Bone-loss linearity vs. deceleration (kinetic model choice)

Whether bone loss under mechanical unloading — in orbit or in bed rest — accrues at a constant monthly rate or instead front-loads early and decelerates. Not a settled fact in either direction: it is a modeling choice that this vault's two central spaceflight-bone-loss papers each make implicitly and each separately flag doubt about, without naming the assumption as shared ground between them.

Matters to this vault because several of its most-quoted figures silently assume linearity. Stavnichuk's pooled −0.8%/month lower-limb rate is the slope of a meta-regression forced through zero, even though the same paper's Discussion notes long-duration missions lost less than intermediate-duration ones. Böcker's 60-day bed-rest dose-equivalence conclusion requires the opposite kinetic — a front-loaded, decelerating bed-rest curve — while citing a reference for "bone loss follows a linear pattern" in the same paragraph, and names its own explanation as unproven. Earlier arithmetic in this vault's bed-rest-analog cluster (claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout, claim-bocker-tibial-losses-match-stavnichuk-only-at-epiphyseal-sites) also converts magnitudes to monthly rates without naming the constant-rate assumption underneath the conversion.

References

• claim-stavnichuk-rate-is-linear-slope-despite-deceleration-hint · claim-bocker-bone-dose-equivalence-rests-on-unconfirmed-front-loading-hypothesis · claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout · claim-bocker-tibial-losses-match-stavnichuk-only-at-epiphyseal-sites

• claim-rittweger-2010-bed-rest-time-course-measured-but-not-published-numerically · claim-rittweger-2010-bed-rest-cortical-loss-prose-points-away-from-front-loading

• Open: question-verify-rittweger-2010-bed-rest-cortical-loss-time-course

• Captures: 2026-10-23-what-genuinely-connects-in-missions-longer-than-30 · 2026-10-27-does-rittweger-et-al-2010s-cortical-bone-loss

• 2026-10-31: the bed-rest side of this fault line got a direct primary read. Rittweger et al. 2010 (the "linear pattern" citation Böcker leans on) measured the 56-day time course at five intra-bed-rest timepoints but never tabulated the numbers, so no fitted curve settles linear-vs-front-loaded from that paper. What its prose does say — delayed epiphyseal onset (day BR.17) and a diaphysis that "continuously decrease[s]" — points away from front-loading, the direction Böcker's dose-equivalence needs. The quantitative question stays open, now narrowed to figure-digitization (Figs. 2/4) or the LTBR 2005 predecessor.

Note (2026-10-23): created during the Stavnichuk–Böcker dose-equivalence bridge promotion, the capture's own verdict was that this fault line, not a shared number, is what the cosine pairing was actually tracking — two papers independently uneasy about the same unresolved kinetic question, from two different directions.