This study is an open-label, long-term study for those patients who participated in the prior proof-of-concept protocol, in which the preliminary efficacy for BYM338 in patients with sIBM was demonstrated after a single 30 mg/kg i.v. dose of BYM338. This study is designed to confirm the efficacy, safety and tolerability of BYM338 in sIBM with long-term dosing.