Exercise performance and \( \dot{V}{\text{O}}_{ 2} \) kinetics during upright and recumbent high-intensity cycling exercise

This study investigated cycling performance and oxygen uptake \(( \dot{V}{\text{O}}_{ 2} )\) kinetics between upright and two commonly used recumbent (R) postures, 65°R and 30°R. On three occasions, ten young active males performed three bouts of high-intensity constant-load (85% peak-workload achieved during a graded test) cycling in one of the three randomly assigned postures (upright, 65°R or 30°R). The first bout was performed to fatigue and second and third bouts were limited to 7 min. A subset of seven subjects performed a final constant-load test to failure in the supine posture. Exercise time to failure was not altered when the body inclination was lowered from the upright (13.1 ± 4.5 min) to 65°R (10.5 ± 2.7 min) and 30°R (11.5 ± 4.6 min) postures; but it was significantly shorter in the supine posture (5.8 ± 2.1 min) when compared with the three inclined postures. Resulting kinetic parameters from a tri-exponential analysis of breath-by-breath \( \dot{V}{\text{O}}_{ 2} \) data during the first 7 min of exercise were also not different between the three inclined postures. However, inert gas rebreathing analysis of cardiac output revealed a greater cardiac output and stroke volume in both recumbent postures compared with the upright posture at 30 s into the exercise. These data suggest that increased cardiac function may counteract the reduction of hydrostatic pressure from upright ~25 mmHg; to 65°R ~22 mmHg; and 30°R ~18 mmHg such that perfusion of active muscle presumably remains largely unchanged, and also therefore, \( \dot{V}{\text{O}}_{ 2} \) kinetics and performance during high-intensity cycling.