Rob,
As you're undoubtedly aware, Shotokan isn't the only Japanese style or group that keeps their back relatively perpendicular to the ground. The various shito ryu groups, including the large Motobu Ha factions in the US, do as well.
Wado leans forward in their junzuki dachi, but have their spines upright elsewhere. You can see it in the wado version of Seishan.
In trying to recall watching the kata of the US Members of the Shorinkan (Shuguro Nakazato's group) I don't recall them leaning forward at the waist.
As a yondan, I trained in kobudo with a fellow who was also a senior instructor in matsubayashi shorin ryu. We trained karate together, too, and we kept our backs straight.
I recall that the San Diego Ryuei Ryu group keeps their back stright and I don't recall seeing Sakumoto leaning, either. The Ryuei Ryu kata that I know, I came to via Hayashi-ha. Backs straight there, too.
So, I don't think it's a "Japanese thing", as a lot of the Okinawan styles or Okinawa influenced styles (like Motobu Ha and Hayashi Ha) also keep their backs stright.
**Warning: SWAG's to follow***
As to reasons why the spine's erect, perhaps it's the influence of the sword. My experience, thus far, is that one's spine is erect in nukitsuke and kiri oroshi.
It could also be the influence of jyu kumite. I find that if I lean into a punch and I've not dropped my opponent, it's difficult to avoid his counterpunch. With my back straight, I can deliver sufficient horspower to the target and, should my technique fail, I can still get out and avoid the counter strike.
Might it also be the influence of Asian medicine and concepts of ki? My tai chi instr's have always taught to keep the spine upright and the shoulders down to keep the qi paths open and unblocked.
Sure, esthetics can figure in, too.
Or some combination of these.
Or, perhaps the upright back is the rule and the lean is the exception in Okinawan and Japanese karate.
Rob
robertmrivers said:
If the lean was eliminated, in my opinion, it was for aesthetic reasons. In Japan, posture is very important. Thus, many Shotokan practitioners keep their back straight. It is not to say they do not hit hard (believe me they do!) but they could harder by leaning and doing a few other things with their stance...
Rob Rivers