The disclosure provides novel molecules related to growth and differentiation factor-8 (GDF-8), in particular mouse and humanized antibodies, and antibody fragments, including those that inhibit GDF-8 activity and signaling in vitro and/or in vivo. The disclosure also provides methods for diagnosing, treating, ameliorating, preventing, prognosing, or monitoring degenerative orders of muscle, bone, and insulin metabolism, etc., in particular amyotrophic lateral sclerosis (ALS). In addition, the disclosure provides pharmaceutical compositions for the treatment of such disorders by using the antibodies, polypeptides, polynucleotides, and vectors of the invention.