Comparison of musculoskeletal responses and its variability after long-term spaceflight and prolonged bed rest conditions.

Despite near-daily exercise, musculoskeletal deterioration occurs following spaceflight, with considerable individual variability. This study quantifies the extent, variability, and recovery of musculoskeletal deterioration after 6-month spaceflight (n = 13; all male; age: 34-56 years) and compares these findings to those observed during 2-month bed rest without any countermeasure (n = 11; all male; age: 22-39 years). Peripheral quantitative computed tomography (pQCT) scans at 4%, 38%, 66%, and 98% of the relative length of the tibia bone from distal to proximal assessed volumetric bone mineral content (vBMC; all measurement sites) and muscle cross-sectional area (Ar.M; 38% and 66%). After spaceflight, Ar.M decreased by 13.3% (SD 5.0%) at the 38% tibial site and by 12.5% (SD 4.9%) at the 66% tibial site, but fully recovered after three months. Bone loss was observed at all measurement sites (p < 0.05) and persisted through 3 months (p ≤ 0.02). The muscle loss observed post-flight was approximately doubled compared to that was observed 2 months of bed rest, while bone loss was comparable. In conclusion, most comparisons showed no inhomogeneity in variance, indicating that the variability in musculoskeletal outcomes after spaceflight is well represented by bed rest. In summary, bed rest serves as a valuable analog for understanding the mechanisms promoting muscle and bone deterioration during spaceflight.