The skeleton's primary mechanical function is to provide rigid levers for muscles to act against as they hold the body upright in defiance of gravity. Many bones are exposed to thousands of repetitive loads each day. During growth and development, the skeleton optimizes its architecture by subtle adaptations to these mechanical loads. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction including:​ mechanocoupling — conversion of mechanical forces into local mechanical signals, such as fluid shear stresses, that initiate a response by bone cells;​ biochemical coupling — transduction of a mechanical signal to a biochemical response involving pathways within the cell membrane and cytoskeleton;​ cell-to-cell signaling from the sensor cells (probably osteocytes and bone lining cells) to effector cells (osteoblasts or osteoclasts) using prostaglandins and nitric oxide as signaling molecules;​ and effector response — either bone formation or resorption to cause appropriate architectural changes. These architectural changes tend to adjust and improve the bone structure to its prevailing mechanical environment. Structural changes can be predicted, to some extent, by mathematical formulas derived from three fundamental rules:​ (1) bone adaptation is driven by dynamic, rather than static, loading;​ (2) extending the loading duration has a diminishing effect on further bone adaptation;​ (3) bone cells accommodate to a mechanical loading environment, making them less responsive to routine or customary loading signals.