Focusing problems at Athletics meets

Hi
Anyone experienced the following problem and got any suggestions on the following problem? I am using the 5D Mrk 3 and both the F4 24-105 & F4 70-200

I was photographing an athletics event from the end of the home straight and repeatedly found that although I had my focus points on the lead runner (in various races) that some of the shots were in focus & some out of focus.
I was shooting using either slow or fast burst modes.

I also tried the different focus tracking modes on the camera, but wasn't sure which was the best.

Can you tell me if there are issues focusing on objects moving towards you quickly, and which is the best modes to try and overcome this issue?

thanks in advance
Edward

I assume you were using AI Servo? Thats what you should have been using anyway.

It is very important that your AF point stays glued to whatever you choose to focus on. If thats say, on the face of the lead athlete, then thats were it must be glued. No wandering about!

It is possible, but it's not easy and does take practise.

What focus points were you using?