Is seiza a "healthy" position?
While seiza may be helpful for training good upper body posture, it obstructs the blood flow to the legs if sitting for longer periods of time. Is this something that can be improved with training - i.e. will the problems associated with seiza diminish over time, and the benefits increase? Is it healthy to sit in seiza for, say, 10-15 minutes each day?
I don't mean to sound rude
But why? What does sitting in this position do for you. What purpose for self defense is sitting in Seiza?
Was just trying to stay on topic, but oh well, here it is.
Alright Dan, I'll try to summarize it all. :)
Thing is, some people have used seiza itself to claim one branch of JSA as superior to another and such things. In the end, it's no longer an interesting discussion to some, including me. If I am in an art that uses it, I will too. If I'm in an art that uses a different style of sitting, I will use that. But by all means if you want a more complete answer, check out these links:
iaido and seiza
and here Advice for Seiza
and here Loose theory about Seiza
and here Another seiza Question
and some others, of varying helpfulness.
Anyhow, it's true, seiza as we know it today isn't the most "combat-ready" position. Seiza in the past however could refer to any correct way of sitting for a particular context (c.f. Bodiford's post in the first link). Some people use seiza for meditation, most often it is a more formal way of sitting for formal functions: meetings, tea ceremony, funerals, etc. If someone practices a style of JSA that is "more formal" or "more refined" that someone is more likely to sit seiza to reflect that. I'll let those who would make judgements about which art is better, do so. I won't even attempt saying which JSA styles might be "more formal," since I don't know, and I don't want to make/lose friends from it.
To answer your question more clearly, some have rationalized seiza in terms of self-defence as: providing balance; emphasizing hip movement; focusing one's attention on hips->upper body as later kata emphasize standing or other postures; training a sort of "explosive" strike from a neutral and harmless looking posture; "sensei told me so."
So I hope that's a bit more of what you were asking.
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I see from my earlier post and one of the few I linked to that my description of a "flat lower back" may not sound similar to, say, Paul Smith's description of a "S-curved back." I think that a straightened spine and tilted pelvis leads to what I describe as a flat lower back but I see how Mr. Smith's description may not sound the same. The spine has a natural S curve and that is what is supposed to be emphasized. I think we may not be talking about the same portion of the lower back. In any event, the second picture from the second link is what we both are trying to describe and reccomend.