When you push until you can’t go any farther or faster, what exactly is holding you back? Whether you’re climbing mountains, running marathons or hoisting weights, physiologists have long debated what really defines our limits. A mounting body of evidence suggests that the brain plays a greater role than previously thought, even for limits that feel purely physical. That doesn’t mean “it’s all in your head,” of course—but it does mean that exploring the brain’s role opens new avenues for pushing the limits of human performance.