I’d probably separate two things here because they tend to get lumped together: myostatin is absolutely real and inhibiting the myostatin/activin pathway can increase muscle mass, but the bodybuilding idea of needing to periodically “reset myostatin” is basically bro-science and his been completely and utterly debunked as absurd and ridiculous.

ACE-031 is interesting because there actually is human research behind it. It’s an ActRIIB-Fc ligand trap, so technically it isn’t even a selective myostatin blocker, it interferes with signaling from myostatin and several related TGF-b family ligands. Human trials showed measurable increases in lean mass and muscle volume, so there’s no question that this pathway plays an important role in regulating muscle growth.

The problem is that the doses studied were considerably higher than 1 mg every 4 or 5 days. In one healthy-adult study they tested single doses ranging up to 3 mg/kg, and 3 mg/kg produced roughly a 3% increase in DXA lean mass and ~5% increase in thigh muscle volume after about a month. In the Duchenne muscular dystrophy study, they used regimens such as 0.5 mg/kg every four weeks and 1 mg/kg every two weeks.

So I’d be pretty skeptical about being able to determine much from 1 mg every 4-5 days. That doesn’t mean that dose does absolutely nothing; it means I’m not aware of human evidence demonstrating a meaningful effect at anything remotely resembling that dose.

There’s also a reason ACE-031 never became an approved muscle-building drug. Development ran into safety problems, including nosebleeds and telangiectasias, and the DMD trials were ultimately terminated. Broadly interfering with this signaling pathway apparently isn’t as simple as “less myostatin = more muscle = awesome.”

The other thing worth mentioning since you brought up cruising between growth phases: the old bodybuilding “myostatin reset” theory has never really been demonstrated.

You’ll still hear that after 8-12 weeks of a growth phase/AAS use, myostatin supposedly rises until it prevents further growth, so you need to cruise for several weeks to “reset” myostatin before another blast will work properly. There’s no good human evidence establishing an 8-, 10-, or 12-week myostatin wall, nor evidence showing that cruising for some particular period resets myostatin and restores anabolic responsiveness. This is real science. Not bro science.

Muscle growth slowing during a prolonged growth phase is obviously real. But there are a shitload of variables involved in that besides myostatin: how much muscle you already carry, diminishing returns, training adaptation, fatigue, calories, recovery, connective tissue, satellite-cell/myonuclear adaptations, etc. Reducing all of that to “myostatin got too high, time for a reset” is taking legitimate molecular biology and turning it into a bodybuilding rule that hasn’t actually been demonstrated.

So ACE-031 and the myostatin pathway are legitimate science. The bodybuilding “myostatin reset” is not.

And personally, I wouldn’t put much weight on monthly InBody changes for deciding whether ACE-031 is working. Hydration, glycogen, sodium and carbohydrate intake can move the estimated lean-mass number around substantially. If you do experiment with it, I’d at least keep those conditions as identical as possible each time you scan.