There's an aikido group that meets over at the Baltimore Zen Center, Sword Mountain, that I've heard good things about. Since they train on Tuesday and Thursday nights -- the same nights I teach karate -- I've never been able to drop by and check them out in person. But their website has a very interesting description of the relationship between martial arts and Zen: "Aikido is Zen practice under duress, the study of one's self within the context of physical threat".
"Zen practice under duress." I think that we could very much apply that to Seido karate. For those who aren't familiar with Seido, our founder, Kaicho Tadashi Nakamura, very deliberately integrated Zen meditation into our practice.
Having "fallen off the cushion" many times over the years, I've found that some of us need something a little less subtle than shikantaza ("just sitting"). We hard-headed folks need that element of "duress", some literal smacks upside the head to help us wake up. It took me many years of practicing Karate Zen before I was prepared enough to develop a somewhat-reliable zazen (seated meditation) practice, as minimal as it is. Your mileage may vary, of course: I'm an exceptionally unsubtle guy.
On the other hand, perhaps training in "Zen under duress" has the advantage of a more robust result. It's one thing to sit on a cushion in a quiet zendo with incense burning, but how is your practice out in the noisy, stinky, always-pushing-your-buttons world? Karate Zen or Aikido Zen introduces that element of difficulty right from the start.
As an instructor, I'm now at the point where I'm often providing that "duress" for others. For example, last Saturday we had two students testing for promotion at my sensei's dojo, including one young lady testing for her advanced brown belt. This is the last test she'll take here in Maryland -- her shodan test will be at our Honbu (headquarters) in New York City -- so it's something of a big deal. My job, while sparring with her during the kumite portion of her test, was to push her out of her comfort zone, mentally as well as physically; not just to test her fighting ability, but to show her that she's stronger than she thinks, that she can keep her center even under difficult conditions.
That's a lesson she can, hopefully, carry into every aspect of her life.