How Muscle Size Is Measured In Research

Studies reporting muscle growth are measuring quite different things depending on the method used. Understanding what each technique captures explains much of the apparent disagreement between trials.

Cross-sectional area from imaging

Magnetic resonance imaging and computed tomography produce slices through a limb, from which the area of a muscle at that point can be traced.

This is considered the most accurate approach available, since it distinguishes muscle from fat and bone directly and does not rely on assumptions about their proportions.

It is also expensive and slow, which is why studies using it tend to be small, and small samples make modest effects harder to detect.

Ultrasound and what it measures

Ultrasound measures muscle thickness at a marked site, and it is inexpensive enough to be used in larger and longer studies.

Its weakness is operator dependence: the angle and pressure of the probe change the reading, and relocating the exact site weeks later introduces error.

It also measures thickness at a point rather than the whole muscle, and muscles do not grow uniformly along their length, so the point chosen affects the result.

Whole-body composition methods

Dual-energy X-ray absorptiometry divides the body into fat, lean and bone compartments, which is useful for overall change and imprecise for a single muscle.

The lean compartment includes water, organs and connective tissue, so shifts in hydration alone can move the reading in either direction.

Bioelectrical impedance devices infer composition from electrical resistance and rest on assumptions about hydration that are frequently violated around training.

Why swelling contaminates short studies

Training draws fluid into muscle, and that swelling can persist for days, inflating any measurement of size taken during it.

Studies of only a few weeks are particularly vulnerable, since a portion of the apparent growth may be fluid rather than tissue.

Careful protocols standardize the interval since the last session precisely for this reason, and studies that do not report it are harder to interpret.

What this means for reading results

Two trials reaching different conclusions may be measuring different quantities with different precision rather than describing different biology.

Effect sizes in this area are generally small relative to measurement error, which is why single studies rarely settle anything and reviews carry more weight.

It also means personal measurements taken with a tape measure sit far outside the precision these methods struggle to achieve, and should be read accordingly.