Myostatin is a protein your muscles make to stop themselves growing. Remove it and muscle keeps building. Belgian Blue cattle, which carry a myostatin defect, are the textbook demonstration: they look like they have been inflated from the inside.
That biology made myostatin inhibitors an obvious drug target, and the scan results deliver. Lean mass goes up or holds steady. Peter Attia's latest post on the class describes the catch: drugs, new and old, that preserve lean mass fail to boost strength. The muscles get larger and the person does not get proportionally stronger.
The mechanism is where the gap opens. Size is a count of tissue on a scan. Strength is what that tissue does when a nervous system recruits it, how well fibres transmit force to tendon, and how much of the added bulk is working contractile protein. A drug that tells muscle to grow addresses the first. It has no obvious hold on the rest. Picture a bigger engine with the same transmission.
Lean mass is the number easy to measure, which is why it appears on so many slides. It is also why the claim deserves a second read. "Preserves muscle" and "keeps you able to climb stairs" are different statements, and only the second is what anyone buying the drug wants.
The timing is awkward for the biomarker enthusiasts. FDA leaders joined aging researchers at ARDD 2026 to examine clinical trials, biomarkers and the regulatory challenges facing longevity therapies. A biomarker earns its place by tracking something a patient feels or does. Lean mass on a scan, in this drug class, tracks less of that than the pitch decks assume.
The practical reader is the one scrolling supplement and protocol content on a Sunday night. Where a product advertises muscle retention, ask what was measured: grip strength, walking speed or chair rises, or only a body-composition scan. If a trial reports the scan alone, you have learned the muscle is bigger. You have not learned it is stronger.
What the evidence establishes today: myostatin blockade can maintain lean mass. What it has not shown: that the added or preserved tissue translates into function. The next real test is any trial that reports strength and physical performance as its primary outcome rather than a footnote to the scan.