Aerobic Exercise and Subsequent Venoconstrictive Thigh Cuffs Preserve Cardiorespiratory Fitness but Not Muscle Function after 30 Days of Bed Rest

PURPOSE: During spaceflight, countermeasures are crucial to counteract cardiorespiratory and musculoskeletal deconditioning, similar to effects seen after prolonged immobilization. Within a bed rest study simulating the effects of microgravity, we investigated the effectiveness of submaximal aerobic exercise and postexercise venoconstrictive thigh cuffs (VTC) for maintaining cardiorespiratory fitness and muscle function. METHODS: Twenty-four males and females were randomly allocated to a countermeasure ( n = 12, age = 34 ± 9 yr, body mass index = 22.9 ± 2.9 kg·m -2 ) or control group ( n = 12, age = 35 ± 8 yr, body mass index = 23.3 ± 2.0 kg·m -2 ). Participants underwent 30 d of strict 6° head-down tilt (HDT) bed rest. Six days per week, the countermeasure group completed 60 min of submaximal cycling in HDT position, followed by 6 h of VTC (50 ± 5 mm Hg). The control group maintained HDT bed rest without any countermeasure. Cardiorespiratory fitness and muscle strength were assessed before and after bed rest via cardiopulmonary exercise testing and isokinetic dynamometry, respectively. RESULTS: Peak oxygen uptake was maintained in the countermeasure group (-106 ± 148 mL·min -1 ) compared with the control group (-607 ± 343 mL·min -1 ; P < 0.001). The countermeasure also mitigated the decrease in plasma volume (countermeasure: -147 ± 95 mL vs control: -286 ± 153 mL; P = 0.014), peak estimated stroke volume (countermeasure: -6 ± 5 mL vs control: -21 ± 13 mL; P < 0.001), and cardiac output (countermeasure: -0.7 ± 0.9 L·min -1 vs control: -3.8 ± 2.1 L·min -1 ; P < 0.001). Both groups showed reductions in fat-free mass and muscle strength after bed rest. CONCLUSIONS: Submaximal exercise with subsequent VTC preserves cardiorespiratory fitness during bed rest, likely by maintaining peak stroke volume and cardiac output. However, this countermeasure did not prevent declines of fat-free mass and muscle strength.