The Böcker matched spaceflight/bed-rest comparison is site- and instrument-matched but not duration-matched, with spaceflight roughly three times longer than the bed-rest arm

The 2026 Böcker et al. pQCT comparison of spaceflight and head-down bed rest (see claim-matched-pqct-comparison-finds-bone-loss-comparable-while-muscle-loss-under-runs) achieves two of the three matching conditions the vault's open question demanded, but not the third. Both arms were scanned at the same four tibial sites, and the two devices used across sites (Star City and Cologne) were cross-calibrated against a shared phantom: "Both devices were cross-calibrated via measurements with the European Forearm Phantom, which showed a difference of 1.4% for total bone mineral density (BMD), a difference of 1.2% for trabecular BMD and a difference of 1.6% for cortical BMD." So the site and instrument legs are genuinely matched.

Duration is not. Mission length (median 178 days, range 165–340) ran roughly three times the 60-day bed-rest arm, and the paper flags this itself as a limitation rather than a design feature:

"Comparing muscle wasting between spaceflight and bed rest is not straightforward, given that space missions in this study lasted three times longer than experimental bed rest, and that space travelers exercised daily whilst bed rest participants performed no countermeasure at all."

The authors justify comparing the two anyway by citing prior literature (Hargens & Vico 2016) for the convention that 60–90 days of strict bed rest is the dose-equivalent of ~6 months of spaceflight — a claim resting one citation layer back from this paper and not independently read at capture, routed to question-verify-hargens-vico-2016-bed-rest-spaceflight-dose-equivalence. This is why the topic question in question-matched-head-to-head-bed-rest-vs-spaceflight-bone-comparison is only half-answered: the study is the best located site-matched, same-instrument comparison, but it is explicitly not the duration-matched design that question asked for. The comparison rests on the dose-equivalence premise rather than on matched exposure, which is exactly the load the conclusion in claim-bocker-concludes-60-days-bed-rest-models-six-month-spaceflight-bone-loss inherits.