on the Suoerman thing , Toriyama freely admits he was inspired by Superman to some degree... I remember we discussed this a few years back in the ''if you like Naruto...'' thread where we were discussing other anime recommendations for Yuki, and we got to plagirisnm vs inspiration due to naruto and dbz and dbz and superman... I think it came down to the fact that if something inspires something greater and isn't a carbon copy then its all good...
as for ridiculous training the gravity thing (based on superman? wasn't that the reason for his strength on earth - krypton's gravity was stronger?) was expanded upon, with the gravity chambers etc and took to another level in the time chamber...
did superman train to get stronger? - no (not as far as I know) he just rets on his abilities like a gifted slob... wheres the lesson in that???... Goku and the Z fighters are inspirational... try hard and get results... in the past when I was training (tai chi, chi, weights etc.) constantly throughout the day I would keep the efforts of goku in mind, and I will do again in the future when I start training again.... some limits are there to be surpassed until you reach the rubicon of death... but even that didn't stop GOKU for F's sake!!
maybe saiyans have some electrical resistance thing going on , as at the end of the day the ki gathered manifesting is usually of our dimension and thus bioelectricity in the physical world, any normal person woukd just burn, fry, blow up if so much electrical energy was introduced into their body, so maybe saiyans can just resist and convert massive amounts of electrical charge without blowing up ... I mean since the Namek sagas they have been releasing the equivalent energy of small scale atomic bombs in power blasts from their palms!!...
beyond that if they do have all their physical body open, their chi/ki/ethereal energy pathways and every single major vortexes (chakras) and subtle vortex energy points open then they are virtually a perpetual energy system that would be unlimited, as they would only be limited by the source of that - which is the universe - and that has infinite reserves!
it is interesting that in Battle of the Gods the concept of 12 universes is introduced (I dont think is a spoiler as its not done with any great expalantion or with any depth at all ) maybe this has something to do with a toriyama version of string theory, and there is some energy conversion thought in there?... but probably not