I very rarely use a backpack when I ride; the only time I do is if it is a short trip, and I am riding one of my bikes without racks or a saddlebag, or if my saddle bag is too small to carry the stuff I want. Any light day pack can carry a light load.

A backpack is about the worst way to carry a heay load on a bike. First of all it raises the center of gravity and might affect the stability of the bike. Secondly, a heavy backpack might affect your back, shoulder, and arm muscles, especially if you are in a bent over position or it is a long trip.