Muscle Mass Measurement
There are several different methods for measuring or calculating the amount of body muscle mass. The only way to actually directly measure muscle mass is via cadaver dissection, which is not usually a viable option! There is no highly accurate and acceptable noninvasive technique of measuring muscle mass. All of these methods described below are estimations, and based on different assumptions, and with varying degrees of accuracy.
In a simple two-compartment model of the human body, we are composed of fat mass and lean (non-fat, fat-free) mass. Fat mass can be calculated using a percentage body fat measure: %BF x total body weight = fat mass weight. The rest of your weight can be considered lean mass, which includes muscle mass, organs, skin and bones etc. Measuring just the muscle mass component of lean body mass is more complex.
Girth measurements can be used to monitor changes in muscle mass. As the girth measure will also include the fat underlying the skin, any changes in your body fat will affect the results. Therefore, girth measures themselves are only a rough guide to muscle mass changes. The calculation below adjusts the girth measures based on skinfold levels in the calculation of muscle mass.
Other possible estimates of muscle mass changes to consider include total weight changes, strength changes, photographic records, resting basal metabolic rate, and muscle protein markers (3-methylhistidine plus others).
- Muscle Mass Calculation - uses a calculation based on measurements of skinfolds and girths
24-hour urinary creatinine method
Another method for determining muscle mass is through the measurement of the 24-hr urinary excretion of creatinine. This method is based on on the correlation between total body creatine and urinary excretion of creatinine, and assumes that nearly all creatine is within muscle tissue, that muscle creatine content remains constant and that creatinine is excreted at a uniform rate.
reference: SB Heymsfield, C Arteaga, C McManus, J Smith and S Moffitt, (1983) Measurement of muscle mass in humans: validity of the 24-hour urinary creatinine method, American Journal of Clinical Nutrition, Vol 37, 478-494.
The following methods are not used routinely to measure muscle mass, though it is possible to get estimates of muscle mass from these measurements. Most of these methods require sophisticated and expensive equipment, not usually available for most people.
- Dual-Energy X-Ray Absorptiometry (DEXA) - using this method you can determine body components including non fat soft tissue.
- Total Body Potassium (TBK) accurately determines the body's total cell mass (that is, the active growing tissues in the body), which in turn can be used to estimate fat-free or lean body mass. When this measurement is combined with measurements from the Total Body Protein, you can determine total organ and muscle mass.
- Magnetic Resonance Imaging (MRI) - with MRI it is possible to get accurate measurements of the composition of body tissue, by identifying muscle, fat and organs etc.
- Total Body Electrical Conductivity (TOBEC) - can be used to estimate lean body mass.
- Computed Tomography (CT) - the high quality images can be processed to differentiate and measure the amounts of fat and lean body tissue.