Terminology
The word backpack was coined in the United States in the 1910s. Knapsack and packsack were used before; they now occur mainly as regionalisms in North America. The word rucksack is a German loanword mainly used in the UK: 'der Rücken' means 'the back' (the part of the body) in German. The name Rucksack is cognate with Danish Rygsæk, Norwegian Ryggsekk, Dutch Rugzak, and Swedish Ryggsäck. Alternative names include Haversack, and Bergen (from the manufacturer's name Bergens, used for a backpack supported by an external frame, usually associated with the British Armed Forces).
Backpacks can often simply be referred to as "packs", especially in outdoors contexts; though sometimes ambiguous compared to other bags such as saddlebags and duffel bags, context is generally sufficient for identification.
Backpack designs
Backpacks in general fall into one of three categories: frameless, external frame, and internal frame. 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 or a zipper, though a few models use a drawstring for the main compartment. Many Backpacks with shoulder straps can affect a humans posture if carrying more than 30 pounds.
Frameless
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.
External frame packs
Two examples of external frame backpack designs dating to the 1860s
The more traditional type of frame pack uses a rigid external frame which is strapped on the back and in turn carries and supports a cloth or leather sack and potentially strapped on items. External frames were traditionally used to carry heavy loads (20 kg / 40 lb and more), giving the wearer more support and protection and better weight distribution than a simple, frameless strapped bag. Wooden pack frames have been 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), and such gear was common in military and mountaineering applications right up to the 20th century; metal versions first appeared in the mid-20th century, and plastic designs towards the turn of the 21st. Modern pack frames are usually made from lightweight metal tubes, generally aluminium but sometimes also using titanium or scandium alloys. The frame typically has a system of straps and pads to keep the sack and the frame from contacting the body. The open structure has the added benefit of improved ventilation and decreased sweatiness. The fabric part of the pack occupies part of the frame's length, but the frame typically protrudes above and below. These areas of the frame allow bulky items (such tents, sleeping bags, and thermal pads) to be strapped on. Thus the main compartment is smaller than that of an internal-frame pack, because bulky items (tents, sleeping bags, thermal pads) are strapped to the parts of the frame not occupied by the main compartment itself. This may result in a less smooth load (annoying in dense forest) and less control over the movement of the center of gravity of the pack. While less popular than internal-frame gear, some manufacturers (such as Kelty, Jansport, and Coleman) continue to produce external packs, and military packs are often external-frame designs as well.
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