The most serious side effect of carrying a heavy backpack is the shift in your spine, shoulders, neck and hips and the extra weight on your ankles and knees. This can lead to life long bone problems.
If you attmpted to carry an extremely heavy load, your height could be reduced over a long period of time. The initial effect would be compression of the soft tissue in your spine which would make you shorter until you recovered. (You are tallest when you wake up in the morning and shrink all day was your spine naturally compresses).
The problem with heavy book bags and children has been widely reported - all you have to do is google it.
