Not too long ago I wrote a blog on muscle quantity vs. muscle quality. I received a lot of questions about muscle density and whether it is the same as muscle quality. If you look back to the original blog, I defined muscle quality as: “… capacity of muscle to generate force.” On the other hand, muscle density can be defined as the amount of lean muscle tissue in a given area compared to the amount of fat in that same area.
Measuring muscle density is needed to determine muscle quality. The quality of muscle is important for individuals who want to increase their strength without increasing the area of muscle. Think of a cyclist who wants to be stronger, but does not necessarily want to increase the overall size of a muscle. An increase in muscle area would create more surface area and ultimately increase the potential for air resistance.
Although a kilogram or pound of muscle weighs the same as a kilogram or pound of fat they occupy different volumes. This is demonstrated in the image below of 2.2 kilograms (4 lbs. 14 oz.) of muscle compared to 2.2 kilograms of fat.
You can see from the above image the difference in the area that 2.2 kilograms of muscle or fat occupy. This difference in volume is due to the fact that the density of muscle is approximately 1.055 g/mL (Ward & Lieber, 2005) while the density of fat is approximately 0.90 g/mL (Fidanza, 2003). In other words, muscle is about 15% denser than fat. Although dual X-ray absorptiometry (DXA) can provide a measure of muscle mass it cannot provide the distribution of muscle in a given area or volume. This measure of mass in a given area can currently only be done using magnetic resonance imaging (MRI) or computed tomography (CT). However, it must be understood that the muscle density calculated by CT is not a physical density measured in g/cm3 or g/mL, but a ‘CT density’ measured in Hounsfield units (Engelke et al., 2018). Basically, a Hounsfield Unit is a relative quantitative measurement of radio density used by radiologists in the interpretation of CT images. These units make it difficult for coaches and athletes to interpret the data. In addition, both MRI and CT image only single slices of a muscle at a time.
To image an entire muscle a series of slices covering the whole muscle must be done and then mathematically stacked together. This can be time-consuming and costly making the use of MRI and CT for the determination of muscle density only practical in research settings. Finally, as mentioned above, because both the MRI or CT images need to be converted into a measure of physical density a calibration step is required, which requires scanning a calibration object with a standardized muscle density value. Furthermore, even if it were feasible to measure muscle density with CT or MRI, it still would not represent muscle quality. To determine a measure of muscle quality you would need a measure of force or power for the muscle that you imaged.
TAKE-HOME MESSAGE
So what is the take-home message? The main point is muscle density and muscle quality are not the same thing. Currently, limitations in technology make it difficult, if not impossible, to measure muscle density or muscle quality for a single muscle. That does not mean we should not try. Researchers are continually pushing technology and it may not be that long before the ability to measure muscle density and muscle quality are within our reach.
REFERENCES
Engelke K, Museyko O, Wang, L, Laredo JD. Quantitative analysis of skeletal muscle by computed tomography imaging – state of the art. J Ortho Trans 2018;15:91-103.
Fidanza F. Body fat in adult man: semicentenary of fat density and skinfolds. Acta Diabetol 2003;40:S242-S245.
Ward SR, Lieber RL. Density and hydration of fresh and fixed human skeletal muscle. J Biomech 2005;38:2317-2320.
About the Author
Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.