Muscle loss due to aging, or sarcopenia, impedes the independence and overall quality of life for older adults, as it causes their ability to perform daily tasks to decline. One contributor to sarcopenia is the reduction of the growth hormone/insulin-like growth factor 1 (GH/IGF-I) axis as a result of aging. While it is known that maintenance of physical activity can help slow aging-related muscle atrophy due to systemic and skeletal muscle specific adaptations, little is known about the specific role of muscle IGF-I activity in these adaptations. Studying the interaction between muscle IGF-I and exercise training and their consequential changes to muscle morphology can provide information that is crucial to improving therapies for aging-induced muscle loss. The main objective of this project is to examine the molecular changes in skeletal muscle morphology in mice with an induced muscle specific IGF-I deletion (MID) and strain matched controls (CON) following endurance exercise training.