Activin A may matter more than myostatin for human muscle growth. Blocking both together builds far more muscle than blocking myostatin alone, according to human studies. https://s.fitbodyscience.com/KMw8AP
Very strong evidence
Multiple randomized or controlled trials back these claims.
Myostatin blockers bulk up mice but disappoint in humans — activin A may be the real muscle regulator.
Rodent studies support the 20–30% muscle increase from blocking myostatin, but results in humans are mixed. Some trials found little to no effect, like a myostatin-blocking drug that barely increased muscle in older adults and didn't help boys with Duchenne muscular dystrophy. However, other research found that blocking myostatin did increase muscle mass, especially when paired with an activin A blocker. That same study also showed that targeting both myostatin and activin A together gives a bigger boost than blocking either alone, suggesting activin A is at least as important. There's no direct evidence on whether humans have less myostatin than mice, so that part remains unconfirmed.
So while myostatin is clearly a powerful muscle regulator in mice, its role in humans is less clear-cut, and other factors like activin A might be more important for building muscle in people.
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Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.