Shear and friction in combination with pressure can also contribute to pressure ulcer development. Shearing can occur when the correct moving and handling techniques are not used, when the individual has been sat up in the bed over a 30-degree tilt and even when you remove bedding or clothing from under the service user, without taking the proper care or protecting the individuals skin first. Shearing occurs when part of the body (the skeleton and underlying tissue) tries to move down the bed through gravity, but the surface of the skin (the skin on the back and buttocks) remains fixed or stuck on the same point on the sheets or mattress. This means that the skin is being pulled and twisted the wrong way which can affect the blood vessels by stretching and blocking them, which will restrict the blood flow to that area and the underlying tissue, which usually leads to the development of more extensive tissue damage. Shear forces can also be exacerbated by the presence of surface moisture through either incontinence or sweating, as these soften the skin and increase the risk of shear or friction forces.