I'm curious as to which muscle groups are of the greatest import in determining one's ability to hand-hold a camera.

Has anybody on APUG undertaken strength training in the interest or reducing camera shake when hand-holding larger MP cameras, such as the RB67/RZ67 or Pentax 67?

I'm moving towards building a medium format system beyond my current Yashicatmat. I'm not overfond of the 6x6 format for certain applicatioins and would prefer a 6x7 system, but all of these are rather more heavy than (e.g.) a Bronica SQ-series camera & lens.