In this article14 sections
Nobody knows: it has never been tested in healthy or trained people. Animals born without myostatin are huge, but in adults, myostatin-blocking drugs have added only modest muscle and rarely more strength. It carries real risks, including fractures, and it is banned in sport.
Myostatin is the body's brake on muscle growth. Switch it off from birth and animals become extraordinarily muscular, which is why every new drug that blocks it gets attention from bodybuilders. Apitegromab, approved in the US in September 2026 for the muscle-wasting disease spinal muscular atrophy, is the first myostatin blocker ever to reach the market, and the questions have followed. This article sets out what the evidence actually says about myostatin blockers and muscle growth: what happens in animals, what the drug trials in people found, what is known about apitegromab specifically, the risks, and the anti-doping position. It is an account of published research. It is not guidance on using any drug, apitegromab cannot legitimately be obtained outside a hospital, and ALTA PEPTIDES does not supply it.
- Mice bred without myostatin have individual muscles weighing two to three times normal (McPherron, Lawler and Lee, Nature 1997).
- Double-muscled Belgian Blue and Piedmontese cattle, 'bully' whippets and one documented German boy all carry myostatin mutations.
- In people, myostatin-blocking drugs have added modest : about 3.3% total lean mass after one injection of ACE-031 in healthy older women, and 0.43 kg of arm and leg lean mass with landogrozumab in adults over 75.
- Extra muscle has rarely translated into extra strength or function: bimagrumab raised lean mass about 6% more than placebo in older adults but did not significantly improve walking or physical performance.
- No myostatin antibody, apitegromab included, has been tested in healthy young or resistance-trained people.
- Myostatin inhibitors are prohibited at all times under section S4.3 of the World Anti-Doping Agency list.
Can apitegromab build muscle in bodybuilders?
In shortUnknown. It has never been tested in healthy or trained people, and similar drugs added only a little muscle.
Assessment based on the trials in this article. 'None measured': the only healthy-volunteer study of apitegromab did not measure muscle.
The honest answer is that nobody knows, because it has never been tested in healthy or trained people. What is known points to a modest effect at most. Apitegromab's trials were in children with spinal muscular atrophy, where it improved motor function, and in adults losing weight on tirzepatide, where it reduced the amount of muscle lost. Other drugs that block the same pathway have been given to healthy and older adults, and they added a little lean mass, roughly in the range of half a kilogram to a few percent, with little or no gain in strength. None of that resembles the dramatic results people associate with myostatin, and the reasons are below.
Why are bodybuilders interested in myostatin?
In shortAnimals born without myostatin are hugely muscular. That's where the hype comes from.
Individual muscles weighed two to three times normal.
Belgian Blue and Piedmontese breeds.
One faulty copy: over-represented among top racers.
Two faulty copies; mother a former pro athlete.
McPherron et al. 1997; Grobet et al. 1997; Mosher et al. 2007; Schuelke et al. 2004.
Because the animal evidence is spectacular. In 1997, Se-Jin Lee's group at Johns Hopkins reported that mice bred without the myostatin gene had muscles weighing two to three times normal, from both more muscle fibres and bigger ones. The same year, researchers found that the double-muscled Belgian Blue and Piedmontese breeds of cattle carry natural myostatin mutations. In 2007, "bully" whippets, unusually muscular racing dogs, were traced to a myostatin mutation too, and dogs with one faulty copy were over-represented among the fastest racers. In 2004, doctors in Germany described a boy with two faulty copies of the gene who was born with very large muscles and unusual strength; his mother, who carried one copy, was a former professional athlete. Pictures of those animals are where the bodybuilding interest comes from.
What have myostatin drugs done in real people?
In shortA little more lean mass, and almost no extra strength.
| More lean mass? | More strength or function? | |
|---|---|---|
| ACE-031 | Yes, +3.3% | Not tested; stopped over bleeding |
| Landogrozumab | Yes, +0.43 kg limbs | Some tests, weak evidence |
| Bimagrumab | Yes, ~6% vs placebo | No significant gain |
| Stamulumab | A few patients | No |
| Domagrozumab | Not the goal | No; stopped 2018 |
| Apitegromab | Less loss on tirzepatide | Better motor scores in SMA |
Percent lean mass by DXA scan. Attie et al. 2013; Rooks et al. 2020; Becker et al. 2015; Wagner et al. 2008; Pfizer 2018; Scholar Rock 2025-26.
Much less. Several companies have tried to block myostatin in people, mostly to treat muscle diseases and age-related muscle loss, and the results have been consistent: a small increase in lean mass, and little effect on strength or function.
- ACE-031 (a decoy receptor) given once to healthy women after the menopause raised total lean mass by 3.3% and thigh muscle volume by 5.1% at four weeks, at the highest level tested.
- Landogrozumab (LY2495655) in 201 adults aged 75 and over who had fallen added 0.43 kg of arm and leg lean mass against placebo, with small improvements in some tests like stair climbing that were judged against a relaxed statistical threshold.
- Bimagrumab in 180 older adults with low muscle mass, all also given diet and exercise advice, raised lean mass about 6% more than placebo but did not significantly improve walking distance or physical performance.
- Stamulumab (MYO-029) in adults with muscular dystrophy showed no improvement in strength or function, with signs of larger muscles in only a few patients.
- Domagrozumab in boys with Duchenne muscular dystrophy missed its main goal, and Pfizer stopped the programme in 2018.
Apitegromab fits the same pattern. In SMA it improved motor scores; in the EMBRAZE obesity trial it cut lean-mass loss on tirzepatide by 1.9 kg. Both are meaningful medical effects, and neither is muscle building in the bodybuilding sense.
Why doesn't more muscle mean more strength?
In shortScans count more than muscle, and strength depends on nerves and tendons too.
- 01Lean mass rises
On the scan: includes water and connective tissue
- 02Fibres get bigger
Not more of them, in adults
- 03Nerves and tendons
Set how much force comes out
- 04Strength
Changes little in the trials
From this article; tendon finding from Mendias et al., PNAS 2008 (mice).
Lean mass on a scan counts everything that isn't fat or bone, including water and connective tissue, so a rise in lean mass is not the same as a rise in working muscle. And even where muscle fibres grow, strength depends on the nerves that drive them, how the fibres are organised, and the tendons that transmit force. In the myostatin-free mice, the extra muscle came with tendons that were smaller, stiffer and more brittle, which is one explanation for why size and strength don't move together. Across the human trials, the gap between the scan results and the strength results is the most consistent finding.
Is apitegromab different from the other myostatin drugs?
In shortIt's the most targeted, but there's still no muscle-growth data in healthy people.
Swipe the table to compare →
| Apitegromab | Older myostatin antibodies | Bimagrumab | |
|---|---|---|---|
| Grabs | Myostatin before it's switched on | Active myostatin | The shared receptor |
| How selective | Myostatin only | Some cross-reaction | Blocks several signals |
| Muscle data in healthy humans | None | Older adults only | Older adults only |
Pirruccello-Straub et al. 2018; Barrett et al. 2021; this article.
In one important way. Most earlier antibodies grabbed the active form of myostatin, which looks similar to related signals such as activin A and GDF11, so they were less precise. Apitegromab grabs myostatin earlier, in its inactive packaged form, which is unique to myostatin. In healthy mice, Scholar Rock's antibodies of this type produced clear muscle growth and better performance. In healthy human volunteers, the only apitegromab trial measured blood levels and safety, and reported no muscle measurements at all. So the selectivity makes it the cleanest myostatin blocker yet, but there is no human evidence that it builds muscle in someone who is healthy, let alone trained.
Would more of it build more muscle?
In shortNo. Doubling the amount in trials did no better: the target was already blocked.
The trial data suggest not beyond the standard amount. In the SMA trial, doubling the amount from 10 to 20 mg/kg did no better, because both levels already locked up the target to a similar degree, which is a ceiling effect. Our apitegromab article sets out those figures. A drug like this can only remove the brake that is there; once it has, there is nothing more to remove.
Why the animal results don't carry over to adults
In shortAnimals born without it grew extra muscle fibres; adults can't.
| Born without it | Drug in an adult | |
|---|---|---|
| Muscle fibres | More, and bigger | Same number, a bit bigger |
| How complete | Total, from day one | Partial |
| How long | For life | Only while it's given |
| Other brakes (activin A) | Still work | Still work |
From this article.
Animals and people born without myostatin lacked it while their muscles were forming, so they built more muscle fibres as well as larger ones. In adults the number of fibres is largely fixed, so a drug can only enlarge the ones already there. Drugs also block the brake partly rather than completely, only while they are being given, and the body has other brakes (activin A among them) that keep working. That is why the drugs that block more of these signals at once, such as bimagrumab, have added more lean mass than the narrow myostatin-only antibodies, and also why they bring more side effects.
What are the risks?
In shortFractures, bleeding with a related drug, brittle tendons in mice, and a lot that's unknown.
SMA trial; label warning.
Nosebleeds and dilated vessels; programme stopped.
At higher levels.
Smaller, stiffer, more brittle.
Never studied.
Never studied.
Scholar Rock 2026; Campbell et al. 2017; Wagner et al. 2008; Mendias et al. 2008.
The known risks come from the trials. In SMA, fractures were more common with apitegromab (9%) than placebo (2%), and the US label warns about fracture risk and possible harm to an unborn baby. ACE-031's development in Duchenne muscular dystrophy was stopped after nosebleeds and small dilated blood vessels in the skin and gums. Stamulumab caused skin reactions at higher levels. In mice with no myostatin, tendons were small, stiff and brittle, which matters for anyone lifting heavy loads. What happens with long exposure in healthy adults, or alongside other performance drugs, has never been studied.
Is apitegromab banned in sport?
In shortYes, at all times, by WADA.
Agents preventing activin receptor IIB activation.
In and out of competition.
Alongside bimagrumab and others.
World Anti-Doping Agency Prohibited List.
Yes. The World Anti-Doping Agency's Prohibited List covers myostatin inhibitors under section S4.3, which includes antibodies against myostatin and its precursor forms, and the class is banned at all times, in and out of competition. Apitegromab has been named on the list since 2023, and bimagrumab, landogrozumab, domagrozumab and stamulumab are named too. Any competitive athlete subject to testing would be committing an anti-doping violation.
Can you get apitegromab?
In shortNo. Online versions are not the real drug.
US, for SMA only.
Not the branded drug; check the CoA.
150 mg vials, laboratory research only.
From this article.
No. Real apitegromab is a large antibody grown in specialised cell-culture plants and given as a hospital infusion, approved only for spinal muscular atrophy in the US. It can't be made the way peptides are; research-grade versions come from specialist antibody manufacturers and are laboratory reagents, not for human use. Products sold online as apitegromab, or as generic "myostatin inhibitors", are not the antibody; at best they are something else, at worst contaminated or harmful, and there is no way to tell. ALTA PEPTIDES supplies research-grade versions of these antibodies as laboratory reagents under our research use policy: recombinant antibodies made by a third-party manufacturer, not the products of the originator companies.
Everything above is the quick version. The sections below go deeper.
How myostatin works
In shortThe science of the muscle brake and the receptors it shares.
Myostatin, also called GDF-8, belongs to the TGF-beta family of signalling proteins. It is made inside muscle as a precursor, promyostatin, which is cut into a latent complex and then released as a short mature protein that binds activin type II receptors (ActRIIB especially) on muscle cells. That signal holds back muscle growth and the activity of the stem cells that repair muscle. Activin A and GDF11 use the same receptors, which is why blocking the receptor (bimagrumab) or using a decoy receptor (ACE-031) has broader effects than blocking myostatin alone. Our myostatin inhibitor guide explains the pathway and the four antibodies now in obesity trials.
The trials in detail
In shortThe full figures from each human trial.
ACE-031: one injection in 48 healthy postmenopausal women; at day 29 the top group had +3.3% total lean mass by DXA and +5.1% thigh muscle volume by MRI (Attie et al., Muscle & Nerve 2013). Landogrozumab: 201 people aged 75 and over with a recent fall; appendicular lean mass rose 0.303 kg against a fall of 0.123 kg on placebo, a difference of 0.43 kg; some timed tests improved against a pre-set threshold of p<0.1, others did not; injection-site reactions in 30% against 9% (Becker et al., Lancet Diabetes & Endocrinology 2015). Bimagrumab: 180 adults aged 70 and over with sarcopenia, both groups on diet and home exercise; lean mass rose about 7% against 1%, but the physical performance score (+1.34 against +1.03) and six-minute walk (+24.6 m against +14.3 m) did not differ significantly (Rooks et al., JAMA Network Open 2020). Stamulumab: 116 adults with Becker, facioscapulohumeral or limb-girdle muscular dystrophy; no improvement in exploratory strength or function measures (Wagner et al., Annals of Neurology 2008). Apitegromab in healthy adults: a phase 1 study of safety, blood levels and target engagement, with no muscle outcomes reported (Barrett et al., Advances in Therapy 2021).
Sources
- McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature 1997;387:83-90. PMID 9139826.
- Grobet L, et al. A deletion in the bovine myostatin gene causes the double-muscled phenotype in cattle. Nature Genetics 1997;17:71-74; McPherron AC, Lee SJ. Double muscling in cattle due to mutations in the myostatin gene. PNAS 1997;94:12457-12461. PMID 9356471.
- Mosher DS, et al. A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genetics 2007;3(5):e79. PMID 17530926.
- Schuelke M, et al. Myostatin mutation associated with gross muscle hypertrophy in a child. New England Journal of Medicine 2004;350:2682-2688. PMID 15215484.
- Mendias CL, Bakhurin KI, Faulkner JA. Tendons of myostatin-deficient mice are small, brittle, and hypocellular. PNAS 2008;105(1):388-393. PMID 18162552.
- Attie KM, et al. A single ascending-dose study of muscle regulator ACE-031 in healthy volunteers. Muscle & Nerve 2013;47:416-423. PMID 23169607; Campbell C, et al. Muscle & Nerve 2017 (ACE-031 in Duchenne). PMID 27462804.
- Becker C, et al. Myostatin antibody (LY2495655) in older weak fallers: a proof-of-concept, randomised, trial. Lancet Diabetes & Endocrinology 2015;3:948-957. PMID 26516121.
- Rooks D, et al. Bimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial. JAMA Network Open 2020;3(10):e2020015. PMID 33074327.
- Wagner KR, et al. A phase I/II trial of MYO-029 in adult subjects with muscular dystrophy. Annals of Neurology 2008;63:561-571. PMID 18335515.
- Pfizer press release, 30 August 2018: termination of domagrozumab clinical studies in Duchenne muscular dystrophy.
- Pirruccello-Straub M, et al. Blocking extracellular activation of myostatin as a strategy for treating muscle wasting. Scientific Reports 2018;8:2292; Barrett D, et al. A randomized phase 1 safety, pharmacokinetic and pharmacodynamic study of the novel myostatin inhibitor apitegromab (SRK-015). Advances in Therapy 2021;38:3203-3222. PMID 33963971.
- World Anti-Doping Agency. Prohibited List, section S4.3 (agents preventing activin receptor IIB activation); WADA 2023 summary of modifications (apitegromab added).
- Scholar Rock press release, 11 September 2026: FDA approval of ISEMBYLD (apitegromab-mstn), including the fracture warning.
Questions people ask
Does apitegromab build muscle in healthy people?
Nobody knows, because it has never been tested in them. Its human trials were in children with spinal muscular atrophy and in adults losing weight on tirzepatide, where it reduced muscle loss. The only trial in healthy adults measured blood levels and safety, not muscle.
Why do myostatin-free animals get so big if the drugs don't?
Animals born without myostatin lack it from before birth, while muscle is still forming, so they grow more muscle fibres as well as bigger ones. A drug given to an adult can only work on the fibres already there, and it blocks the brake only partly and for as long as it is given.
Is apitegromab banned in sport?
Yes. Myostatin inhibitors are on the World Anti-Doping Agency's list under S4.3, prohibited in and out of competition, and apitegromab has been named on the list since 2023.
What are the risks of myostatin blockers?
Known so far: more fractures with apitegromab in its SMA trial (9% against 2% on placebo), nosebleeds and small dilated blood vessels with ACE-031, and skin reactions with stamulumab. In mice with no myostatin, tendons were smaller, stiffer and more brittle. Long-term effects in healthy people are unknown.
Can you buy apitegromab for bodybuilding?
No. The branded medicine is a hospital infusion available only for spinal muscular atrophy in the US. Research-grade apitegromab is a laboratory reagent for research and is not for human use.
Information, not advice. This article summarises published research; nothing here is guidance on human or veterinary use. Products are sold under our research use policy.
