Effects of an 8-month isolation on body composition and cardiopulmonary exercise testing: SIRIUS-21 space analog mission

Isolation is a significant stressor in spaceflight, affecting both physiological and psychological health. Ground-based analog studies, such as the SIRIUS-21 program, simulate space mission conditions to explore countermeasures for these impacts. From November 2021 to July 2022, six international crew members (50 % female) participated in an 8-month isolation study at the Institute for Biomedical Problems, Moscow. Conditions included restricted social contact, confined living spaces, and extravehicular activities (EVAs) via Virtual Reality and physical models. A structured endurance and resistance exercise program aimed to counteract isolation-related changes in body composition and cardiorespiratory fitness (CRF), assessed through cardiopulmonary exercise testing (CPET). Body composition was measured using bioelectrical impedance analysis (BIA) and physical activity (PA) monitored via Garmin devices at baseline (BDC), mission days (MD) MD 53, MD 85, MD 153, MD 185, MD 212, and recovery (R). CPET was conducted at BDC, MD 27, and R. Data analysis used SPSS v28. Time spent in total PA increased significantly, from 0.25 h/day at BDC to 1.38 h/day at R (p < 0.05); moderate PA rose from 0.24 h/day to 1.28 h/day (p < 0.05). Fat mass decreased from 24.80 % at BDC to 22.70 % at MD 212 (p < 0.05) and trended to 21.20 % at R, with fat-free mass and muscle mass maintained. A significant negative correlation (r = −0.900) between the change in PA (MD 85–BDC) and Fat percentage (MD 53–BDC) indicates that increased PA is associated with a substantial reduction in fat%. Similarly, a significant positive correlation (r = 0.900) between the change in PA (MD 85–BDC) and changes in FFM and MM (MD 53–BDC) suggests higher PA levels contribute to increased FFM. Notably, a positive correlation (r = 0.900) was also observed between PA (MD 85–BDC) and MM changes (MD 185–BDC). CPET showed improved aerobic capacity with predicted oxygen uptake rising from 37.77 ml/min/kg to 41.28 ml/min/kg. Fat oxidation declined 17.06 ± 11.57 % at BDC to 14.16 ± 7.67 % at R, and carbohydrate oxidation rose over time from 82.93 ± 11.57 % to 85.83 ± 7.67 %, with significant differences during warm-up. METs indicated moderate to vigorous intensity, and median respiratory quotient suggested greater carbohydrate use due to increased PA and delayed anaerobic threshold. These results demonstrate that structured exercise effectively mitigates physical deconditioning during prolonged isolation while preserving CRF. This has direct relevance to long-duration space missions, highlighting the significance of integrating exercise protocols in mission planning, and confirming ground-based analogs as essential platforms for testing interventions before Moon, Mars, and deep space exploration.