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A backpack

The simplest form of backpack (also rucksack or knapsack) is a cloth sack carried on one's back and secured with two straps that go over the shoulders and below the armpits. The shoulder is better suited for bearing heavy weights for long periods of time than the hand, so backpacks are often used for that purpose.

Backpacks designed for backpacking (often just called packs) are more complex than ordinary varieties. They come in two main designs: internal-frame and external-frame. In addition to the two shoulder straps, modern packs always have a padded hip belt, which carries most of the pack's weight. This reduces shoulder fatigue, since the pelvis is sturdier, and also improves the hiker's agility. Packs generally have many pockets on the outside, and the main compartment may even be subdivided. Almost all packs, especially external-frame models, have lash points on the exterior, so that bulky items may be strapped on. Packs are typically about 3 feet (1 m) tall.

Internal- vs. external-frame backpacks

External-frame packs are the older of the two designs and have been in use for at least the past 50 years. An external-frame pack is constructed around a metal (usually aluminium) frame. The frame has a system of straps and pads to keep the sack and the metal parts from contacting the body, with the added benefit of improved ventilation and decreased sweatiness. The fabric part of the pack is stretched along part of the frame's length, but the frame protrudes above and below. The main compartment is smaller than that of internal-frame packs, because bulky items (tents, sleeping bags, thermal pads) are strapped to the outside of the pack.

The frame of internal-frame packs is contained entirely inside the pack and consists of strips of either a specially designed polymer or a metal that molds to one's back to provide a good fit. Usually a fairly complex series of straps works with the frame to distribute the weight and hold it in place. Internal-frame packs may provide a few lash points, but it is difficult to lash a large item so that it stays rigid and does not bounce. Sleeping bags are a particularly difficult case, as they are very bulky and take up a large part of an internal-frame pack's storage space. Compression sacks allow sleeping bags to be reduced in size dramatically.

Internal-frame packs are best suited for skiing and other forms of locomotion involving upper-body movement, but have become increasingly popular for ordinary backpacking as well. Today, internal-frame packs are generally more common. External-frame models have vanished completely from the British Isles, but in the United States, some manufacturers continue to produce them.

Comparison of backpack models
External frame Internal frame
Large metal frame to which the pack is secured Highly reduced semirigid frame in the inside of the pack
Good ventilation Tight fit and less bouncing
Large capacity for bulky strap-on items Roomy internal storage
May cost approximately US$80-130 May cost approximately US$150-250 (or more)ca:Motxilla

de:Rucksack ja:ザック


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