Without gravity loading the skeleton, osteoclasts (bone-resorbing cells) keep outpacing osteoblasts (bone-building cells), so weight-bearing bone mineral density (BMD) falls at a fairly steady rate measured on real ISS crews at roughly 1–1.5% per month in the hip and spine. Skeletal muscle — especially the postural and leg muscles that fight gravity on Earth — atrophies faster still, since protein synthesis drops once the mechanical load that normally triggers it disappears.
boneLoss/month = 1.2% × (1 − 0.85 × resistanceLoad)
muscleLoss/month = 5.0% × (1 − 0.80 × (0.6·resistanceLoad + 0.4·aerobicLoad))
load = clamp(minutes_of_that_type / 45, 0, 1)
- Resistance exercise (like the ISS's ARED — Advanced Resistive Exercise Device) mechanically loads bone directly and is the dominant lever against BMD loss; ~45 min/day of full-intensity resistance work is close to what fully offsets the loss in real astronaut data.
- Aerobic exercise (treadmill, cycle ergometer) helps cardiovascular fitness and contributes to muscle preservation, but loads bone far less directly — it alone barely slows BMD loss.
- Combined protocol — the real ISS regimen — pairs both, which is why flown astronauts on full compliance often return with near-zero net bone loss, while those with poor compliance or no countermeasure show the unmitigated 1–1.5%/month curve.
- Fracture risk ramps from Nominal → Watch → Elevated as cumulative BMD loss crosses clinically meaningful thresholds — beyond ~10% loss, hip and vertebral fracture risk on return to full gravity rises measurably even though the astronaut felt no symptoms in orbit.
This model tracks whole-body aggregate loss; the companion 3D simulator breaks the same physiology down by skeletal site, since femoral neck, spine and calcaneus lose density at different real-world rates while the forearm — not weight-bearing even on Earth — barely changes.