The present invention provides compositions and methods which involve specifically antagonizing GDF8 and Activin A. In certain embodiments, compositions are provided which comprise a GDF8-specific binding protein and an Activin A-specific binding protein. For example, the invention includes compositions comprising an anti-GDF8 antibody and an anti-Activin A antibody. In other embodiments, antigen-binding molecules are provided which comprise a GDF8-specific binding domain and an Activin A-specific binding domain. For example, the invention includes bispecific antibodies that bind GDF8 and Activin A. The compositions of the present invention are useful for the treatment of diseases and conditions characterized by reduced muscle mass or strength, as well as other conditions which are treatable by antagonizing GDF8 and/or Activin A activity.
