The invention provides pharmaceutical compositions comprising particular isoforms of human insulin-like growth factor I (IGF-I). An isofoim of the invention is an IGF-I polypeptide corresponding to the mechanical-stimulation-specific, stretch-inducible IGF-I isoform present in stretched rabbit EDL muscle, the polypeptide comprising peptides encoded by IGF-I exons 4, 5 and 6 when transcribed in the reading frame of the stretched rabbit EDL muscle isoform and the polypeptide having the ability to induce growth of muscle tissue. The isoforms of the invention have utility in treating muscular disorders, especially muscular dystrophy. Methods of treatment of such muscular disorders in humans and animals with the isoforms of the invention are also provided.