In this two-part talk, I will first describe our human locomotion experiments and optimization calculations, demonstrating that energy optimality can predict many aspects of humans locomotion behavior. Energy optimality not only (roughly) explains steady locomotion behavior in a straight line, but also unsteady locomotion with changing speeds, non-straight-line locomotion in complex curves, and other less conventional tasks. Next, I will describe our attempts to characterize the controller humans use to walk and run stably. To infer the controller, we use human responses to natural intrinsic noise and externally applied perturbations during walking and running. We will show that some human responses to such perturbations can also be explained (roughly) by energy-optimal perturbation recovery.