Böcker's bone dose-equivalence conclusion rests on an explicitly unconfirmed hypothesis that bed-rest bone loss front-loads in the first 60 days and decelerates thereafter

space-medicineaerospace-physiologybone-densitybed-rest-analogstudy-design

Unlike the muscle figure, which is fitted to an external dataset with a named logarithmic function (claim-bocker-dose-equivalence-not-benchmarked-against-stavnichuk), Böcker's 60-day bone dose-equivalence conclusion (claim-bocker-concludes-60-days-bed-rest-models-six-month-spaceflight-bone-loss) is not derived from a fitted decay curve at all. It is inferred, after the fact, from a 3x-shorter bed-rest exposure producing a bone-loss magnitude the paper reports as comparable at most measurement sites — with a significant difference remaining at the proximal epiphysis (vBMC.totTib98, p = 0.01 at R + 1/R + 3) — against the spaceflight arm:

"Previous bed rest studies showed that muscle loss follows a logarithmic pattern, whereas bone loss follows a linear pattern. However, the duration of bed rest in these studies was longer for muscles than for bones. The results of our study suggest that the greatest bone loss occurs within the first 60 days and slows considerably thereafter, leading to comparable results despite the different durations (approximately 6 months of spaceflight and 60 days of bed rest), which need to be proved in possible future studies."

The "linear pattern" Böcker attributes to the prior bed-rest literature (ref 22, Rittweger et al. 2010, 56-day bed rest) is the opposite of the front-loaded, decelerating kinetic their own dose-equivalence conclusion implicitly requires — and the authors say so themselves, flagging their own explanation as unproven rather than asserting it as a finding. This is the bed-rest-side mirror of Stavnichuk's own linear-model tension on the spaceflight side: both papers separately flag doubt about whether bone loss accrues at a constant rate over the timescales each is modeling.

[unverified-mechanism] — this claim's resolution depends on reading ref 22 directly for its longitudinal time-course of cortical loss during bed rest, which has not been done; see watch_flag above and question-verify-rittweger-2010-bed-rest-cortical-loss-time-course.