When Carson Palmer, a professional American football player, hurt his arm a few years ago, he took a week off from throwing the football. But in his head, Palmer practiced every day. The following weekend, Palmer had the best game of his life.

    For more than a century, scientists have been trying to understand how this mental training works. In the 1930s, researchers proved by experiment that when you're imagining an action, your brain sends signals to your muscles(肌肉) which are too weak to tighten the muscles but might help train the body to perform. In other words, mental practice might create a pattern in your head, like an inner how-to guide for a particular skill.

    Sports psychologists have conducted hundreds of studies comparing imagined and physical practice for actions. On the whole, the research shows that mental training works. A 2012 study, for example, compared 32 amateur golfers who practiced hitting the balls to another 32 who merely held a golf club in their hands and visualized（想象）their swings. Under the same training rules, both groups improved their skills by getting the ball about 4 inches closer to the hole.

    Visualization has advantages over the real thing: You can do it anywhere, even when injured. It is safe-a major plus for high-risk performers such as gymnasts and surgeons. And you can practice for longer periods of time because you're not restricted by physical tiredness, That's not to say it's easy, we've had Olympic-level athletes sitting in our lab, visualizing the movements for two hours," says Tadhg Macintyre, a sports psychologist at the University of Limerick in Ireland. “When we're done, they're absolutely tired.”

    It doesn't work for everyone, though. "If you're anovice, the effect can be harmful,” warns Macintyre. If you're trying to visualize a free throw, and you don't even know the proper movement, then you're probably going to mentally practice the wrong skill.”