Measuring Muscle Mass

AARP started sending mail about a month ago. I’ve decided I do not need a trunk-organizing insulated cooler. I did, however, decide to get a mega lab panel—which, thankfully, calculated my biological age to be in the low 40s. Today, I’m celebrating a milestone birthday—yes, that’s right, the big 5-0.

Turning 50 has given me time to think about what I want this next decade to look like. In my 30s, my focus was keeping stress low. In my 40s, my singular goal was to avoid any additional joint surgeries—and I’m happy to say I accomplished that. For my 50s, I’ve decided my main goal will be to maintain muscle mass.

Skinny Fat

Sarcopenia, or age-related muscle loss, is a very real threat for aging adults. Even more concerning is sarcopenic obesity, often referred to as being “skinny fat.” This occurs when loss of muscle mass is paired with an increase in adipose (fat) tissue. When fully clothed, a person may appear to be a “normal weight” and have a BMI in the healthy range; however, in a swimsuit there may be noticeable excess body fat with little visible muscle tone.

We have known for some time that being “skinny fat” is metabolically unhealthy—often as unhealthy as obesity, and in many cases more metabolically harmful than having a BMI in the overweight range while being physically active and maintaining adequate muscle mass. Although weight may appear normal on the scale, the body is metabolically weaker and less resilient.

Evaluating Muscle Mass

There are several technologies available to help estimate muscle mass and evaluate for the presence of sarcopenic obesity. Each has strengths and limitations.

Bioimpedance Scales

Many people own a bioimpedance scale—often purchased for several hundred dollars. This technology works by sending a small electrical signal through the body. The signal travels quickly through water and muscle and is resisted by adipose tissue.

Bioimpedance can provide a rough estimate of body fat percentage, but it is truly a ballpark measurement. The typical error range is ±3–5% body fat which means that the error for muscle percentage is similar. For example, if a scale reports 20% body fat, the true value could realistically be anywhere from 15–25%. This explains why body fat readings on bioimpedance scales can fluctuate dramatically from day to day without any meaningful change in diet or activity.

Calipers

The use of skinfold calipers to measure body fat is an oldie-but-goodie method. While calipers do not directly measure muscle mass, they can be useful for estimating subcutaneous body fat. When performed perfectly by a skilled examiner, calipers have an error range of approximately ±3–4% body fat, similar to bioimpedance. In real-world settings, however, the error is more realistically ±5–7%, largely due to technique variability.

Although calipers do not measure muscle directly, one can infer that lower body fat percentages are generally associated with relatively higher lean mass—particularly when measurements are tracked consistently over time.

Dual-Energy X-ray Absorptiometry (DXA or DEXA)

DXA is the most commonly used clinical method for assessing muscle mass. Studies comparing DXA to MRI have shown similar accuracy and measurements. One study in nonhuman primates confirmed that both DXA and MRI accurately measured muscle size, though MRI appeared slightly more precise.

DXA remains widely used because it is accessible, relatively low radiation, and provides regional body composition data.

MRI (and CT Scans)

MRI has been used to evaluate muscle size and density, although it has not been fully validated as a standard diagnostic tool for sarcopenia. It is also more expensive than other methods of assessing muscle mass. One key advantage of MRI is its ability to provide additional information beyond size alone, including muscle inflammation (edema), fibrosis (scarring), and fat content within muscle tissue.

MRI is particularly valuable for assessing fat infiltration within muscle, not just overall muscle size. In contrast, CT scanning—despite being able to measure muscle mass—delivers a relatively high radiation dose and therefore should not be used solely for this purpose.

Current MRI-based approaches to assessing muscle mass typically focus on the mid-thigh, the lumbar region at the L3 (third lumbar vertebra), or the paraspinal muscles running along the spine, which may be measured at the neck or other spinal levels. If a person chooses to have muscle mass assessed by MRI, the cost is typically not covered by insurance and is usually paid out of pocket.

Ultrasound

Ultrasound has been used primarily in research settings. A 2019 study demonstrated that ultrasound could detect muscle bulk changes comparable to MRI just six weeks after baseline in individuals undergoing exercise interventions.

My Thoughts

Although there are ways to directly measure muscle mass and objectively track fitness as we age, I’m not convinced the additional expense is always necessary. If I can do 20 push-ups and perform at least one pull-up, I know I’m doing a good job maintaining my muscle strength.

I also know that consistently varying my workouts helps maximize benefits and prevents plateaus, which can otherwise contribute to catabolic remodeling and muscle loss. As long as I strength train at least once per week—and ideally twice per week—I can maintain and even build muscle mass over time.

Often, you hear people say they don’t want to get older. A friend of mine, who lost a dear friend to cancer in her early 30s, helped me understand what a gift it truly is to grow older—and that we should always be proud of our age. Aging gracefully, and aging with strength, should be a priority.