It depends on your size and fitness, says Giovanni Cavagna, who studies locomotion at the University of Milan. Unfortunately, there are no hard and fast rules. "Exceeding 30 percent of your body weight is a bad idea," says Neal Henderson. The reason: The more you carry, the harder you have to work, yet your performance decreases. "When we put Brian Robinson on a 15 percent grade with a third of his body weight in his pack, he was not feeling too great," says Henderson. And if it makes Brian suffer, imagine how it'll feel for you.