Conceptual model of the consequences of reduced skeletal muscle capillarization on muscle mass, glucose metabolism, and aerobic capacity in aging with a sedentary lifestyle. In this model, aging-related decreases in physical activity contribute to reduced skeletal muscle capillarization, which can limit the delivery of oxygen, amino acids, glucose and insulin. This likely contributes to declines in muscle mass, aerobic capacity and muscle metabolism, all of which may interact with each other and ultimately reduce physical function and further contribute to physical inactivity in a cyclic manner.