High-Intensity Resistive and Rowing Exercises Do Not Prevent Orthostatic Intolerance after 70 Days of Bed Rest

Abstract

More than 60% of US astronauts participating in Mir and early International Space Station missions (>5 months) were unable to complete a 10-min 80 head-up tilt test on landing day. This high incidence of post-spaceflight orthostatic intolerance may be related to limitations of the in-flight exercise hardware that prevented high-intensity training. PURPOSE: To determine whether a countermeasure program that included intense lower-body resistive and rowing exercises designed to prevent cardiovascular and musculoskeletal deconditioning during 70 d of 6 head-down-tilt bed rest (BR), a spaceflight analog, also would protect against post-BR orthostatic intolerance. METHODS: Sixteen men participated in this study and performed either no exercise (Control, n=10) or an intense exercise protocol with resistive and aerobic components (Exercise, n=6). On 3 d/wk, Exercise subjects performed supine lower-body resistive exercise and a 30-min continuous bout of upright seated rowing (≥75% max heart rate) separated by 4 hr. On 3 other d/wk, Exercise subjects performed high-intensity interval rowing. Orthostatic intolerance was assessed using a 15-min 80 head-up tilt test performed 2 d before (BR-2) and on the last day of BR (BR70). Plasma volume was measured using a carbon monoxide rebreathing technique on BR-3 and before rising on the first recovery day (BR+0). RESULTS: After 70 d of BR, tilt tolerance time significantly decreased 34±31% (mean ±SD) in the Control subjects (BR-2: 15.0±0.0, BR70: 9.9±4.6 min) and by 56±16% in Exercise subjects (BR-2: 12.2±4.7, BR70: 4.9±1.9 min), with no difference between groups. Plasma volume also decreased from pre to post-BR in both groups (Control: -0.56±0.40, Exercise: -0.48±0.33 L), with no between group differences. CONCLUSIONS: Despite the combination of high intensity aerobic with resistive exercise during prolonged BR, orthostatic intolerance and plasma volume losses were evident, similar to previously results observed in shorter duration BR that used less comprehensive exercise protocols. These data suggest the need to continue to investigate novel or combinations of countermeasures, including periodic orthostatic stress, fluid repletion, and/or lower-body compression garments to minimize orthostatic intolerance following spaceflight.