Muscle and bone form a functional unit. Muscle forces essentially regulate bone strength. Moreover, the muscle function is a crucial factor to prevent falls and fractures. Therefore, a diagnosis of osteoporosis is inadequate, if the muscle is not taken into consideration. An improvement in muscle function not only results in an increase in bone strength, but also prevents falls, thus reducing the risk of fracture.

Function of bone

The most important function of bone is to transmit force and hence enable locomotion. The highest forces acting on bone are caused directly by muscle contractions and are transmitted by tendons to the bone. Due to lever arms the muscle are acting one these forces can easily exceed the 10 fold of the body weight.

To fulfil these mechanical requirements bone must be able to adapt to changes in magnitude and direction of those forces in able to make sure that voluntary contraction cannot cause pain or even fractures. In addition bone has to be able to adapt to changes in metabolism an other external parameters.

Mechanostat

Bone strength (not bone mass) is a regulated quantity. In the sixties Harold Frost described a feedback control loop called mechanostat that explains the adaptation of bone to mechanical forces.

An essential characteristics of control loops is negative feedback. Muscle forces result in a deformation (strain) of bone. Osteocytes, the most abundant bone cells are able to detect this deformation.

If the bone strain exceeds a certain threshold osteoblasts add bone to make it stronger and the strain returns to values below the threshold. If a second, lower threshold is not exceeded regularly, osteoclasts remove bone so that the lower threshold is exceeded again. This mechanism optimises bone strength with a minimum of bone mass.

Diagnosis

Fracture risk depends on bone strength and on the forces that act on it. The main goal of bone densitometry using pQCT is to determine bone strength. Bone strength is defined by the material properties and geometry. With the exception of some diseases bone’s material properties are allmost constant and bone strength hence mainly depends on bone geometry. Changes in bone metabolism is detected at first in trabecular bone.