I practised kendo for 10 years in Denmark. Only sport I ever found interesting. There were several things that caused me to stop, but among them was back pain.
@Kozushi is correct, in observing, that many of the top kendoka have broad shoulders, and I would add stocky builds. I am neither of those, slender build. And this probably did not contribute well to my health in the sport. We practised 3 times a week, at times up to 3-4 hours, and sometimes more when there was national team training. There was no strength component taught, only the sport. I wish that I had used some kind of functional training my self on the side, or that such a thing had been taught at practise, since I believe that it may have atleast spared me the back problems. And would have made me more resilient to handle the strenous activity that kendo is at a high amateur level is. When I did discover S&S, several biomechanical lights went off in my head linking it to kendo, and would have liked to have tested my ideas, but now I atleast get to use S&S for my job as a clinical massage therapist. Glad to see someone is enjoying S&S in tandem with kendo:-D
edit: S&S is described as a GPP, and there was no GPP focus when I practised. Maybe other GPPs' will also enable one to play a rough/hard sport well. But I am not surprised if S&S works well with kendo. It would be interesting to investigate any direct transference of biomechanical patterns from swings and getups to kendo.