Individual Fitness as a Moderator for the Effects of Countermeasure Exercise Training in Isolation on Cardiorespiratory Kinetics

PURPOSE
Countermeasure exercise is paramount to maintaining physical fitness during long-term space travel, and its efficacy can be investigated in highly controlled terrestrial spaceflight simulations. These analogs offer good operational fidelity and include important aspects like reduced daily activity, isolation, and limited communication. In addition to assessing peak values from incremental exercise, heart rate (HR) and oxygen uptake (V̇O2) kinetics can be derived from moderate-intensity exercise protocols, reducing medical risks, and serving as indicators for physical fitness. However, the participants’ individual fitness should be considered for the prescription of exercise training and a possible transfer of findings to microgravity.
METHODS
Two SIRIUS campaigns simulated journeys to the moon, isolating six (34 ± 6 years, 3 females, V̇O2peak: 41 ± 7 ml × kg × min-1) and five (35 ± 6 years, 2 females, V̇O2peak: 32 ± 2 ml × kg × min-1) participants for 120 days (S-19) and 240 days (S-21), respectively. Countermeasure exercise included either alternating continuous and interval endurance treadmill running (S-19) or a mixture of endurance and strength exercise (S-21). Changes in cardiorespiratory fitness over the missions’ time course were assessed with an exercise test protocol. The protocol included pseudo-random work rate changes (PRBS; kinetics analysis) and constant work rate phases at Pre, mission day (MD) 29/30, 44/57, 87/90, 117/118, and Post. Additional incremental ramp exercise during isolation was performed in S-19 only. HR was measured beat-to-beat, and V̇O2 was assessed breath-by-breath. Due to data availability, the analysis included only the first 120 days of both studies. The remaining data might be available for presentation at the congress. ANOVAs with repeated measures on the factor MD were performed separately for S-19 and S-21. Both campaigns were then analyzed with the co-variate V̇O2peak at Pre.
RESULTS
Over the time course of S-19, peak velocity increased while HRpeak was reduced (P=0.025) and V̇O2peak increased (P=0.012). Main effects for MD were also observed for HR kinetics (P=0.028) and V̇O2 kinetics (P=0.048). In S-21, no effects were visible for HR (P=0.327) or V̇O2 kinetics (P=0.401) until MD 118. However, the co-variate V̇O2peak showed a near significant difference between the two campaigns (P=0.055).
CONCLUSION
Peak values and kinetics of HR and V̇O2 were improved by countermeasure exercise training in isolation, indicating its positive effect on physical fitness. While no clear advantage for a specific exercise modality could be detected (e.g., interval, continuous, combination with strength training), the best physical fitness was observed in-mission after several weeks of exercise training. BDC measurements were reported as severely more stressful for SIRIUS participants, which might explain the fast return to pre-mission values in the post-test and should be considered for future mission planning. While the applied training improved fitness in the absence of microgravity, spaceflight will likely induce additional degenerating effects. Kinetics proved to be an important indicator for monitoring these effects during moderate-intensity exercise, avoiding the risks associated with incremental exercise. Participants of the two campaigns differed in their physical fitness, likely moderating the efficacy of the applied countermeasure exercise. Therefore, physical fitness should be considered for selecting participants in analog studies, for the individualization of exercise prescription, and for transferring results from analog studies to cosmonauts and astronauts who undergo a more rigorous physical fitness selection.