The disclosure provides novel molecules related to growth and differentiation factor-8 (GDF8), in particular epitopes specific to GDF8 and other specific antagonists of GDF8 in particular anti-GDF8 antibodies or antigen binding protein or fragment thereof which may inhibit GDF8 activity and signal in vitro and/or in vivo. The disclosure also provides for an immunoassay used to detect and quantitate GDF8. The disclosure also provides methods for diagnosing, preventing, ameliorating, and treating GDF8-associated disorders, e.g., degenerative orders of muscle, bone, and insulin metabolism. Finally, the disclosure provides pharmaceuticals for the treatment of such disorders by using the antibodies, polypeptides, polynucleotides, and vectors of the invention.