Exercises to keep breathing?
This question seems trivial - just keep breathing, but yet I find it pretty difficult to keep breathing during a hard training, which leads to tiredness at the end of the class. I can see the same problem when more advanced students do juyuwaza or randori. Yet, there are people who breath and seem relaxed through the whole session and they seem to be as fresh as in the beginning of the class.
My question is: how to train this ability to keep breathing even when you do a hard technique or even when you do difficult ukemi.
I realize this is connected to being relaxed, not fear to be thrown of to be injured, but are there any exercises that could help?
From my understanding running and maybe multiple bokken cuts should help to keep the breathing in shape. Any other ideas?