The absence of dystrophin and resultant disruption of the dystrophin glycoprotein complex renders skeletal muscles of dystrophic patients and dystrophic mdx mice susceptible to contraction-induced injury. Strategies to reduce contraction-induced injury are of critical importance because this mode of damage contributes to the etiology of myofiber breakdown in the dystrophic pathology. Transgenic over-expression of insulin-like growth factor-I (IGF-I) causes myofiber hypertrophy, increases force production, and can improve the dystrophic pathology in mdx mice. In contrast, the predominant effect of continuous exogenous administration of IGF-I to mdx mice at a low dose (1.0-1.5 mg/kg/day) is a shift in muscle phenotype from fast glycolytic toward a more oxidative, fatigue resistant, slow muscle without alterations in myofiber cross sectional area, muscle mass or maximum force producing capacity. We found that exogenous administration of IGF-I to mdx mice increased myofiber succinate dehydrogenase activity, shifted the overall myosin heavy chain isoform composition toward a slower phenotype, and most importantly, reduced contraction-induced damage in tibialis anterior (TA) muscles. The deficit in force producing capacity after two damaging lengthening contractions was reduced significantly in TA muscles of IGF-I treated (53 ± 4%) compared with untreated mdx mice (70 ± 5%, P < 0.05). The results provide further evidence that IGF-I administration can enhance the functional properties of dystrophic skeletal muscle and, when compared to results in transgenic mice or viral-mediated over-expression, highlight the disparities in different models of endocrine factor delivery.