Umm - post-war uchi-deshi did sumo practice too. Terry Dobson told me about how they used to try out tanto-dori techniques and do sumo. One thing I remember him saying is "I could never beat Chiba in sumo." This is not, however, some <secret teaching> of internal power. Sumo was, until recently, the most basic sport of guys in Japan. Everyone did it You don't even need a ball. It's really a stretch to impute, in any fashion, that the after-hours recreation (or in-class warm-up, if that occurred) of day-to-day sumo had anything to do with, for example, the shikko practice of Sagawa, or the kind of body-grounding training that some types of internal strength regimen recommend.
Ellis Amdur