The dreaded sled pull, one of the eight exercise stages in a Hyrox race. | Amaury Paul/AFP via Getty Images Exercise can be a humbling experience.  A simple push-up can debase the most dignified humans, transforming them into helpless little halibuts, unable to unmoor their bellies from the floor. Squats and lunges can leave their victims crouching like tiny goblins or trembling against gravity’s will. Have you ever willingly watched yourself run at full speed? Only if you have self-esteem of steel. But for certain people, the most physically demanding experience of their lives is no...