the mind, recognizing that the body may be going too hard, starts sending fewer of the messages that tell the muscles to contract. The muscles contract less frequently and more feebly. In a sensation familiar to anyone who exercises, your legs die beneath you.

“I think the training effect of this theory is potentially very profound,” Tucker says. “Training is no longer simply an act of getting the muscles used to lactate or teaching the lungs how to breathe harder.” It’s also about getting your brain to accept new limits by pushing yourself, safely. “Once your brain recognizes that you’re not going to damage yourself,” Foster says, “it’ll be happy to let you go.”