Stand in front of a mirror. Look at your shoulders. Now breathe in and out deeply. What do you see?
If your physiology is remotely human, what you should see is your shoulders rising as your lungs expand and falling as you breathe out. Under normal circumstances, this takes little effort. However, if a heavy bag is carried on your shoulders, each time you breathe in you need to lift the whole weight of that bag, essentially using your diaphragm. It can easily be seen that this will exacerbate the exhaustion caused by heavy exercise. Worse, your breathing is restricted, so your muscles cannot recover from this extra effort as quickly.
One way to resolve this problem is to attach the bag to the body somewhere other than the shoulders. Sadly, there are few other natural protuberances on the human body. The arms can be used but, of course, these are directly connected to the shoulders, so the problem remains. The usual solution in most cultures is to carry heavy loads on the head. However, this is somewhat precarious, so the civilisations of Europe, North America and Australasia in the late 20th Century developed an innovative solution.
This was to attach the bag at its bottom and then use the structure of the bag, rather than the structure of the body, to support the bag. In other words, the bag does not 'hang' from the human body, it 'stands' upon the waist, with its own spine.
The first design to do this was the 'external frame' backpack. This was a complex affair. The bottom of the bag was supported by steel tubes, rather like tent-poles. These also supported the back of the bag (the vertical side nearest the wearer). The weight of the bag is then taken on a waist-strap. Shoulder-straps remain but, when the bag is worn correctly, do not take the weight - they are simply for ease of donning and to prevent the bag from falling over backwards when in use.
However, this was a complex design, requiring adjustment based on the wearer's height (measured from the seventh vertebra - the sticky-outy one - to the top of the hips). Although favoured by American hikers1, it requires a great deal of assembly and clipwork.
A mountaineer called Greg Lowe (also responsible for plastic buckles and compression straps) realised in 1967 that the horizontal bars at the base actually have little effect except in very large bags. When only the vertical bars are required, it is easy to incorporate these into the bag itself. This design is the 'internal frame' backpack, with all the benefits of an external frame, but not requiring tent-building skills to assemble or adjust.
Using an Internal Frame Backpack
Even the simplified internal frame design can have an alarming array of straps to wear and adjust. The result is that many users either use the bags with all the weight at the shoulders (meaning that they are no better than a normal bag) or so poorly adjusted that they risk twisting their back.
The first step in donning an IFP is to pack the bag with the weight as even as possible, with heavy items nearest the wearer. This should keep the centre of gravity as close to the wearer as possible.
Next, loosen all the straps. Undo the waist strap and (if present) the strap linking the shoulder-straps. Don the bag, putting both arms through the shoulder-straps. At this point, the bag should be hanging with all its weight on your shoulders.
Breathe in deeply. Close the waist-strap and pull it as tight as possible (in most designs, this simply involves pulling the strap from the ladder-lock to one side). Now breathe out. As you do so, the weight of the bag should shift from your shoulders to your waist. As a quick test, try lifting both the shoulder-straps at once with your little fingers. There should be no weight on the straps.
The final step is to adjust the bag to fit you. It will be leaning back, as the shoulder-straps are too loose. Lean forwards and pull down as hard as possible on the tabs on the front of the shoulder-straps. This should tighten the bag against your back and prevent it from moving in comparison to your body. The bag should now follow your movements if you run or jump, without hindering you by moving against your body. Further, all its weight should be directed through your calf and thigh muscles, which are the most powerful in your body.
What Are All These Other Straps Then?
You wouldn't feel like a pro if there weren't other straps to play with, and the bag designers have thoughtfully provided plenty for you.
There may be a second set of straps between the shoulder-straps and the main bag. These allow the user to adjust the bag to sit higher or lower on the back - useful for adjusting to fit the user's height.
Some bags have a strange loop on the back. This is to hold an ice-axe or similar equipment.
Many bags have a pair of straps on the top and/or the bottom. These are for holding a sleeping-bag and sleeping-mat. It is usual to attach the sleeping-bag to the bottom of the bag through the compressor straps (the 'spider'), and the sleeping-mat to the top, as the mat moves less.
Some bags now come with a smaller, separate bag attached to the back by a zip. This can be used as a day-pack or for hand-luggage.
Another option is an enclosing case (which can also be bought separately). As well as providing additional waterproofing, this prevents the straps getting snagged when checking in the bag as luggage.
The ultimate in useless strappage is the little plastic clip to link the shoulder-straps in front of the chest. It is assumed that this was designed with some purpose in mind, possibly to prevent the shoulder-straps from slipping over the shoulders. However, this is mere supposition, and its true function has been lost.
Wrapping the Bag For Travel
When checking in the bag as luggage for a plane or bus journey, it is best to tie the straps out of the way. This is quickly and easily achieved by bending the waist straps backwards and clipping them together at the back of the bag, then wrapping the shoulder-strap-joining strap (if present) around the shoulder-straps to hold them together.
If the sleeping-bag straps are not being used, they can be tied in a neat knot, or sometimes removed altogether.