Backpacks in general fall into one of four categories: frameless, external frame, internal frame, and bodypack. A pack frame, when present, serves to support the pack and distribute the weight of its contents across the body more easily (generally by transferring much of the weight to the hips and legs), so most of the weight does not rest on the shoulders, restricting range of motion and possibly causing damage from pressure on the straps. Most are capable of being closed with either a buckle mechanism, a zipper, or a dry-bag type closure though a few models use a drawstring fitted with a cord lock for the main compartment. Many backpacks with shoulder straps can affect the posture of a person carrying more than 14 kg (30 lbs), as well as being less restrictive of the upper body range of motion. Most backpacks are capable of being closed with either a buckle mechanism, a zipper, or a dry-bag type closure though a few ,odels use a drawstring fitted with a cord lock for the main compartment.
The simplest backpack design is a bag attached to a set of shoulder straps. Such packs are used for general transportation of goods, and have variable capacity. The simplest designs consist of one main pocket. This maybe combined with webbing or cordage straps; while more sophisticated models add extra pockets, waist straps, chest straps, padded shoulder straps, padded backs, and sometimes reflective materials for added safety when the wearer is out at night. In general, these packs can be produced inexpensively.
Some outdoors packs, particularly those sold for day hikes, ultralight backpacking and mountaineering are sometimes frameless as well.
The safety of some of the designs of these backpacks have been criticized by the UK NHS, many bag designs being deemed unsafe for children.
External frame packs were designed carry heavy loads (20kg / 40lb and more), 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, though some archaeologists believe the frame found with the body was part of a snowshoe. Such gear is common in military and mountaineering applications; metal versions first appeared in the mid-20th century.
The frame of an E-frame pack is typically made from aluminum or exotic light-weight metal alloy tubing or graphite-fiber. The frame is provided with a system of straps and tautly-stretched netting which prevents contact between the metal frame and user's back. In addition to comfort, this stand-off provides the additional benefit of creating air-circulation between the frame and user. For this reason E-frame packs are generally conceded to be a cooler load than I-frame packs designs. Similar to I-frame pack designs, E-frame packs also provide a fabric sack portion but which by design is smaller, leaving frame portions protruding above and below to accommodate attachment of items of any dimension. I-frame pack design's lock-in (by fixed dimension of sack-compartments) an expectation of the size of tent/sleeping-bag/air mattress. In contrast, E-frame pack-styles extend their versatility even further, since they allow total dismount of the entire manufacturer's preconceived sack portion from the frame, permitting the frame to be utilized in transportation of a user's own configuration of tiered stuff-sacks or non-conventional load eg: transport of a quartered game-animal from a field location. Military packs are often external-frame designs due to their ability to carry loads of improvised geometry and weight. The ability to adapt to improvised load(s) is the primary advantage that E-frame designs possess over I-frame pack designs. Manufacturers of E-frame pack designs include Kelty, Jansport, and Coleman.
The internal frame backpack is a recent innovation, invented in 1967 by Greg Lowe, who went on to found Lowepro, a company specializing in backpacks and other forms of carrying bags for various equipment. An internal-frame pack has a large cloth section in which a small frame is integrated. This frame generally consists of strips of either metal or plastic that mold to one's back to provide a good fit, sometimes with additional metal stays to reinforce the frame. Usually a complex series of straps works with the frame to distribute the weight and hold it in place. The close fitting of the back section to the wearer's back allows the pack to be closely attached to the body, and gives a predictable movement of the load; on the downside, 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. Internal-frame packs may provide a few lash points (including webbing loops and straps for sleeping bags and other large items), but as the frame is fully integrated and not available on the outside, it is difficult to lash a large, heavy item so that it stays fixed and does not bounce, so most cargo must fit inside. 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, they ride better in activities that involve upper-body movement such as scrambling over rocky surfaces and skiing. The improved internal frame models have largely replaced external frame backpacks for many activities.
A backpack fitted with pocket(s) that are suspended on the wearer's front side (chest) and loaded in such a way that the load in the front and the load in the back are about equal is called a bodypack. The majority of the load on a bodypack is carried by the hips. The ideal load carrying system should not disturb the wearer's natural posture, balance and movement of the body. The load must be dispersed onto the skeletal structure in a balanced way, and should not produce forces on the body forward, aft, right, or left.
A frameless backpack is a non-rigid bag strapped to the shoulders. It is light, inexpensive and widely available. It is only suitable for light loads (less than 5 kg).
While an internal frame backpack is a semi-rigid frame in the inside of the pack. It has a tight fit and it is less bouncy. It has a roomy internal storage with occasional lash points for external items.
On the other hand, an external frame backpack is a large rigid (metal, plastic, or wood) frame to which the pack is secured. It is heavier and it has good ventilation. It has a large capacity for bulky strap on items.
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