A drooping/forward head puts stress on the spine and fatigues the muscles of the neck/shoulders.
They were never meant for hanging things! the weight of a backpack can pull on shoulders and put added stress on the upper back.
A heavy backpack can change the way you walk. This, and the effort to propel yourself forward can cause aching hips and knees.
The postural change of leaning forward to carry the load stresses the spine and overworks the muscles of the lower back.