VO2 kinetics of constant-load exercise following bed-rest-induced deconditioning

The purpose of this study was to determine the effects of bed-rest-induced deconditioning on changes in O2 uptake (VO2) kinetics, O2 deficit, steady-state VO2, and recovery VO2 during the performance of constant-load exercise. Five male subjects (36-40 yr) underwent 7 days of continuous bed rest (BR) in the head-down (-6 degrees) position. Two days before (pre) and the day after (post) BR each subject performed one submaximal exercise test in the supine and one in the upright position consisting of 5 min of rest, 5 min of cycle ergometer exercise at 700 kg.m/min, and 10 min of recovery from exercise. VO2 was measured continuously in all tests from 2-liter aliquot gas samples collected every 30 s. Following BR steady-state VO2 was unchanged in supine and upright exercise. In the supine position BR did not change total exercise VO2, O2 deficit, or total recovery VO2. However, compared with pre-BR, total exercise VO2 decreased (P less than 0.05) from 7.41 +/- 0.11 to 7.23 +/- 0.17 liters, O2 deficit increased (P less than 0.05) from 1.15 +/- 0.05 to 1.41 +/- 0.07 liters, and total recovery VO2 increased (P less than 0.05) from 5.17 +/- 0.11 to 5.47 +/- 0.17 liters during the post-BR upright test. Despite the ability to attain similar steady-state VO2 within 5 min, bed-rest-induced deconditioning resulted in a reduction of total VO2 capacity and an increase in the O2 deficit during submaximal constant-load exercise. This change in VO2 kinetics is found only with exercise in the upright rather than supine position implicating orthostatic mechanisms in the delayed response to submaximal exercise.