Measuring Muscle Mass
Resource Page on Measuring Muscle Mass with Tim Egerton
Measuring Muscle Mass
A goal of many training programmes is increasing and measuring muscle mass. It is also an important measurement in many research studies. This resource page provides a critical analysis of the various methods available for measuring muscle mass, muscle volume and muscle cross sectional area.
For additional resource pages on topics related to Sport and Exercise Science and topics related. to Strength and Conditioning, please visit the following links:
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Measuring Aerobic Capacity Resource Page
Measurement of Work Economy Resource Page
Measuring Blood Lactate Resource Page
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Measuring Muscles: Mass, Volume and Cross Sectional Area
Increasing muscle mass is a goal of many training programs. Muscle mass and volume are closely related to its cross-sectional area. A great deal of research has been devoted to the study of changes in muscle mass, volume, and cross-sectional area (CSA) in response to different interventions. Exercise scientists have developed many methods for measuring these variables and each has strengths and weaknesses, as well as different degrees of accuracy.
What does this page provide on measuring muscle mass?
This page explains which methods for studying changes in muscle mass, volume, and cross-sectional area are available, when they are used, why they are used, and how accurate they are. By the end, you will be able to critically appraise studies investigating muscle mass, volume and CSA.
Measuring Muscle Mass - What part of the muscles are we measuring?
Muscle mass, volume and CSA can be measured in many different ways. Assessment of muscle mass and volume includes the measurement of variables such as global muscle mass, local muscle mass and muscle thickness. Regarding CSA, there are two types of measurement: physiological cross-sectional area and anatomical cross sectional area. There are several different methods that can be used to carry out these CSA measurements, including magnetic resonance imaging (MRI), computed tomography (CT), ultrasound, and girth. Knowing the differences between the different measurement types and measurement methods is important for being able to interpret the literature.
Measuring muscle globally and locally
The exact nature of the study into a program designed to bring about hypertrophy determines whether global or local measures of hypertrophy are most appropriate. Any hypertrophy arising from a nutritional intervention might be expected to be global in nature. It seems unlikely that increased protein intake, for example, would result in selectively increasing skeletal muscle mass in one specific part of the body. Of course, if this nutrition intervention were to be combined with an arm training protocol then we might expect selective hypertrophy in the arm musculature. For this reason, we are more likely to be interested measures of local hypertrophy when studying the responses to certain types of training protocols. A good example might be an investigation into the effects of different repetition and loading schemes. Such a study might involve subjects performing high volume, low load biceps curls on one side of the body and low volume, high load biceps curls on the other side of the body. Measuring local muscle hypertrophy using methods such as corrected girth measurements would seem appropriate here, as it would provide insight into how the different sides of the body adapted to the differing stressors.
Measuring muscle sarcoplasm and myofibrillar protein
The sarcoplasm is a thick aqueous solution contained within each muscle cell. Myofibrils are cylindrical structures within muscle fibers containing contractile proteins. If sarcoplasmic hypertrophy occurs to a greater extent than myofibrillar hypertrophy then, although the cross-sectional area of a muscle will increase, the density of muscle fibers per unit area will decrease. Similarly, if myofibrillar hypertrophy is more prevalent than sarcoplasmic hypertrophy, then the density of myofibrils in a muscle will increase.
The concept of sarcoplasmic hypertrophy occurring in the absence of myofibrillar hypertrophy has been used to explain findings from research demonstrating discordance between hypertrophy and strength gains. It may therefore be of interest to measure the sarcoplasmic versus myofibrillar hypertrophic response to certain training interventions. Stable Isotope Tracers are needed to measure differences in sarcoplasmic versus myofibrillar hypertrophy. These measure the synthesis rate of proteins within these different fractions. However, it is worth expressing a word of caution here. Sarcoplasm actually appears to remain constant as a fraction of cell volume. There are no examples within the research literature whereby skeletal muscle hypertrophy occurs due an increase in sarcoplasm content in the absence of growth of the myofibrillar pool.
Identifying key concepts - measuring muscle mass
The following terms and concepts are key for an understanding of how muscle cross-sectional area is measured:
1. Anthropometry – this is the measurement of the human body. Surface anthropometry makes use of surface dimensional measurements that describe the human phenotype. These measurements may include mass, stature, skeletal breadths, segment lengths, girths and skinfolds.
2. Physiological cross-sectional area – this is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point.
3. Anatomical cross-sectional area – this is the area of the cross section of a muscle perpendicular to its longitudinal axis. Differences in physiological and anatomical cross sectional area therefore occur due to muscle pennation angle.
4. Pennation angle – this is the angle between the line of muscle fiber alignment and the longitudinal axis of the muscle. In a non-pennate muscle the fibers are parallel to the longitudinal axis and so physiological and anatomical cross-sectional area coincide. Pennation angle can change in response to training. Therefore, changes in muscle cross-sectional area after a training intervention will be very likely to be different depending on whether the measurement type used is physiological or anatomical cross-sectional area.
5. Reactive hyperemia – when a muscle has recently been exercised, this causes reactive hyperemia, or a “muscle pump”. Reactive hyperemia is defined as the transient increase in organ blood flow following a brief period of ischemia. During resistance-training, vasodilation of the muscles that have been exercised results in an increase in blood plasma in the interstitial spaces. The interstitial space is the area surrounding the muscle cells and an increase in interstitial fluid after resistance training may produce an acute increase in girth measurements.
6. Global hypertrophy – this is a systemic increase in muscle mass. In contrast, local hypertrophy relates to increases in the cross-sectional area of specific muscles compared to other muscles.
7. Regional hypertrophy – this is the non-uniform increase in cross-sectional area along the length of a muscle, along with possible lateral and medial differences in hypertrophic adaptation within a muscle.
8. Sarcoplasmic hypertrophy – this is an increase in the volume of the non-contractile muscle cell fluid. In other words, it is an increase in Sarcoplasm within the muscle cell.
9. Myofibrillar hypertrophy – this is an increase in the contractile protein content of muscle cells.
10. Stable Isotope Tracing – this is used to observe the movement of certain materials in chemical, biological or physical processes. In relation to muscle, we are interested in the movement of certain amino acids in order to measure the synthesis and degradation of protein. The term muscle protein balance has been used to describe the difference between the rates of synthesis and the rates of breakdown.
TYPES OF MUSCLE CROSS-SECTIONAL AREA MEASUREMENT
There are two types of cross-sectional area measurement: physiological cross-sectional area and anatomical cross sectional area. Physiological cross-sectional area is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point. Anatomical cross-sectional area is the area of the cross section of a muscle perpendicular to its longitudinal axis.
Direct vs. indirect measurements
Direct measurement of physiological cross sectional area is very difficult to perform. Instead, physiological cross sectional area tends to be estimated using equations based on muscle volume and muscle fiber length. This yields an acceptable degree of accuracy, so long as muscle volume is measured directly using methods such as magnetic resonance imaging (MRI), and muscle fiber length measurements are based on data from dissected muscles.
MRI is a type of scan that involves using strong magnetic fields and radio waves in order to produce detailed images of organs and structures inside the body. The scan usually involves individuals lying inside a large tube containing powerful magnets. Data from dissected muscles is obtained by firstly dissecting the body part of interest on a cadaver and preserving the cadaver using a method such as the Thiel soft fix embalming method (Thiel, 1992a; Thiel, 1992b; Thiel, 2002). This is a preservation method whereby skin and muscles remain flexible and features remain clearly identifiable. Following this, muscle fascicle lengths are measured using calipers.
Accuracy of each type of measurement
Anatomical cross sectional area is a less accurate measurement in comparison with physiological cross sectional area because it ignores changes in pennation angle. Greater weight should therefore be placed on findings from studies measuring physiological cross sectional area.
TYPES OF MUSCLE MASS AND VOLUME MEASUREMENT
Whole body muscle mass can be estimated from a range of measurement methods, such as dual emission x-ray absorptiometry, hydrostatic weighing, and skinfold measurement. These methods can be used to can be used to identify changes in fat mass and non-fat mass. Changes in non-fat mass are usually considered to represent an estimate of changes in muscle mass. An increase in muscle mass is representative of hypertrophy. Sometimes measurements are taken acutely following a training session as an estimate of the hypertrophic response to that session. Such measurements include muscle protein balance and nitrogen balance.
Muscle protein balance
The term muscle protein balance has been used to describe the difference between the rates of synthesis and the rates of breakdown of muscle protein. Stable isotope tracing is used to measure muscle protein balance. This involves using a labeled amino acid as a tracer. It is assumed that this amino acid is reflective of most of the amino acids involved in synthesizing muscle protein. The tracer moves into the muscle protein; and if an intervention is able to increase the rate of synthesis of muscle proteins, more of the tracer will go in – and you can measure the change. You are measuring the rate of incorporation of the labeled amino acids that go into the muscle cell.
But to measure muscle protein balance you also need to be measuring the rate at which the proteins are leaving the cell. To do this we need to look for the appearance of amino acids into the muscle pool itself and also look for the appearance into the blood of amino acids that are being broken down and released from the muscle. These are acute measurements made within the first few hours after a bout of training. This is an inherent limitation with respect to the measurement of hypertrophic adaptation, since hypertrophy is a long-term outcome seen after a period of training.
The assumption is that acute changes in muscle protein balance following resistance training are predictive of what will happen with respect to long term hypertrophy. Increased muscle protein balance following a nutritional or training intervention is thought to add up in the long run, ultimately resulting in measureable hypertrophy. However, there are conflicting research findings with respect to the relationship between acute muscle protein balance responses and chronic hypertrophic adaptation.
Nitrogen balance
Nitrogen balance studies look at how much nitrogen you ingest versus how much nitrogen you get rid of. The assumption is that protein accounts for all the nitrogen measured in the body. It gives you no insight into which tissues were built up or broken down to change protein content within the body. If what is going in is greater than what is going out then there must be a net retention of nitrogen. We don’t know which pools of nitrogen / protein may be built up or lost within a given experiment. Tissues other than muscle may increase their protein content and this method provides no means of differentiation. Nitrogen balance is probably the wrong variable upon which to base training or nutritional recommendations for hypertrophy. Muscle protein balance is perhaps a more valid measure of the acute response to training. However, measurements that allow for chronic adaptations to be measured are preferred.
METHODS OF MUSCLE CROSS-SECTIONAL AREA MEASUREMENT
There are several different methods currently in use for measuring muscle cross-sectional area, including girth, magnetic resonance imaging (MRI), computed tomography (CT), ultrasound, surface anthropometry and muscle biopsy. The gold
standard methods are currently considered to be magnetic resonance imaging (MRI) and computed tomography (CT). However, these methods are expensive and often inaccessible. So while greater weight may be placed on the findings from studies that use these technologies, it is likely that other measurement methods will be observed more often.
Magnetic resonance imaging
Magnetic resonance imaging is a type of scan that involves using strong magnetic fields and radio waves in order to produce detailed images of organs and structures inside the body. The scan usually involves individuals lying inside a large tube containing powerful magnets.
Computed tomography
Computed tomography (CT) is a type of scan that involves using x-ray equipment in order to produce detailed images of the inside of the body. The term tomography is derived from the Greek word tomos, which means a cut, slice or section. Each picture produced from a CT scan shows the various tissues in a thin slice of the body. Each individual picture from the CT scan is a 2-dimensional image. However, computer programs are used to piece the pictures together so that various tissues can also be seen in a 3-dimensional image. It is also important to remember that CT scans involve the use of ionizing radiation. As such there is a limit to the number of CT scans that can be safely performed on a single individual.
Ultrasound
Ultrasound is a type of scan that involves using high frequency sound waves to produce an image of part of the inside of the body. There are different types of ultrasound scan. These include external ultrasound, internal ultrasound and endoscopic ultrasound. Internal ultrasound and endoscopic ultrasound involve the insertion of an ultrasound device inside the body. External ultrasound is more interest in terms of measuring muscles. A transducer is placed on the skin and is moved over the body part being assessed and lubricating gel is used to allow the transducer to move smoothly. Pulses of ultrasound are sent from a probe in the transducer, through the skin and into the body. Ultrasound travels freely through fluid and soft tissues. However, it is reflected back from the more dense structures of the body to be displayed as an image on the monitor.
Muscle biopsy
Muscle Biopsies are a valid means of measuring hypertrophy in individual muscle fibers. This allows for the determination of regional hypertrophy, as several biopsies can be taken from different parts of a single muscle. Muscle biopsies can be taken from any muscle that is accessible. Accessibility is largely dependent on the proximity of major blood vessels and major nerves. A commonly biopsied muscle is the Vastus Lateralis. A small amount of local anesthetic is injected underneath the skin to freeze the area. Then a small incision is made, usually 5 to 6 millimeters in length. Though that incision a Bergstrom biopsy needle is usually advanced. The needle has a small opening and suction is used to pull the muscle into the needle. The muscle is then clipped and the needle is pulled back with a piece of muscle typically ranging from about 30 to 100mg. Having obtained a small piece of muscle tissue, it is sectioned so that the fibers can be seen and the circumference of individual fibers can then be measured.
Girth
Girth measurements have been used in research to track muscle hypertrophy during training interventions. Changes in muscle girth, as measured by use of a tape measure recording the circumference of a muscle, can be considered to be a gross estimate of muscle hypertrophy. A limitation of this method is that it is unable to distinguish between fat and muscle. As such, while an increased girth measurement may be indicative of increases in muscle mass, it is possible that an increase in fat mass rather than an increase in muscle mass may be responsible for a change in this measurement. In addition, other factors influence the reliability of this method, including the point at which circumference is measured, whether the muscle is in a flexed or extended position, whether the muscle is relaxed or under tension, and whether the individual has recently exercised the body part being measured, which can cause reactive hyperemia.
Surface anthropometry
While girth measurements have been used to track muscle hypertrophy, as noted above, the inability to discriminate between fat mass and muscle mass limits the validity of girth measurements as a means of assessing hypertrophy. Skinfold measurement can therefore be used to help improve the accuracy of girth measurements by adjusting for changes in fat mass. Correcting girth measurement for subcutaneous adipose tissue involves the combined use of skinfold and girth measurements. This is a useful surrogate for muscularity and allows for a local rather than global estimate of muscle mass.
A formula for corrected girth measurement is available. It involves subtracting the skinfold measurement multiplied by pi from limb girth measurement. More recently, two separate equations for the estimation of thigh muscle cross sectional area based upon corrected girth measurements have been studied. Both equations demonstrate good reliability but underestimate thigh muscle cross-sectional area when compared to computed tomography measurements. This error was consistent, however, which means these corrected girth measurements were able to measure increases in cross sectional area to a similar degree of accuracy as computed tomography. However, these equations have been validated for thigh muscle cross-sectional area. In order to have confidence in research findings based on corrected girth measurements, it seems necessary that the equation being used should have been validated in relation to the muscle group being assessed. Thus, equations that have been developed and validated for the thigh musculature are unlikely to be appropriate for investigations into hypertrophy of other muscles.
Methods of Muscle Mass and Volume Measurement
Introduction
Where global measures of hypertrophy are of more interest, a number of methods are available. These include dual emission x-ray absorptiometry (DEXA) scanners, Bod Pods, hydrostatic weighing, bio-impedance, and multiple-site skinfold measurement. Currently, the gold standard for measuring whole-body muscle mass is a DEXA scan.
Dual emission x-ray absorptiometry
Dual emission x-ray absorptiometry (DEXA) scans are currently considered the gold standard for measuring body composition. A DEXA scanner partitions body composition into 3 compartments: muscle, fat and bone. Any increase in what is termed the lean compartment is considered to be an increase in muscle mass. As this is a global measure, it provides information relating to changes in whole body muscle mass as opposed to localized muscle hypertrophy. Nevertheless, it is a valid means of assessing global increases in muscle content.
Hydrostatic weighing
Hydrostatic weighing is an underwater weighing technique that can be used to estimate body composition. Because body fat is less dense than water, and fat free mass has a greater density than water, hydrostatic weighing can be used to determine body density. From this determination of body density, and after having corrected for residual volume, it is possible to estimate body fat percentage. Residual volume refers to the amount of air remaining in the lungs following maximal expiration. If residual volume is estimated, rather than measured directly, the accuracy of subsequent body fat percentage estimation is compromised.
The procedure for hydrostatic weighing is as follows:
- Measure body mass to the nearest 0.1kg
- Measure or estimate residual volume
- Set the scale to zero in the underwater weighing tank (so that measurements are of the weight of the subject and not of the combined weights of the chair and the subject)
- Once seated in the tank, instruct the subject to exhale as fully as possible, slowly bend forward until fully submerged and then remain motionless until the scale settles and a reading of underwater weight can be taken.
When this procedure is followed, the following equation can be used to determine body density:
Body Density = BM / [((BM – UW) / water density) – residual volume – 0.1]
Where BM is Body Mass and UW is underwater weight.
Following the determination of body density, either the Siri equation (Siri, 1961) or the Brozek equation (Brozek et al., 1963) can be used to estimate body fat percentage.
Siri Equation Body Fat % = [(495 / Body Density) – 450] x 100
Brozek Equation Body Fat % = [(4.570 / Body Density) – 4.142] x 100
BodPod
BodPod machines use air displacement as a means of assessing body composition. This is based on the same principles as for hydrostatic weighing, except that air displacement is used rather than water displacement. Air displacement and water immersion methods both provide estimates of fat mass versus non-fat mass, which can be used as a proxy marker of muscle mass. As these are also global measures, it is not possible to determine where changes are occurring.
Bio-impedance
Bio-impedance devices are commonly used to provide an estimation of body fat percentage. They measure the electrical impedance through body tissues, which can be used to provide an estimate of total body water. This in turn can be used to estimate body fat percentage. Bio-impedance measurements have large amounts of error associated with them. The other previously discussed methods are preferred methods of determining skeletal muscle hypertrophy.
Multiple-site skinfolds
In skinfold testing, a pinch of skin (a skinfold) is taken at one or several sites and a caliper is used to measure the thickness of each skinfold. Numerous equations have been developed, using different populations and different skinfold sites, for the estimation of body fat percentage from skinfold measurements. Multiple-site skinfold measurements have large amounts of error associated with them. The other previously discussed methods are preferred methods of determining skeletal muscle hypertrophy.
24-hour urinary creatinine
A strong correlation is known to exist between total body creatine and urinary excretion of creatinine. On the assumption that nearly all creatine is within muscle tissue, that muscle creatine content remains constant and that creatinine is excreted at a uniform rate, it is proposed that urinary creatinine is proportional to muscle mass (Heymsfield et al., 1983). The use of urinary creatinine for the estimation of total body muscle mass appears to be valid in adult males ingesting a meat free diet (Wang et al., 1996). However, this type of diet may not be representative of the diet of most athletic populations. Further research into the this method may therefore be needed before we can have confidence in its validity as an estimate of total body muscle mass.
Stable isotope tracing
Stable Isotope Tracing is used to observe the movement of certain materials in chemical, biological or physical processes. In relation to muscle, we are interested in the movement of certain amino acids in order to measure the synthesis and degradation of protein. The term muscle protein balance has been used to describe the difference between the rates of synthesis and the rates of breakdown.
Measuring muscle protein balance involves using a labeled amino acid as a tracer. It is assumed that this amino acid is reflective of most of the amino acids involved in synthesising muscle protein. The tracer moves into the muscle protein; and if an intervention is able to increase the rate of synthesis of muscle proteins, more of the tracer will go in – and you can measure the change. You are measuring the rate of incorporation of the labeled amino acids that go into the muscle cell. But to measure muscle protein balance you also need to be measuring the rate at which the proteins are leaving the cell. To do this we need to look for the appearance of amino acids into the muscle pool itself and also look for the appearance into the blood of amino acids that are being broken down and released from the muscle.
Acute changes in muscle protein balance in response to particular training interventions are limited as a measure of hypertrophy, since this methodology is reliant on the assumption that acute training responses can accurately predict chronic adaptation. Alternative methods such as those previously discussed are therefore required if we are to investigate hypertrophy in longitudinal training studies.
SUMMARY - Measring Muscle Mass
Hypertrophy is the increase in muscle mass. It can be measured either locally by reference to muscle cross-sectional area or globally by reference to whole-body muscle mass. There are two key measurement types for muscle cross-sectional area: physiological cross-sectional area and anatomical cross-sectional area. Physiological cross sectional area is the best measurement of muscle cross-sectional area, as it corrects for changes in pennation angle. The gold standards for measuring physiological cross sectional area are magnetic resonance imaging and computed tomography. The gold standard for measuring whole-body muscle mass is the DEXA scan.
This means we may be more likely to have confidence in research findings on hypertrophy, where changes in Cross-sectional area or muscle mass are measured using Magnetic resonance imaging, computed tomography or DEXA scanners. The appropriateness of each of these measurement methods will depend on the nature of an investigation. For example, we might expect an increase in whole body muscle mass in response to a nutritional intervention. In this situation a DEXA scan would be the most appropriate measurement method. Where local hypertrophy is expected, such as might occur in a training programme targeting a specific muscle group, MRI or CT scans would be more appropriate.
REFERENCES - Measuring Muscle Mass
- Brozek, J., Grnade, F., Anderson, J. T. & Keys, A. (1963). ‘Densitometric analysis of body composition: revision of some quantitative assumptions’. Annals of the New York Academy of Sciences, 110, pp. 113-40
2. Heymsfield, S. B., Arteaga, C., McManus, C., Smith, J. & Moffitt, S. (1983). ‘Measurement of muscle mass in humans: validity of the 24-hour urinary creatinine method’. American Journal of Clinical Nutrition, 37 (3), pp. 478-94
3. Siri, W. E. (1961). Body Composition from Fluid Space and Density. In: Brozek and Hanschel (Eds.). Techniques for Measuring Body Composition. Washington, DC: National Academy of Science
4. Thiel, W. (1992a). ‘An arterial substance for subsequent injection during preservation of the whole corpse’. Annals of Anatomy, 174 (3), pp. 197-200
5. Thiel, W. (1992b). ‘The preservation of the whole corpse with natural color’. Annals of Anatomy, 174 (3), pp. 185-95
6. Thiel, W. (2002). ‘Supplement to the conservation of an entire cadaver according to W. Thiel.’. Annals of Anatomy, 184 (3), pp. 267-9
7. Wang, Z. M., Gallagher, D., Nelson, M. E., Mathews, D. E. & Heymsfield, S. B. (1996). ‘Total-body skeletal muscle mass: evaluation of 24-h urinary creatinine excretion by computerized axial tomography’. American Journal of Clinical Nutrition, 63 (6), pp. 863-9
Additional Notes
This resource page on measuring muscle mass was developed by Tim Egerton whilst working on a previous project. It is unlikely Tim will provide further resource pages on measuring muscle mass. This is because Tim is now primarily focused on his current business Foxwood Personal Training in York. There are three main services for Foxwood Personal Training. More information on these can be found through the links below:
You can also get in touch via DM on the Foxwood Personal Training Instagram page. I look forward to hearing from you. You may wish to ask questions about the above services for Foxwood Personal Training, York. Or you may have some questions regarding this resource page on measuring muscle mass. Either way, do get in touch. I will be more than happy to answer your questions.