Project Summary Building off of our current R01, we propose to characterize the effect of systemic hormone concentrations on basic skeletal muscle characteristics. Initially, young and otherwise healthy female surgical patients will be recruited to determine an associative mapping between muscle fiber composition and E2 concentration using histology and protein/gene expression of skeletal muscle biopsies. Next, young and healthy female subjects would be recruited to identify potential muscle fiber composition changes over a limited time window during the menstrual cycle, using a state-of-the-art 7 Tesla scanner to perform magnetic resonance spectroscopy. Finally, young and healthy female subjects would be recruited to identify the potential effect of the endocrinological state on muscle plasticity by monitoring circulating myomiRs and muscle activity during exercise. If successfully completed, these aims would reveal a relationship between a subject’s endocrinological state and acute changes in basic muscle phenotype and the associated changes in neuromuscular control. Taken together these aims have the potential to elucidate the underlying biological mechanisms that may drive the sex-disparity seen in musculoskeletal injury and to guide future research in injury prevention strategies.