Disclosed is an isolated human antibody or antigen-binding fragment thereof that specifically binds to, or blocks the biological activity of wild-type mature human GDF8 comprising SEQ ID NO:340, but does not bind to, or block the biological activity of a chimeric GDF8/TGFB1 construct having amino acids 48-72 of mature human GDF8 replaced with the corresponding amino acid sequence of TGFB1, wherein the chimeric GDF8/TGF 1 construct comprises the amino acid sequence of SEQ ID NO:352, and wherein the antibody or antigen-binding fragment comprises: (a) HCDR1/HCDR2/HCDR3 amino acid sequences selected from the group consisting of SEQ ID NOs: 116/118/120； 362/364/366； and 378/380/382； and (b) LCDR1/LCDR2/LCDR3 amino acid sequences selected from the group consisting of SEQ ID NOs: 124/126/128； 370/372/374； and 386/388/390. Also discloses antibodies which bind to the same epitope or compete with said antibody.
