**1.2. Mechanical stimuli in bone**

Mechanical stimuli are one of the important regulation factors of bone remodeling. According to recent findings, osteocytes might play a role in the mechanical regulation of bone, receiving mechanical input signals and transmitting these stimuli to other cells in bone. Osteocyte is believed to comprise a sensory network that monitors mechanical load and tissue damage, and triggers appropriate adaptive responses, either formation or resorption. Mechanisms by which osteocytes could sense mechanical load have been understood by means of fluid movements throughout the lacunar-canalicular system, with some combination of shear stress and streaming potentials providing the proximate stimuli. The relations between mechanical stimuli and mature bone have been examined by experimental studies. There are suggested that higher rates of mechanical loading would evoke grater adaptive responses than lower rates of loading in mature bone (LaMonte et al., 2005). Exercise can substantially alter the physical states of a bone and generate adaptive responses. In a randomized controlled clinical trial, Fuchs et al.(2001) showed that, in jumpers, jumping significantly increases bone mineral content in femoral neck and lumbar skeleton relative to controls. McKay et al. (2000) found that jumping three times weekly for 8 months significantly augmented a real bone mineral density in the femoral trochanteric region. These studies have hypothesized that loading induced stimuli is the main signalgenerating factor and play a key role in mechanobiology. However, disuse uncouples bone formation from resorption, leading to increased porosity, decreased bone geometrical properties, and decreased bone mineral content which compromises bone mechanical properties and increases fracture risk. The removal of routine bone stresses (e.g. from immobilization, inactivity, or reduced gravity) has deleterious consequences on bone integrity. Reduced skeletal loading causes net bone loss by unbalancing bone formation and bone resorption (Takata and Yasui, 2001; Caillot-Augusseau et al., 1998). Rat hindlimb immobilization, human spaceflight, and human bedrest can all cause increased bone resorption and decreased bone formation (Caillot-Augusseau et al., 1998; Weinreb et al., 1989; Li et al., 2005; McGee et al., 2008). Therefore, the process of mechanotransduction of bone, the conversion of a mechanical stimulus into a biochemical response, is known to occur in osteoblast, osteoclast and osteocytes in response to it.
