Anxiety can improve sports performance as seen in the inverted-U hypothesis. When you map out anxiety (also known as arousal) on the x-axis, and sports performance on the y-axis, what you see is that a moderate amount of arousal leads to optimum performance. But when you go to the extremes of arousal (anxiety), with comatose on one extreme and panic attack on the other extreme, then you see a significant decline in performance. So what you end up seeing is a curvilinear relationship between arousal (anxiety) and performance that looks like an inverted-U.
So basically, moderate amounts of anxiety (arousal) leads to optimum performance, while the extremes (comatose vs. panic attack) lead to declining performance. So you want a moderate heart beat, but not be pulseless (comatose) and not have racing hearbeat (panic attack).
