of 4% were reported for bilateral muscle mass of the arms. Precision of DXA is high. 40 According to Hangartner, the precision error, expressed in %CV, for lean body mass was 1.2% 40 . Strengths and weakness of the DXA technique are summa- rized in Table 1. Figure 2Relations between appendicular lean soft tissue (ALST) and to- tal-body skeletal muscle (SM) mass. 25 Measurement of muscle mass 271 Journal of Cachexia, Sarcopenia and Muscle2018;9: 269–278 DOI: 10.1002/jcsm.12268

Note that DXA half-body analysis in obese subjects appears to be closely comparable to whole-body analysis for fat mass, non-bone lean mass, and percent fat, though there are no data on the comparability at appendicular sites. 43 Dual energy X-ray absorptiometry is a candidate for pro- viding a reference technology for assessing lean mass (as a proxy of muscle mass) and body composition in research and clinical practice. There is however a need for standardiza- tion. Standardization can be approached using phantoms or humans. Existing body composition phantoms are not anthro- pometric and cannot be used as absolute reference standards for soft-tissue composition. Therefore, a recent International Society for Clinical Densitometry report concluded that‘No phantom has been identified to remove systematic difference in body composition when comparing in vivo results across manufacturers’. As a consequence,‘an in vivo cross- calibration study is necessary when comparing in vivo results across manufacturers’. 41 Still for a unique standardization, the use of phantoms would be preferable because anin vivocross calibration is in- fluenced by age, gender, ethnicity, healthy versus diseased subjects, and so on. 44 Ideally, the calibration materials and equations used to derive lean mass should be standardized across manufacturers or cross-manufacturer algorithms should be developed by industry to standardize the output. It is also important to standardize the local regions of inter- est, such as trunk, arms, and legs, which are significantly dif- ferent across manufacturers. 45,46 Computed tomography Computed tomography (CT) was thefirst method introduced that could quantify regional skeletal muscle mass with high ac- curacy. 17 CT determines the cross-sectional distribution of the X-ray absorption coefficient, which after normalization to the absorption of air and water is called CT value and measured in Hounsfield units (HU). CT slices of predefined width can be analysed for different tissues, using manual segmentation or automated software. For example, muscle area, or in case of the analysis of a stack of images, volume of individual muscles, or a group of muscles can be determined. By definition, the HU value of air isμ1000 and of water 0. Bone, skeletal muscle, ad- ipose tissue, and visceral

width can be analysed for different tissues, using manual segmentation or automated software. For example, muscle area, or in case of the analysis of a stack of images, volume of individual muscles, or a group of muscles can be determined. By definition, the HU value of air isμ1000 and of water 0. Bone, skeletal muscle, ad- ipose tissue, and visceral organs have specific Hounsfield unit ranges, allowing for their identification in the cross-sectional images. The tissue area/volume (cm 2 /cm 3 ) of the cross-sec- tional/stack of images is subsequently calculated by multiply- ing the number of pixels/voxels for a given tissue by the pixel area/voxel size. Muscle mass can be derived by multiply- ing muscle volume by 1.04 that is the assumed constant den- sity (kg/cm 3 ) of adipose tissue-free skeletal muscle. 47 Compared with DXA, CT is a 3D imaging technique that al- lows for quantitative assessment of individual muscles. More- over, the muscle tissue composition can be quantified, either by separate segmentation of muscle and adipose tissue or by analysing muscle density, that is, the HU distribution within the segmented muscle. 48 In vivoprecision errors for muscle volume or mass mea- surements have rarely been reported, but reanalysis preci- sion errors are low due to its high resolution (typically 50 microns or less). 49 This is important because with advanced 3D imaging, precision of muscle area and mass depend more on image segmentation than on repositioning. For reanalysis, intraclass correlation coefficients (ICC) between 0.98 and 1.00 (P<0.001) 50 in quantifying both adipose tissue and muscle mass 51 were reported. Major disadvantages of CT are limited access to the radio- logical departments that operate it and considerably higher cost and radiation exposure than for DXA. Despite calibration of HU to water, calibration of CT across models and scanner manufacturers is still required when comparing scans from different devices. In addition, very obese patients may not fit into the scanner and image quality will be poor. Also, the operation of a CT scanner requires highly qualified personnel. The widespread implementation of CT imaging in thefield of sarcopenia has

of HU to water, calibration of CT across models and scanner manufacturers is still required when comparing scans from different devices. In addition, very obese patients may not fit into the scanner and image quality will be poor. Also, the operation of a CT scanner requires highly qualified personnel. The widespread implementation of CT imaging in thefield of sarcopenia has been hampered by the previously mentioned Table 1Strengths and weakness of measuring muscle mass by dual energy X-ray absorptiometry Strengths Weaknesses Non-invasive with small doses of radiation (<1μSv for whole-body scans). 41 Projectional technique, individual muscles cannot be assessed separately. Relatively cheap, compared with CT scan or MRI. Not portable, which may preclude its use in large-scale epidemiological studies and studies in the home setting. Rapid Availability is limited in some care settings. Allows measurement of three body compartments. Body thickness and abnormalities in hydration status (e.g. water retention, heart, kidney, or liver failure) can affect muscle mass measure. 42 Low precision errors Very tall and very obese people cannot be measured. Cannot quantify fatty infiltration of muscle. It is a bias in the diagnosis of sarcopenia obesity. Does not measure skeletal muscle mass in non-limb regions of the body (e.g. trunk). Several devices and several software packages and software versions resulting in different results. CT, computed tomography; MRI, magnetic resonance imaging. 272 F. Buckinxet al. Journal of Cachexia, Sarcopenia and Muscle2018;9: 269–278 DOI: 10.1002/jcsm.12268

limitations. An alternative to whole body clinical CT scanners may be the use of CT scanners dedicated and limited to pe- ripheral investigations, which is cheaper and has lower expo- sure to radiation is presently better suited for small-scale research studies in which accurate measurements of muscle quantity and quality are needed. Magnetic resonance imaging The introduction of MRI in the 1980s expanded the initial use of CT as a means of developing 3-dimensional images of skel- etal muscle, adipose tissue, and other organs. This develop- ment is usually referred to as structural or anatomic imaging. 17 The resolution is very high, and MRI is safe with- out any radiation exposure. With the advancement of the MRI technique, the time for reliable image acquisition has de- creased significantly. In addition, most modern MRI scanners can accommodate obese subjects. Limitations in the use of MRI in clinical and research settings are largely related to the high cost, the technical expertise required for analysis, and the effect of respiratory motion on image quality for whole-body assessments. Multiple slices are required to as- sess the composition of the total body, including total body skeletal muscle mass. 52 Finally, the existence of multiple pro- tocols for data acquisition impacts the standardization of this technique for the study of muscle mass. 42 Bearing all these considerations in mind, MRI is presently better suited for small-scale research studies in which accurate measurements of muscle quantity and quality are needed. Estimation of lean body mass and muscle mass with bioimpedance analysis Bioimpedance analysis (BIA) was pioneered in the 1950s and 1960s by Hoffer, Nyboer, and Thomasset. 53–55 Since then, BIA has become a broadly applied approach used in body compo- sition measurements and healthcare assessment systems. 56 BIA is based on the notion that tissues rich in water and electrolytes (i.e. skeletal muscle) are less resistant to the passage of an electrical current than lipid-rich adipose tissue (i.e. bone). 17,57 All BIA systems exploit these tissue-specific conductivity differences to quantify body-compartments. In bioimpedance measurements, the human body is divided into five inhomogeneous segments, two for the upper

This study reviews methods to assess muscle mass and aims to develop a reference standard.