I would like to share this experience with some members here, and I am sure there are others who have the same experiences before me.
The first "heavy" gear I owned was the 70-200mm F2.8 IS. It opened up my view on "sharpness and bokeh". Since then I believe only on good lens, and unfortunately for Canon system, it means "heavy" gears. (why can't Canon produce quality lens like Leica? )
My collections now include almost all the Canon Primes, up to 300mm f2.8. On top of it, after holding on to a 1d MkIIn and now a Mk III, I thought anything less than those are inadequate.
So when I shoot in Singapore, my bag has a min of 2 big primes and a MkIII (that's minimum). and also when I travel oversea, imagine shooting for a few days with such a weight.
A few years ago, I have no problem walking, hiking, climbing, jumping for a few days with such weight, but recently problems started ... first with back pain (which can easily be cured with a back massage) but recently the knees are giving me painful warnings ... so much so it hurts and affect my sleep.
Before it gets worse, I am now in the process of cutting down the weight of the gears I am carrying for shoot. I am now looking into some other "lighter" systems, like the RF, films, PnS, etc...
I think I should share this experience with the members here to remind everyone that our health is still prime important. Do not be fooled by thinking that you are healthy and young, you can "handle" all these heavy gears without any long term health issue.
When it comes, it comes without any warning.
So those above 40+, please take my words seriously ....