The precise mechanisms that underlie this important benefit of exercise remain uncertain. We are currently performing experiments to fill this gap in knowledge. We are probing the processes that account for the sustained effect of exercise on phosphorylation of the key signaling protein called AS160 (also known as TBC1D4) and evaluating the role that AS160 plays in mediating exercise-induced improvement in insulin sensitivity. We recently performed experiments to evaluate exercise effects on muscle from both healthy rats (with normal insulin sensitivity) and obese and insulin-resistant rats. Both groups had greater AS160 phosphorylation and elevated insulin-stimulated glucose uptake. We are now trying to identify the mechanisms for the exercise effect on AS160 and determine if this effect on AS160 is required for increased insulin sensitivity after exercise. We are also performing experiments to determine exercise effects on glucose uptake by single muscle fibers of different fiber types.