The Mechanostat is a term describing the way in which mechanical loading influences bone structure by changing the mass (amount of bone) and architecture (its arrangement) to provide a structure that resists habitual loads with an economical amount of material. As changes in the skeleton are accomplished by the processes of formation (bone growth) and resorption (bone loss), the mechanostat models the effect of influences on the skeleton by those processes, through their effector cells, osteocytes, osteoblasts, and osteoclasts. The term was invented by Harold Frost: an orthopaedic surgeon and researcher described extensively in articles referring to Frost and Webster Jee's Utah Paradigm of Skeletal Physiology[1][2][3][4][5] in the 1960s. The Mechanostat is often defined as a practical description of Wolff's law described by Julius Wolff (1836–1902), but this is not completely accurate. Wolff wrote his treatises on bone after images of bone sections were described by Culmann and von Meyer, who suggested that the arrangement of the struts (trabeculae) at the ends of the bones were aligned with the stresses experienced by the bone. It has since been established that the static methods used for those calculations of lines of stress were inappropriate for work on what were, in effect, curved beams, a finding described by Lance Lanyon, a leading researcher in the area as "a triumph of a good idea over mathematics." While Wolff pulled together the work of Culmann and von Meyer, it was the French scientist Roux, who first used the term "functional adaptation" to describe the way that the skeleton optimized itself for its function, though Wolff is credited by many for that.
According to the Mechanostat, bone growth and bone loss is stimulated by the local, mechanical, elastic deformation of bone. The reason for the elastic deformation of bone is the peak forces caused by muscles (e.g. measurable using mechanography). The adaptation (feed-back control loop) of bone according to the maximum forces is considered to be a lifelong process. Hence, bone adapts its mechanical properties according to the needed mechanical function: bone mass, bone geometry, and bone strength (see also Stress-strain index, SSI) adapt to everyday usage/needs. "Maximal force" in this context is a simplification of the real input to bone that initiates adaptive changes. While the magnitude of a force (the weight of a load for example) is an important determinant of its effect on the skeleton, it is not the only one. The rate of application of force is also critical. Slow application of force over several seconds is not experienced by bone cells as a stimulus, but they are sensitive to very rapid application of forces (such as impacts) even of lower magnitude. High frequency vibration of bone at very low magnitudes is thought to stimulate changes, but the research in the area is not completely unequivocal. It is clear that bones respond better to loading/exercise with gaps between individual events, so that two loads separated by ten seconds of rest are more potent stimuli than ten loads within the same ten seconds.
Due to this control loop, there is a linear relationship in the healthy body between muscle cross sectional area (as a surrogate for typical maximum forces the muscle is able to produce under physiological conditions) and the bone cross sectional area (as a surrogate for bone strength).[6][7]
These relations are of immense importance, especially for conditions of bone loss like osteoporosis, since an adapted training utilizing the needed maximum forces on the bone can be used to stimulate bone growth and thereby prevent or help to minimize bone loss. An example for such an efficient training is vibration training or whole body vibration.