Exercise during artificial gravity preserves cardiorespiratory fitness but not orthostatic tolerance following 60 days of head-down bed rest (BRACE)

Abstract

Exercise is a critical countermeasure to prevent cardiovascular deconditioning during spaceflight; however, exercise does not protect astronauts from postflight orthostatic intolerance. Artificial gravity (AG) by short-arm centrifugation can attenuate reductions in orthostatic tolerance following prolonged head-down bed rest (HDBR), but AG does not protect cardiorespiratory fitness. The European Space Agency hypothesized that exercise and AG countermeasures could be applied simultaneously to protect both cardiorespiratory fitness and orthostatic tolerance following prolonged HDBR. Twenty-four healthy men (age: 29 ± 6 yr, peak oxygen uptake: 47.5 ± 6.0 mL·min-1·kg-1) completed 60 days of HDBR and were randomized into either sedentary control (n ¼ 8), exercise (n ¼ 8), or exercise þ AG (n ¼ 8) groups. Exercise participants performed 30 min of high-intensity interval cycling on 49 of 60 days during HDBR. The exercise þ AG group performed the same 30-min exercise program while spinning in supine position in a short-arm centrifuge to generate a head-to-foot acceleration. Peak oxygen uptake (HDBR X group: P < 0.001) was reduced in the control group following HDBR (D ¼ -24 ± 5%) but was protected by the exercise (D ¼ 0 ± 6%) and exercise þ AG (D ¼ 4 ± 6%) countermeasures. Time to presyncope was reduced in all groups (control: D ¼ -9.0 ± 3.4 min, exercise: D ¼ -12.4 ± 5.2 min, exercise þ AG: D ¼ -4.5 ± 8.8 min) following HDBR (main effect: P < 0.001). Activation of the muscle pump during exercise likely minimized the redistribution of blood volume into the legs and consequently, the simulated orthostatic stress experienced during centrifugation, preventing benefits of exercise þ AG on orthostatic tolerance following HDBR. Therefore, AG by short-arm centrifugation should be implemented at rest or postexercise to protect orthostatic tolerance.