This is scaring me. I'm a workout person, very recently getting into the bodybuilding thing to some extent. Your comment about having some trouble with a 70-200 for several hours at age 68 is heartbreaking. I cannot imagine that happening 40 years form now; I thought that if I got really strong over the next few years, and then kept good strength and flexibility, I would never have such problems with moderate-weight camera gear (e.g. a pro body, 70-200/2.8 and a flash unit). Or is it the joint/rotator cuff injuries that are really responsible for your fatigue these days (and not so much an issue of naturally diminished strength due to aging)? Because, if a former bodybuilder's upper body strength is normally that diminished over time, I can only imagine what would happen to a non-fitness/weight training enthusiast as they age (which is why I believe fitness is essential...to protect us from such issues to an extent as we get older).