A backpack—also called knapsack, schoolbag, rucksack, rucksac, pack, sackpack, booksack, bookbag or backsack—is, in its simplest frameless form, a fabric sack carried on one's back and secured with two straps that go over the shoulders, but it can have an external frame, internal frame, and there are bodypacks. Backpacks are commonly used by hikers and students, and are often preferred to handbags for carrying heavy loads or carrying any sort of equipment, because of the limited capacity to carry heavy weights for long periods of time in the hands. Large backpacks, used to carry loads over 10 kilograms (22 lb), as well as smaller sports backpacks (e.g. running, cycling, hiking and hydration), usually offload the largest part (up to about 90%) of their weight onto padded hip belts, leaving the shoulder straps mainly for stabilising 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 wearer's own center of mass.