Inclusion Criteria:
1. Male or female participants, 18 to 60 years of age at the time of informed consent.
2. Genetic diagnosis of FSHD Type 1 or FSHD Type 2, confirmed with the appropriate documentation from an accredited laboratory
3. Clinical severity score of 1.5 to 3.0 (Ricci score; range 0 to 5), inclusive, at screening
4. Baseline 10-meter walk/run test time ≤5 seconds
Exclusion Criteria:
1. Prior history of a hypersensitivity reaction to a mAb or recombinant protein bearing an Fc domain (eg, a soluble receptor-Fc fusion protein), apitegromab, or excipients of apitegromab
2. Treatment with other investigational drugs in a clinical trial within 3 months or 5 half-lives, whichever is longer, before screening
3. Previous treatment with apitegromab, or with other anti-myostatin therapies, including activin receptor antagonists
4. Current or prior use of anabolic steroids, growth hormones, glucagon-like peptide-1 receptor agonist or other substances with known effects on muscle.
5. Use of therapies with potentially significant muscle effects (eg, androgens, insulin-like growth factor, growth hormone, systemic beta-agonist, botulinum toxin, or muscle relaxants or muscle-enhancing supplements) or potentially significant neuromuscular effects (eg, acetylcholinesterase inhibitors) within 60 days before screening
6. Use of systemic or corticosteroids within 60 days prior to screening. Inhaled or topical steroids are allowed.
7. Pregnant or breastfeeding.
8. Contraindications for MRI that may include, but are not limited to, certain implanted electronic devices, cochlear implants, metallic foreign bodies, vascular clips, and metallic implants; or claustrophobia, contrast agent allergies, inability to lie still, or external medical devices that may not be removed.
History of alcoholism, or illicit drug use (drugs that are illegal and have not been prescribed).
Taking medications that impede coagulation or platelet aggregation or has a history or active coagulopathy disorder.
Any acute or comorbid condition interfering with the well-being of the participant within 7 days prior to screening, including active systemic infection, the need for acute treatment, or inpatient observation due to any reason
