jackcbr.
On first impression your idea sounds OK, but in reality it does not work, and is also not a fair representation of load carrying.
For pure strength training, I did exactly what you suggest, but its also funny that water soon becomes too light!, once you become accustomed to the weight, and you then need to move to bricks, or iron dumbells etc (I still have some of my original bricks all wrapped in towels and taped up to avoid ripping the bergen lol).
The reality of course, is that if you packed your pack solely with water (for training) then you end up with a very low centre of gravity, and will thus need to lean forwards too much, which can not only seriously strain your abdomen, but also slows you down, and hampers breathing on steep uphill stretches. It is also not good for your mental training either IMHO as it would allow you to jettison some or all of your load through tiredness or laziness, which you cannot do with your normal gear!
Last of all, it is not accustoming you to your intended normal load, and so your method of walking or gait will will be wrong, as will your technique for crossing stiles/walls.
I have to say that the above method with bricks/dumbells was used by me for many years on daily 10 mile cross country walk/jogs.......Did it make me fit. Yes it did, but it also left me with some permanent injuries caused by wearing issue boots, which were later to be associated with long term injuries.............Not that it was known when I wore them!
I stand corrected, as I said, just a thought that came into my head, nothing to back it up. thanks for putting me straight, I won't give it a go.
