The internal frame backpack is a recent innovation, invented in 1967 by Greg Lowe, who went on to found Lowe Alpine and Lowepro, companies specializing in backpacks and other forms of carrying bags for various equipment.[8] An internal-frame pack has a large fabric section around an internal frame composed of strips of either aluminum, titanium or plastic, sometimes with additional metal stays to reinforce the frame. A complex series of straps works with the frame to distribute the weight and hold it in place. The internal frame permits the pack to fit closely to the wearer's back and minimizes shifting of the load, which is desirable when participating in activities that involve upper-body movement such as scrambling over rocky surfaces and skiing. However, the tight fit reduces ventilation, so these type of packs tend to be more sweaty than external frame packs. The internal construction also allows for a large storage compartment; a few lash points (including webbing loops and straps for sleeping bags and other large items) may be present, but as the frame is completely integrated, it is difficult to securely lash larger and heavier items which do not fit inside the compartment to the outside of the pack. Internal frame packs originally suffered from smaller load capacity and less comfortable fit during steady walking, but newer models have improved greatly in these respects. In addition, because of their snug fit, the improved internal frame models have largely replaced external frame backpacks for many activities.
The other type of external frame which recently was proposed, is made from composite plastic which is not flexible like current backpack straps and also it is a kind of material that can be shaped like human spine curvature. In this type of backpack, load directly transfers to the shoulders through the non-flexible straps. This non-flexible structure diminishes the momentum at lumbar region of the back. Strap curvature is shaped close to spine curvature and there are two flexible drawstrings to prevent backpack movement in transverse plane. The straps of this backpack are wide enough to distribute the pressure on shoulders and also a white glass wool layer is added to the internal part of them to absorb dynamic forces, which could be produced through walking. This backpack type is an experimental sample that need further options to be prepared for usage. One of the benefits of backpack with external frame is preventing the spine to incline forward during walking that would be helpful in preventing damage of long term backpack carrying.[4]
External frame packs were designed to carry heavy loads (>20 kg or 40 lb), giving the wearer more support and protection and better weight distribution than a simple, frameless strapped bag. Wooden pack frames were used for centuries around the world. Ötzi the Iceman may have used one in Copper Age Alpine Italy,[5][6] though some archaeologists believe the frame found with the body was part of a snowshoe. Such packs are common in military and mountaineering applications;[7] metal versions first appeared in the mid-20th century.
I actually use this tote as a diaper bag, I have two children in diapers and it really works great for that. Aside from the way I use it, I think it's an over all great bag. A few issues: the elastic pockets on the sides seem to have lost their tension already, I don't put anything large or heavy in them to cause this, also the pocket in the back.. Wish it was larger, I don't really see the point of the zipper in the bottom of the pocket. The only other issue I have is the straps, wish they had some sort of padding on them, they tend to be a little hard on your hands if you're carrying anything heavy.