TIME COURSE OF CARDIORESPIRATORY DECONDITIONING WITH 17 DAYS OF 6?? HEAD DOWN TILT BEDREST 866

Abstract

The purpose of this project was to assess the time course of exercise cardiorespiratory changes in response to 17 days of 6° head down tilt bedrest. Seven males (age, 42.3 ±8.7 yr.; height, 181.4 ±6.5 cm; weight, 79.8 ±10.6) completed a continuous exercise test to volitional exhaustion on a supine cycling ergometer prior to bedrest (CON), on days 2 or 3 of bedrest (BR1), 8 or 9 (BR2), and 13 or 14 (BR3), as well as on days 3 or 4 of recovery (R1), and 7 or 8 (R2) to determine the submaximal (150 W) and maximal cardiorespiratory responses to exercise. Submaximal oxygen consumption(VO2) and minute ventilation (VE) did not change from CON during bedrest or recovery. Heart rate (HR: b·min-1) was significantly elevated (p<0.05) from CON (124±4) during BR2 (134±4), BR3(138±4), and R1 (135±2), but had returned to CON levels by R2(128±3). VO2max (L·min-1) was decreased(p<0.05) from CON (3.24±0.20) during BR1 (2.99±0.17; -7.3%), BR2 (3.00±0.17; -7.1%), BR3 (2.92±0.20; -9.0%), and R1(3.02±0.20; -6.6%), but was not different (p > 0.05) than CON by R2(3.13±0.19; -3.3%). HRmax did not change from CON during bedrest or recovery. Initial changes in VO2max (BR1) were significantly correlated with urine balance during bedrest day 1 (r=0.91, p < 0.05). These results suggest that the time course for changes in the cardiorespiratory responses to exercise are not linear and are related to the initial changes in body fluid volumes during 6° head down tilt bedrest.