Backpacks come in either external or internal frame designs. Internal types dominate in the market today, due to two main factors. Most-conspicuously, but functionally secondary, is the popularity (or more accurately, prestige) of mountain climbing, rock-climbing, ice-climbing & etc. More-pragmatically & realistically, the problem with external frame designs is that they really need to be custom-fitted & adjusted. Mass-manufacturing cannot address this need well, and the design still inherently places complexity-demands on the user, even when it is implemented well.
Internal frame types are really more ‘internally-stiffened’, rather than having a true frame. These packs evolved from pack-sacks, rucksacks and strapped carriers resembling the common shoulder-strap book-bag. But they have evolved a great deal; use foams to achieve form-fitting, and mold closely to the back & torso. Additionally, they tend to be highly stylish and fashion-forward (which is a major factor with most outdoor equipment).
Climbing & clamoring etc favor a pack that does not shift, is not mobile and hugs the body closely. External designs achieve their desirable & laudable goals, by providing for controlled mobility; forms of suspension are incorporated allowing greater comfort and better load-distribution, like in an automobile. This suspension inherently requires extra space for its mounting & its ‘action’, and also allows more relative movement between pack & packer, requiring further space. Without a suspension, your car would be basically a steel wagon … and could not practically go much faster nor smooth the bumps any better than a wagon intended to be drawn by a horse.
Climbers, though, place themselves in predicaments where very fine control of body & load-movement is essential. They also need to maneuver in minimal, tight confines. The internal frame pack, molded (‘plastered’) to the back and much easier to mass-market, met their needs … and this product now virtually defines the cool & modern backpack.
Internal frames also contributed to the light-weight and ultra-light backpacking trends. At the root of it, the shift to internal designs penalized heavier loads. What is a reasonable load with a decent external frame setup, tends to become excessively punishing, with an internal type. Attempting to move faster with an unsuspended load, soon invokes the image of an ox-cart going down the road at 20 mph … never mind 50 or 60.
External frames and the suspension-systems they incorporate allow for both greater loads and for higher speeds, exactly as we see with automobiles. It is impossible to propel rigid, unsuspended vehicles at high speeds … and the same principles apply to backpacks. To go fast with the popular internal frame pack-solution, necessitated unloading it. Your riding lawn mower can be modified to go 20 mph … but it’s pretty darn scary. A 4×4 ATV the same size & weight can go 60+, over much rougher ground, and maintain good control … because of its bodacious suspension
Unfortunately, more-sophisticated suspensions tend to be seen only on those external frame packs intended for carrying heavy and large loads. A casual, light-load external frame pack does not require such a well-developed suspension, to serve its minimal goals adequately. However, if one wants to travel faster, harder or farther with a light load, an advanced light-load external frame pack will repay the effort & expense. There’s just not much market for such a product.
For literally thousands of years, the more-complicated external frame pack was virtually the only ‘serious’ way that beasts of burden carried their loads. Without the external frame and the suspension it allows, pack-animals cannot meet the better-equipped competition. Packing-methods resembling modern internal frame solutions result in smaller loads, lower speeds, and greater susceptibility to injury. Performance, efficiency & economics all improve sharply, with external pack frames for animals.
Humans in sustained burden-carrying roles tended to be equipped with yokes, similar to that of oxen. Primarily due to the superiority of this yoke, oxen easily out-competed and out-performed horses in low-speed traction-roles for thousands of years, until quite late in the Agricultural Era. The yoke provides a highly-sophisticated & effective pendulum-suspension, and by curving around the human neck places the weight perfectly over the spinal axis (even better than the best shoulder-strap backpacks). The main cost with yokes, is in the very broad path-width they require.
Of course, pack-frames for horses and other animals were not mass-produced. Cross-trees were hand-fitted, sawn & notched piece-by-piece; the suspension-fabric draped directly on the animal’s back and marked-to-cut for best-fit. The topography of the load-bearing back was accommodated optimally … and in cases that did not use such techniques, the permissible loads and achievable speeds suffered.
Backpackers can treat themselves like valuable beasts of burden, in days-of-yore, but they also must then take on the role of savvy pack-train wrangler, as well as that of the animal. Fitting of external frames is the big challenge, but drills & drill-bits, clevis-pins, retainer-rings, mesh-nylon, nylon-web, plastic side release buckles and even fabric-glues make for a DIYer’s dream. All that really remains is the knowledge & principles of fitting the frame and in particular the suspension-elements, to the individual.
The easy way to understand external frame pack-fitting, is to imagine (or look at) an adult pack on a child. And a youth-pack, on an adult. The wrongness is then clear. These things have to fit right, and if they’re wrong, they’re just plain wrong.
The height or length of the pack has to match the back. Plenty of very sturdy people have a short back. Arguably, short backs are inherently stronger, and speculatively it could be that across our history of work (and manual combat), short backs have been selected & enhanced. Yet the industry has tended to build short packs for smaller & weaker people; aiming at lower performance & price-points.
The other vital measurement is the width of the neck. In the case of muscular people, this width not only reflects a more-robust neck, but a sloping ramp of trapezius musculature against the sides of the neck. And gym-workers tend to go straight for the trapezius … you know who you are.
A lightly built or slender person tends to have a flat shelf for a shoulder, as well as a smaller-diameter neck. This allows the pack strap to sit flat on the shoulder, and closer to the center-axis, both of which are good. The buff dude or robust gal must work-around the fact that the straps ‘want’ to slide-off the sloping trapezius, in addition to being further away from the spinal axis, both of which are demerits.
The last element is the suspension-fabric. Even low-end frames are curve to match the S-form of the human spine … and the side-to-side horizontal members are arched, to allow for the taut fabric between the frame and your back. Lacing of the fabric to the frame allows for better contact, while rigid cinch-bars allow for easier tightening, at a cost to conformance. Some stretch is good.
Only the straps and the suspension-fabric should touch.