The Finish Line
The 400mm focal length will typically provide this finish line view from a full frame DSLR when standing just outside of the track fence.
When runners are directly approaching the camera position, the stub-leg syndrome (fully bent leg appearing cut off at the knee) should be strongly avoided. Unfortunately, if the runner's leg pace matches the camera's frame rate (which can happen at the 5D III's 6 fps), you can have opposite legs stubbed in consecutive frames. No problem in this shot.
The aperture was intended to be f/2.8 - f/3.2 was an accident. Just in case you were wondering.
Camera and Lens Settings
400mm f/3.2 1/2000s ISO 500 5760 x 3840px