I fail to see how a backpack would destroy your joints any faster...
If you train with one all the time.. even when you condition and when you stretch as well as when you train the basics your body would adjust, would it not? Would it forever be an added weight? Could your body not get stronger as a result and then when you don't have it, it makes it that much better?
It destroys your joints because its an added load.
Its likebeing 20# overweight and taking drops -- its usually not advised. The dead weight adds to the amount of muscle that is necessary to compensate for it to keep it safe. On high impact, quick moves it is difficult to have "perfect" landing form to make sure your joints don't do funny things like have your knees caving inwards.
If you are going to train with a backpack it should be functional (to be able to perform with a light pack on...) and not as a method of increasing the difficulty.
Your misconception is a common one to someone who is new to exercise physiology. Training skills with increased weight does not help the unweighted skill in many cases. If the motor pattern has to change drastically to accommodate for the weight then its like training something totally different (and often times, not practical). This is why movements like the squat are so beneficial -- they can be weighted very heavily and have very high translation into a broad variety of movements (squatting heavy helps jumping, pistols, getting up from chairs, cleans, snatches, push jerks, push presses, kip-ups, kipping pull ups...the list can go on for a long time..)
you have a nice point because some runners use ankle wieghts to trian and condistion with. Also if you go to a baseball game and look at the next batter up the have a wieght on the end of there bat while they wait for the other batter. So i'm sure that if you really condisoned with it and used it everytime you go out and run you should be fine.
Training with ankle weights for running is silly. No competitive runner who knows what they are doing would ever do that...it programs motor patterns improperly and will actually cause your running speed to DECREASE when you remove the weights.
The same is true for baseball players. The added weight doesn't make swinging the bat easier but it helps with the perception of the weight of the bat. There are studies on this in particular. The bat doesn't actually get lighter, nor does it even swing faster...but the batter has more confidence because it "feels" lighter.
This is a purely perceptual change that causes increased confidence which causes increased technical ability -- it has nothing to do with a baseball player actually getting stronger because of the weight. Long term batting practice with a heavier bat can actually decrease bat swing speed since swinging a bat is such a technical action.
