---
title: "Bone-loss linearity vs. deceleration (kinetic model choice)"
type: "entity"
entity_kind: "concept"
status: "hub"
canonical_name: "Bone-loss linearity vs. deceleration assumption"
aliases: ["constant-rate bone-loss assumption","front-loaded bone loss"]
first_seen: "2026-10-23T00:00:00.000Z"
writer_model: "claude-sonnet-5"
connects_to: ["spaceflight bone loss","bed rest analog","bone-loss meta-analysis","Jörn Rittweger","Mariya Stavnichuk"]
seek_code_commit: "17d9798"
---


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. [[claim-stavnichuk-rate-is-linear-slope-despite-deceleration-hint|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. [[claim-bocker-bone-dose-equivalence-rests-on-unconfirmed-front-loading-hypothesis|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.