The role of inter-subject variability for the effects of countermeasure exercise training on cardiorespiratory fitness in analog and spaceflight studies

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Cardiorespiratory fitness (CRF) must be maintained during spaceflight to meet the physical demands of re-entry or future planetary landings in partial gravity (Norsk et al., 2020). Therefore, the efficacy of potential countermeasure exercise training programs is investigated in terrestrial analog studies. With a perspective on future space travel, long-term terrestrial spaceflight simulations are utilized for their operational fidelity and highly controlled environment. However, the lack of microgravity enables CRF improvements, whereas reduced values are still reported after sojourns on the international space station (ISS; Hoffmann et al., 2016). An additional factor requiring consideration is the inter-subject variability of physical fitness between most analog studies’ participants and cosmonauts and astronauts. We hypothesize that the magnitude of countermeasure exercise effects on CRF is dependent on pre-mission physical fitness, which might impair the transfer of terrestrial findings to an application in spaceflight.
CRF was investigated with an exercise test protocol, including moderate-intensity pseudo-random work rate changes to assess heart rate (HR) and oxygen uptake (V̇O2) kinetics and incremental exercise for peak values. The same protocol was implemented in 60 mission days (MD) of head-down tilt bed rest with reactive sledge-jump countermeasure exercise (RSL; N=22; 11 Ctrl; 30±6 years; V̇O2peak: 47±9 ml×kg×min-1), 30 MD of simulated spaceflight in isolation with bicycle and strength exercise (HERA; N=16, 40±9 years; V̇O2peak: 39±4 ml×kg×min-1), two times 120 MD of simulated spaceflight in isolation with endurance treadmill running and strength exercise training (SIRIUS-19: N=6; 34±6 years; V̇O2peak: 41±7 ml×kg×min-1 & SIRIUS-21: N=5; 35±6 years; V̇O2peak: 32±2 ml×kg×min-1), and 174±22 MD on the ISS with treadmill and strength training (ISS; N=10; 44±3 years; V̇O2peak: 37±5 ml×kg×min-1). The exercise test protocol was performed pre and post (all studies) and at MD 27 (± 4; SIRIUS only), 47 (± 8), 86 (± 4), and 117 (± 1; all SIRIUS & ISS). ANCOVAs with repeated measures on the factor MD were calculated for the variables HRpeak, V̇O2peak, HR kinetics, and V̇O2 kinetics, including the group variable study and pre-mission V̇O2peak as a covariate (CoV).
From pre to post, V̇O2peak showed an MD effect (P = 0.003; ηp2 = 0.193), a MD*study*CoV interaction (P < 0.001; ηp2 = 0.325), and a significant between-subject effect for the CoV (P < 0.001; ηp2 = 0.441). V̇O2peak improved in HERA but was reduced after RSL and ISS. In SIRIUS-19 and SIRIUS-21, improvements in V̇O2peak during the mission were diminished at Post. No effect was found for HRpeak.
V̇O2 kinetics showed a
significant MD*study*CoV interaction (
0.002; ηp2 =
0.320) and effects for CoV (P = 0.020; ηp2 = 0.119) and CoV*study (P < 0.001; ηp2 = 0.605) with slowed responses in HERA, RSL, SIRIUS-21, and ISS but accelerations in SIRIUS-19. HR kinetics showed a significant MD*study*CoV interaction (P = 0.045; ηp2 = 0.199) and a significant between-subject effect for study*CoV (P < 0.001; ηp2 = 0.387). HR kinetics were accelerated after SIRIUS-19 and SIRIUS-21 but slowed following RSL and ISS.
Countermeasure exercise training effects on CRF differ depending on the study design and the participants’ initial level of physical fitness. While the beneficial effects of exercise are mitigated the most by head-down tilt bed rest and microgravity, high outcome variability is also observed in isolation studies. The current findings emphasize the need for individualized exercise prescription and the consideration of individual fitness for participant selection in analog studies. An improved matching might enhance the transfer of findings from analog studies to spaceflight.
Original languageEnglish
Title of host publication7th Human Physiology Workshop
Number of pages1
PublisherDeutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)
Publication date03.12.2022
Publication statusPublished - 03.12.2022

ID: 11948348

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