Re: Zazen necessary for training
A few years ago I had a good aikido teacher. She quit Aikido, because her Zazen practice was so exhausting, that she could not combine both.
Well, I personally think, that it is not important, which type of meditation you do. Having some time before training to calm down and leave all your daily burden outside the dojo is very good.
And taking some more time to find your true way is helpful, too. Zazen, Seiza, Yoga, a simple quiet prayer or whatever you want. Nothing of them is the alone one , that is necessary. But you should choose (at least) one of them.
My 2 ct Dirk