Q:

Why do runners immediately stop after a run? Does opening your mouth more while running affect oxygen intake?

Hello all. I've got two questions regarding the way runners run.
Firstly, what I've always thought is that stopping immediately after completing the run (1k / 2k / 3k / 5k) is bad and we should like carry on at a slow pace for some distance.
But why is it that the runners in this video stop immediately after they've completed the run? Isn't it bad for their bodies? (Once  my friend even told me that they would get "rectum pus" whatever that is.)
Secondly, what I've always thought is that we should open our mouth "bigger" to allow for more intake of oxygen during the run. What I've seen in the same video is that they do not do so. Does anyone know why is it so?

A:

You're right that it's very important to cool down after
exercising. Cool down should consist of three phases: gentle
exercise (to gradually lower the heart rate and prevent dizziness),
stretching (to reduce muscle stiffness), and refueling. Here's
another useful article on cooling down. 
In general, as xpda points out, a lot of runners neglect the cool
down because they're so exhausted at the end of a race (I know I'm
guilty of this from time to time). A cool down is designed to
restore your body to pre-exercise conditions, but it still takes
effort. Stretching can be somewhat painful if you've really
worked your muscles. 
In the video you linked to, however, the runner was immediately
swept into celebration following the race, so he didn't exactly have
the opportunity to cool down. 
According to this article - it's best to breathe through both
the nose and the mouth while running. So perhaps it's unnecessary to
open the mouth very wide if you're also breathing through your nose.

A:

In a race, you may not be able to jog for a while afterward, especially if you put everything into a sprint in at the end.
The size of the mouth is not normally the limiting factor for air intake, so you don't need to open it any wider. Incidentally, CO2 output is as important as oxygen intake.

