Title Page Wise-2005: Effect of Aerobic and Resistive Exercises on Orthostatic Tolerance during 60 Days Bed Rest in Women

Cardiovascular deconditioning after long duration spaceflight is especially challenging in women who have a lower orthostatic tolerance (OT) compared with men. We hypothesized that an exercise prescription, combining supine aerobic treadmill exercise in a Lower Body Negative Pressure (LBNP) chamber followed by 10 min of resting LBNP, 3 to 4 times a week, and flywheel resistive training every third day would maintain orthostatic tolerance (OT) in women during a 60-day head-down-tilt bed rest (HDBR). Sixteen women were assigned to two groups (exercise, control). Pre and post HDBR OT was assessed with a tilt/LBNP test until presyncope. OT time (mean ± SE) decreased from 17.5±1.0 min to 9.1±1.5 min (-50±6%) in control group (p<0.001) and from 19.3 ±1.3 min to 13.0 ± 1.9 min (35±7%) in exercise group (p<0.001), with no significant difference in OT time between the two groups after HDBR (p=0.13). Nevertheless compared with controls post HDBR, exercisers had a lower heart rate (mean±SE) during supine rest (71±3 versus 85±4, p<0.01), a slower increase in heart rate and a slower decrease in stroke volume over the course of tilt/LBNP test (p<0.05). Blood volume (mean±SE) decreased in controls (-9±2%, p<0.01) but was maintained in exercisers (-4±3%, p=0.17). Our results suggest that the combined exercise countermeasure fails to protect OT but improves cardiovascular response to subtolerance levels of orthostatic stress.