Effect of simulated weightlessness on exercise-induced anaerobic threshold

Ventilation (V̇E), CO₂ output (V̇CO₂), oxygen uptake (V̇O₂), respiratory exchange ratio (R), and the ventilatory equivalents for V̇O₂ and V̇CO₂ were measured during graded exercise before and after 10 d of continuous bed rest (BR) in the -6° headown position to determine the effect of deconditioning on the anaerobic threshold (AT), i.e., the highest workrate or V̇O₂ which was achieved without evidence of lactic acidosis, as judged from the profile of ventilatory and gas exchange responses. Ten healthy male subjects performed a supine graded cycle ergometer test before (pre) and after (post) BR which consisted of 4 min of unloaded pedaling at 60 rpm followed by an increased workrate of 15 W · min^-1 until volitional fatigue (max). V̇E, V̇CO₂, V̇O₂, R, V̇E/V̇O₂ and V̇E/V̇CO₂ were measured every 30 s and used collectively to identify the AT. Plasma (PV) and blood (BV) volumes were measured pre- and post-BR by T-1824. Following BR, V̇O₂max decreased from 2.42±0.17 to 2.25±0.13 L · min^-1 (7.0%, p < 0.05). BR significantly (p < 0.05) reduced at AT from 1.26±0.09 to 0.95±0.05 L · min^-1 V̇O₂; from 52.2±2.0 to 42.6±1.6% V̇O₂max; and from 93±9 to 65±6 W. A correlation coefficient (r) of -0.11 (NS) was found between the change in V̇O₂max and change in AT. A decrease in BV of 8.8% (p < 0.05) was due to the 11.0% reduction in PV; red cell volume remained constant. The change in AT following BR was significantly (p < 0.05) correlated with the changes in BV and PV (r =+0.73 and +0.80, respectively). These data suggest that: the reduction in AT induced by BR deconditioning cannot be completely explained by the reduction in V̇O₂max; the change in AT following BR is manifested by a decrease in both absolute and relative workrate; and the decrease in AT is associated with the reduction in intravascular fluid volume.