SUBMAXIMAL EXERCISE VO(2) AND Q(C) DURING 30-DAY 6-DEGREES HEAD-DOWN BED REST WITH ISOTONIC AND ISOKINETIC EXERCISE TRAINING

Background: Maintaining intermediary metabolism is necessary for the h ealth and well-being of astronauts on long-duration spaceflights. Whil e peak oxygen uptake (V over dot O-2) is consistently decreased during prolonged bed rest, submaximal V over dot O-2 is either unchanged or decreased. Methods: Submaximal exercise metabolism (61 +/- 3% peak V o ver dot O-2) was measured during ambulation (AMB day-2) and on bed res t days 4, 11, and 25 in 19 healthy men (32-42 yr) allocated into no ex ercise (NOE, N = 5) control, and isotonic exercise (ITE, N = 7) and is okinetic exercise (IKE, N = 7) training groups. Exercise training was conducted supine for two 30-min periods per day for 6 d per week: ITE training was intermittent at 60-90% peak V over dot O-2; IKE training was 10 sets of 5 repetitions of peak knee flexion-extension force at a velocity of 100 degrees s(-1). Cardiac output was measured with the i ndirect Fick CO2 method, and plasma volume with Evans blue dye dilutio n. Results: Supine submaximal exercise V over dot O-2 decreased signif icantly (p < 0.05) by 10.3%* with ITE and by 7.3%* with IKE; similar to the submaximal cardiac output decrease of 14.5% (ITE) and 20.3%* ( IKE), but different from change in peak V over dot O-2 (+1.4% with ITE and -10.2% with IKE) and decrease in plasma volume of -3.7% (ITE) an d -18.0% (IKE). Reduction of submaximal V over dot O-2 during bed res t correlated 0.79 (p < 0.01) with submaximal Q over dot (C), but was n ot related to change in peak V over dot O-2 or plasma volume. Conclusi on: Reduction in submaximal oxygen uptake during prolonged bed rest is related to decrease in exercise but not resting cardiac output; pertu rbations in active skeletal muscle metabolism may be involved.