A backpack is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders, but there can be exceptions. Lightweight types of backpacks are sometimes worn on only one shoulder strap. 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 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 centre of mass.