Abstract
PurposeLong stays in space require countermeasures for the degrading effects of weightlessness on the human body, and artificial gravity (AG) has been proposed as an integrated countermeasure. The aim of this study was to assess the cardiorespiratory and neuromuscular demand of AG elicited via daily centrifugation during 60 days of bed rest.MethodsTwenty four participants (33 ± 9 y, 175 ± 9 cm, 74 ± 10 kg, 8 female) were subjected to 60 days of strict six-degree head-down tilt (HDT) bed rest and were randomly allocated to one of three experimental groups: 30 min of daily centrifugation with an acceleration of 1 g at the center of mass and 2 g at the feet applied continuously (cAG) or intermittently in 6 epochs of 5 min each, separated by 3 min breaks (iAG), or non-centrifuged control (CTRL). Cardiorespiratory demand during centrifugation was assessed at the beginning (HDT3) and end (HDT60) of the bed rest phase via spirometry and heart rate monitoring, leg muscle activation was monitored via electromyography.ResultsOn average, analyses of variance revealed that heart rate during centrifugation increased by 40% (iAG) and 60% (cAG) compared to resting values (p < 0.001), while oxygen uptake did not change significantly (p = 0.96). There was a preference for calf over knee extensor muscle activation (active time soleus 57 ± 27%, gastrocnemius medialis 45 ± 27% and vastus lateralis 27 ± 27%, p < 0.001), with large inter-individual differences in leg muscle active time. AG could not prevent the increase in resting heart rate after bed rest. For most of the recorded parameters, there were little differences between cAG and iAG, with the increase in heart rate during centrifugation being a notable exception (greater increase for cAG, p = 0.01).ConclusionDaily 30 min bouts of artificial gravity elicited by centrifugation put a substantial demand on the heart as a pump without increasing oxygen consumption. If centrifugation is to be used as a countermeasure for the deteriorating effects of microgravity on physical performance, we recommend combining it with strenuous exercise.
Highlights
The deconditioning of astronauts during long-duration space missions is a well-recognized risk (Williams et al, 2009)
For the deterioration of physical performance associated with gravitational unloading and physical inactivity, an effective countermeasure has already been established during a previous bed rest study: in the 60 day bed rest study, a short intensive jump training program consisting of countermovement jumps and repetitive hops was tested and was able to prevent the large musculoskeletal and cardiovascular deconditioning effects, in particular the loss of bone mineral mass and density, lean muscle mass, maximal leg strength and power as well as peak oxygen uptake (Kramer et al, 2017a,b, 2018)
For cAG, average heart rate during centrifugation was about 60% higher compared to baseline values, whereas for iAG it increased by 40%, with a significant centrifugation∗group interaction effect (p = 0.003 for HDT3 and p = 0.01 for HDT60) and a significant main effect of centrifugation (p < 0.001)
Summary
The deconditioning of astronauts during long-duration space missions is a well-recognized risk (Williams et al, 2009). In addition to the expected benefits for cardiovascular function, it has been suggested that short-arm centrifugation could help to prevent muscle atrophy, bone demineralization, and impairment of neuromuscular and sensorimotor coordination (Linnarsson et al, 2015). These studies were too short in duration to thoroughly study the efficacy of short-arm centrifugation on neuromuscular function and bone health
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