A site- and instrument-matched pQCT comparison finds bone loss comparable between 6-month spaceflight and 2-month bed rest, while muscle loss shows the expected ~2x under-run

A 2026 npj Microgravity study (Böcker, Lau, Mittag, Bloch, Vico & Rittweger) ran 6-month spaceflight subjects (n=13 space travelers, 16 datasets counting repeat missions) and 2-month head-down-tilt bed-rest subjects without any countermeasure (n=11) through the same peripheral quantitative computed tomography (pQCT) protocol at four homologous tibial sites (4%, 38%, 66%, 98% of tibial length). This is the first located comparison to remove the unmatched-population confound by measuring both conditions on cross-calibrated instruments at identical sites. Its central result splits bone from muscle:

"The muscle loss observed post-flight was approximately doubled compared to that was observed 2 months of bed rest, while bone loss was comparable."

"When comparing spaceflight and bed rest without countermeasures, changes were generally twice as pronounced in spaceflight as in bed rest with regards to muscle, but they were comparable in relation to bone, particularly at the diaphyseal sites."

The site-by-site statistics substantiate "comparable": most individual bone comparisons were not significant, with only one epiphyseal metric diverging at the earliest timepoint — "there were only significant moderate differences for vBMC.totTib98 (p = 0.01) at R + 1/R + 3" — and a handful of further divergences (vBMC.totTib38, vBMC.CtTib38, vBMC.totTib98, vBMC.TbTib98, all p ≤ 0.04) surfacing only later at the R+90 recovery timepoint, i.e. in recovery kinetics rather than in the initial magnitude of loss.

This directly tests the near-equality that claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout could only treat as a possible unmatched-study artifact, and finds it holds under matching — weakening the artifact explanation rather than resolving it toward "under-run." The clean ~2x gap that claim-bed-rest-bone-turnover-markers-run-half-spaceflight-magnitude documents for turnover markers reappears here for muscle, not bone. It qualifies the flat "quantitatively smaller" summary of claim-head-down-bed-rest-is-the-standard-ground-analog-for-spaceflight-bone-loss and sits alongside the DXA figure in claim-bed-rest-bmd-loss-about-one-percent-per-month-hip-pelvis-heel. The matching has a caveat of its own — the durations are not matched — carried in claim-bocker-matched-comparison-is-site-matched-not-duration-matched. Lead author context: entity-jorn-rittweger.