Your ability to stay calm and relaxed during exertion will determine your performance!
The term relaxed doesn’t mean slack — it simply means being able to use only the muscles necessary for the movements you’re performing.
The best way to stay calm is to control your breathing during effort, and possibly go further by understanding the relationship between breath — or its absence — and panic.
Try holding your breath for just one minute, and you’ll see that your need for air won’t first appear as a real physical necessity, but as a mental reaction of panic!