DXA
DXA is a low-dose X-ray scan that estimates how much of the body is fat, lean tissue and bone.
It works by passing two X-ray energies through the body and inferring composition from how much of each is absorbed. It is among the more reliable methods available and is common in research.
It remains an estimate built on assumptions, and results vary between machines, between software versions, and with how hydrated the person was on the day.
What it is not
DXA is not a direct measurement of muscle and not an exact figure. Comparing a DXA result from one clinic with one from another, or across a software update, can produce a difference that reflects the instrument rather than the person.