Thanks for the review. Does the cone grip help you keep your elbow against your body? Yes, the description of your fitness is very pertinent. I too shoot with the 400mm f/5.6L hand held, which is like a toy lens compared with one of the f/4 or f/2.8 lenses. One of my concerns is not being able to hand hold the 600mm lens, and I am indeed working with dumbbells to increase arm and upper core strength. I am guessing that I am going to need to get to at least 15# in endurance style sets, but what do I know? At any rate, once I get to my desired fitness level, I am going to rent first the 600mm and then if that's too heavy the 500mm.