On Earth, gravity loads the skeleton every time you stand or walk, and that mechanical strain signals osteoblasts to keep building bone at the same rate osteoclasts resorb it. In microgravity that loading signal disappears: resorption continues while formation slows, and ISS crews without countermeasures lose roughly 1–1.5% bone mineral density per month in weight-bearing sites such as the hip and spine (NASA/ESA longitudinal DXA studies).
This simulator drives a trabecular bone patch with a simplified but literature-grounded remodeling balance:
dBMD/dt = -R0 * ( 1 - Eeff*E - Deff*D - S*E*D )
R0 = 0.05 %/day baseline unmitigated loss (~1.5%/month)
E = exercise adherence, 0-1 (ARED resistive-exercise dose)
D = diet adequacy, 0-1 (from calcium + vitamin D intake)
Eeff = 0.85 max mitigation exercise alone can provide
Deff = 0.25 max mitigation diet alone can provide
S = 0.15 synergy bonus when both are high
- Resistive exercise dominates. Mechanical loading is the only signal that directly tells osteoblasts to build bone, so E carries the largest weight — matching findings that ARED-equipped crews lose far less bone than earlier missions without it.
- Diet supports but cannot replace loading. Calcium and vitamin D supply the mineral and absorption needed for new bone matrix, but studies (e.g. LeBlanc et al.) found nutrition alone does not prevent spaceflight bone loss — it needs mechanical strain to act on.
- Combined countermeasures can approach zero net loss or even a small gain, which is why current protocols prescribe both together rather than either alone.
The 3D view is a small trabecular ("spongy") bone patch, the kind found inside a vertebra or the femoral neck. Each strut has its own resorption threshold; as BMD falls, the thinnest and weakest struts vanish first and the remaining struts thin out — the same connectivity loss seen in osteoporotic bone imaging.