Bone remodels along lines of stress, according to Wolff's law
Expansion
Wolff’s law: bone adapts to the loads placed on it, laying down bone where stress is greatest and resorbing it where it is not.
The mechanism is cellular: osteocytes, embedded in lacunae and connected through canaliculi, sense fluid shear during loading. Loading suppresses their production of sclerostin, which otherwise inhibits bone formation, so formation increases.
Loading increases bone density most when it is:
- High impact and dynamic rather than static
- Varied in direction
- Applied at rates above habitual
This is why weight-bearing and resistance exercise build bone while swimming and cycling do relatively little, and why gymnasts have exceptionally high bone density.
Unloading causes rapid loss: bed rest, spinal cord injury and spaceflight cause bone loss of 1 to 2 per cent a month, far faster than postmenopausal loss, with hypercalcaemia and hypercalciuria.
Stress fractures occur when remodelling cannot keep pace with repetitive loading, and are associated with the female athlete triad of low energy availability, menstrual disturbance and low bone density.