Meaning of "run flat out"

What does "I can run flat out for a half mile before my hands start shaking" mean?

It was said in The Bourne Identity.

I am not a native English speaker. I know the meaning of the individual words, but have never heard the expression "run flat out" before.

Solution 1:

Flat out is an expression used to describe one of three things, depending on the context:

1. Doing something as hard or as quickly as you can: e.g. I worked flat out to finish the assignment.
2. Doing something without hesitating or any second thought: I flat-out disagree with your view.
3. Lying down completely stretched out: After winning the race, I was exhausted and collapsed flat-out on the ground.

In this case, it's the second or the first use, depending on the context, though in either case he's making a statement about his physical condition. The speaker is saying one of two things:

1. He can run as quickly as he can for half a mile before his hands start shaking
2. The speaker can, without a moment's hesitation, run half a mile, without a warm-up, before his hands start shaking.