Myostatin Inhibitors Can Build Bigger Muscles — But Not Stronger Ones

The foundational assumption behind myostatin inhibition is that myostatin — the protein that limits muscle growth — is the main bottleneck standing between your current muscle mass and more of it. Block myostatin, grow more muscle, get stronger. The pathway is clean in animal models. The pathway is not clean in humans.

A pattern is accumulating across clinical trials: myostatin inhibitors reliably increase lean tissue volume measurable on MRI. They do not reliably produce corresponding gains in grip strength, walk test performance, or functional outcomes. The muscles appear to be getting bigger. They aren’t getting stronger.

The Compounds and What the Trials Show

Apitegromab blocks the precursor proteins that activate myostatin. In the EMBRAZE trial — evaluating apitegromab combined with tirzepatide for body composition — the drug produced 2.2 kg additional lean mass preservation versus placebo. That’s a real and meaningful mass difference. Both groups showed identical sit-to-stand improvements: 2.4 additional reps per group, no significant between-group difference.

Bimagrumab takes a different mechanism — it blocks the ActRII receptor that both myostatin and activin A use to suppress muscle growth. It consistently increases muscle volume on MRI across multiple trials. In a separate study context, patients showed no improvement in 6-minute walk tests, grip strength measurements, or functional outcomes despite the measurable volume increase.

The pattern holds across both compounds, different mechanisms, different patient populations. More muscle by scan measurement; no more strength by functional assessment.

Four Hypotheses for the Disconnect

1. Non-contractile tissue masquerading as muscle

Muscle volume on MRI includes connective tissue, fat infiltrates, and water, not just contractile fiber. A drug that triggers tissue growth could be expanding the non-contractile components that fill the muscle compartment — producing a bigger muscle on the scan without adding contractile capacity. The MRI measurement and the functional test are measuring different things.

2. Fiber alignment disruption with rapid growth

New muscle fiber must integrate with the existing tendon attachment structure and align appropriately with the force production axis. Rapid pharmacologically-induced growth could be producing fiber that isn’t properly aligned with the mechanical load path — producing volume without proportionate force output. This is speculative but mechanically plausible.

3. Neuromuscular junction underdevelopment

Muscle fiber volume and neuromuscular junction development aren’t necessarily linked. A fiber that grows faster than its neuromuscular junction can mature is a fiber that can’t receive and execute motor signals at full capacity. The neural drive to the expanded muscle mass may be insufficient to express the mechanical potential of the added tissue.

4. Fibrosis contributing to volume measurement

Fibrotic tissue — scar-like deposits within the muscle — contributes to MRI volume measurements and looks similar to healthy muscle in some imaging protocols. Drug-induced changes to the extracellular matrix could be producing non-functional volume that appears as lean mass gain.

The Resistance Training Comparison

These trials don’t have a resistance training arm, but the comparison is implicit. Resistance training produces both muscle hypertrophy (volume increase) and strength gain. The volume increase from resistance training is predominantly contractile — it involves adding sarcomeres, improving fiber alignment with load, and developing the neural pathways that drive the muscle. The quality of the volume increase is different.

This suggests the limitation of myostatin inhibitors isn’t just a dosing or compound selection problem — it may be structural: pharmacological myostatin inhibition doesn’t trigger the mechanotransduction cascade that resistance training uses to produce functional hypertrophy.

The Implication

None of the current myostatin inhibitors look like a replacement for resistance training as a tool for building functional strength. They may still have clinical value in contexts where the goal is preserving lean mass under muscle-wasting conditions — sarcopenia, cachexia, disease-related muscle loss — where the question is whether any muscle remains, not whether that muscle is optimally functional.

For healthy individuals pursuing performance: the current evidence doesn’t support myostatin inhibition as a strength-building intervention. The mass gain is real. The strength gain is not following from it.

Source: Peter Attia MD

Content created with AI assistance and reviewed for accuracy.