Efficacy and Safety of Bimagrumab/BYM338 at 52 Weeks on Physical Function, Muscle Strength, Mobility in sIBM Patients

Key Inclusion Criteria: * Diagnosed with sporadic inclusion body myositis; * Must be able to walk (assistive aids allowed, including intermittent use of wheelchair); Key Exclusion Criteria: * Must not have other conditions that significantly limit ability to move around; * Must not be using corticosteroids. Must not have used systemic corticosteroid (at daily dose \>=10mg prednisone) for the past 3 months; * Must meet cardiovascular requirements; * Must not be pregnant or nursing; * Must not have a chronic active infection (e.g., HIV, hepatitis B or C, tuberculosis, etc.);