Figure 5 .

The mechanostat model of bone development according to Frost.^{25, 26} The central regulatory unit of bone development is the feedback mechanism between bone deformation and bone stability (black boxes and thick black arrows). Osteocytes act as sensors of bone deformation. If deformation exceeds a preset threshold, osteocytes cause osteoblasts and osteoclasts to adapt the bone tissue to the increased load. Thereby bone stability increases and the bone is deformed less by the same load. In children, the loads on the bones increase continuously as the result of growth in length and increasing muscle force. Consequently, bone stability is continuously adapted "upwards". However, during inactivity, bone stability can also be adapted "downwards"—that is, the bones get weaker. Nutrition, hormones, and other factors modulate the regulatory feedback loop at various sites (grey boxes and arrows).