Backpacks are a problem because they carry the load fairly high, making you more top-heavy, with more risk of falling over, and less maneuverability. Also they make your back sweat, weigh down your torso and strain your shoulders. Messenger bags ride a little lower, but the best place to carry a load is in a pannier because it's closest to the ground, and not dragging down your body. Second best is on top of the rear rack.