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So I made a small diagram explaining the physics behind this shit. If we simplify the backpack to a lever with the back as its fulcrum (green), the downwards force of the mass (red) would get partially translated into lateral forces on the hip belt and the shoulder/sternum straps. This would relieve some of the vertical strain on the hips and shoulders and engage the abdominal muscles to compensate.
Now in the "bodypack" example the OP posted, the lever is balanced, removing any lateral forces. Sounds good right? Wrong! This completely wastes the stabilizing potential of the core muscles and instead places the full force on the hips and shoulders. This compresses the spine and bruises the shoulders and hips.
