Purpose: This study aimed to examine the effect of the degree of inclination from upright to supine postures on cycling performance, fatigue, and oxygen uptake (VO2) kinetics. Methods: In experiment 1, 10 subjects performed graded and fatigue (exhaustive constant-load heavy exercise with 10 s all-out efforts interspersed every minute) tests at four cycling postures: upright, 30° recumbent (R), 15° R, and supine. In experiment 2, nine different subjects performed two bouts of constant-load heavy exercise in the same four cycling postures. Bout 1 was brought to failure, and bout 2 was limited to 6 min, so that the breath-by-breath VO2 data from the first 6 min of each bout were averaged and curve fit. Results: The time sustained during the graded test was significantly shorter in the supine compared with the other three postures and also shorter in the 15° R compared with the upright. The rate of fatigue was higher in the supine compared with the other three postures, and the normalized EMG activities of three leg muscles at end exercise were larger in the supine (and in some cases 15° R) compared with upright posture. The time sustained (min) during high-intensity constant-load cycling was significantly longer during upright (12.8 ± 5.3) and 30° R (14.2 ± 6.1) compared with 15° R (8.5 ± 1.7) and supine (6.8 ± 2.0) postures, but the amplitudes of the slow component of the VO2 response (L/min) were larger during 15° R (0.57 ± 0.10) and supine (0.61 ± 0.15) compared with 30° R (0.39 ± 0.12) and also larger in the supine than upright (0.43 ± 0.13) postures. Inert gas rebreathing analysis revealed similar cardiac output responses at 60 s into the exercise among postures. Conclusion: Lowering the recumbent angle to 15° resulted in shorter performance, larger fatigue, and altered VO2 kinetics.
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