Bed Rest Accurately Mimics Spaceflight-induced Musculoskeletal Variability

Kurzfassung

INTRODUCTION Despite countermeasures, prolonged microgravity exposure still leads to muscle atrophy and bone loss, primarily affecting the anti-gravity muscles of the lower limbs. These effects are subject to inter-individual as well as intra-individual variability, both in space travelers as well as subjects participating in bed rest studies. For the success of future deep-space missions, it is imperative to maintain crew health and identify suitable analogue scenarios on earth to acquire an in depth understanding of factors accelerating muscle atrophy and bone loss. To this end, we compared musculoskeletal adaptations after spaceflight and bed rest to determine how effective bed rest mimics its general deterioration as well as its inter-individual variability. METHODS Volumetric bone mineral content (vBMC) was assessed distal and proximal by peripheral quantitative computer tomography (pQCT) at four standardized sites corresponding to 4%, 38%, 66%, and 98% of tibial length of 13 space travelers with mission durations of approximately 6 months and 11 subjects, which underwent 60 days of strict head-down tilt bed rest (RSL study). Additionally, we analyzed the muscle cross-sectional area (Ar.M) at the diaphysis. All subjects were examined twice prior mission/bed rest and at three times post mission/bed rest including recovery (R+1, R+14, R+90). We calculated the individual percent change pci as the relative change from baseline to each post measurement. For detection of any differences in variability, we performed Levene´s test for comparing the variances of pci. Additionally, we analyzed the occurrence of inter-individual variability by pci exceeding the 95%-confidence interval defined by group-site mean values ± 1.96 measurement uncertainty. RESULTS Space travelers demonstrated an Ar.M loss of 13.3 ± 5.0% at Tibia 38 and 12.5 ± 4.9% at Tibia 66 at R+1, respectively. The muscle atrophy was significant smaller after bed rest (Tibia 38: 6.2 ± 4.0%, p < 0.001; Tibia 66: 7.9 ± 2.7, p = 0.009). Further analysis suggested a bed rest duration of 80 days to induce comparable muscle losses. In contrast, bone loss was similar between space travelers and bed rest participant at R+1 with one exception (vBMC at Tibia 66, p = 0.02). Levene´s test results revealed significant inhomogeneity for only two out of 36 comparisons (p = 0.03 and p = 0.047), both of which were bone site comparisons. DISCUSSION This data suggests that the variability of musculoskeletal adaptations is almost uniformly comparable for space missions and bed rest across all measurement sites and dates. The inter-individual variability was also comparable between the two conditions, being most pronounced for the diaphyseal bone sites and tending to be smaller for the muscles than the bone sites. In conclusion, 60-day bed rest studies adequately simulate the variability of musculoskeletal response after spaceflight and can serve as a suitable analogue for this organ system.