Backpack

A backpack (also called rucksack, knapsack, packsack, pack, bookbag or Bergen) is, in its simplest form, a cloth sack carried on one's back and secured with one or two straps that go over the shoulders. Because of the limited capacity to carry heavy weights for long periods of time in the hands, backpacks are generally preferred to handbags for longer trips.

Large backpacks, used to carry loads over 10 kg (22 lbs), usually offload the largest part (up to about 90%) of their weight onto padded hip belts, leaving the shoulder straps mainly for stabilizing the load. This improves the potential to carry heavy loads, as the hips are stronger than the shoulders, and also increases agility and balance, since the load rides nearer the person's own center of mass.

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 fitted with a cord lock 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
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 and such gear was common in military and mountaineering applications right up to the 20th century. Metal framed 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 aluminum 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 and thus the main compartment is smaller than that of an internal-frame pack, because internal space is sacrificed to allow for bulky items to be strapped to the parts of the frame not occupied by the main compartment itself. This can result in a less smooth load and less control over the movement of the center of gravity of the pack, and can also result in bruising caused by the uncushioned frame rubbing or hitting against the body. While less popular than internal-frame gear, some manufacturers such as Kelty, Jansport, and Coleman continue to produce external packs. Military packs are often external-frame designs as well due to their increased durability.

Internal frame packs
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 carrying solutions 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 but with the downside of the tight fit reducing ventilation between the pack and wearer. 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 large, heavy items securely. Therefore most cargo must fit inside the pack. 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.