The present invention provides methods comprising the in vivo delivery of small nucleic acid molecules capable of mediating RNA interference and reducing the expression of myostatin, wherein the small nucleic acid molecules are introduced to a subject by systemic administration. Specifically, the invention relates to methods comprising the in vivo delivery of short interfering nucleic acid (siRNA) molecules that target a myostatin gene expressed by a subject, wherein the siRNA molecule is conjugated to a lipophilic moiety, such as cholesterol. The myostatin siRNA conjugates that are delivered as per the methods disclosed are useful to modulate the in vivo expression of myostatin, increase muscle mass or enhance muscle performance. Use of the disclosed methods is further indicated for treating musculoskeletal diseases or disorders or diseases or disorders that result in conditions in which muscle is adversely affected.