Orthostatic responses following 30-day bed rest deconditioning with isotonic and isokinetic exercise training

To determine if intensive isotonic or isokinetic exercise training during 30 d of -6 degrees head-down bed rest (BR) would accentuate orthostatic intolerance, 19 men (32-42 years) were divided into a no-exercise control group (N = 5), and isotonic (Quinton ergometer, N = 7) and isokinetic (Lido ergometer, N = 7) exercise groups. Training was two 30-min periods per day for 5 days per week. Changes (* = p less than 0.05) in peak VO2 uptake (L/min) from control day 2 to BR day 29 were: isotonic +1.4%, isokinetic -10.2%*, no exercise -20.1%*. Changes in resting plasma volume (ml) from control day 1 to BR day 30 were: isotonic -3.7%, isokinetic -18.0%*, and no exercise -17.2%*. A 60 degrees head-up tilt test was administered on control day 1 and BR day 30; the test was terminated at 60 min or when presyncopal signs and/or symptoms occurred. Changes in means tilt tolerance were: isotonic, 42 to 34 min (delta = -8 min*); isokinetic, 53 to 30 min (delta = -23 min*); and no exercise, 46 to 30 min (delta = -16 min*). Mean day 30 group tolerances were all significantly lower than day 1 tolerances, but the reductions were not different between groups. Because there was no obvious relationship between type of exercise, exercise energy expenditure, change in peak VO2, or change in resting plasma volume and the consistent reduction in post-BR tilt tolerances, it appears that the orthostatic intolerance was due mainly to the reduction in body hydrostatic pressure from the -6 degrees head-down body position, and was not related to the level of physical fitness. Thus, factors other than training status are probably involved.