At the end of the incremental test, three VO2 response could be described : a VO2 plateau, a VO2 peak or sometimes a VO2 drop1 (Figure 1). Astrand1 considered that this drop was the proof of the maximal VO2 attainment. The VO2 drop was also described at the end of constant work rate exercise. This phenomenon was observed in 93 and 54% of subjects during constant work rate exercise in running at . 140% of the maximal aerobic power (PVO2max)3 and at 95% PVO2max4. Therefore, the purpose of this study was to clarify the etiology of the VO2 drop phenomenon at the end of exhaustive exercises eliciting VO2max (i.e. appearance frequency and characteristics) according to: 1) the type of exercise at the same relative intensity at 90% PVO2max (cycling, n=32; running, n=17; swimming, n=8), 2) the intensity of exercise (between 84 and 115% PVO2max in running, n=8), 3) the protocol (constant work rate test at 100% PVO2max vs. incremental test, n=25), 4) the age of subjects during exercise at 90% PVO2max (14.7 +/- 1.1 vs 34.5 +/- 6.5 years, n=11and n=32), 5) the fitness of subjects during exercise at 90% PVO2max (> or < 70 mlO2.kg-1.min-1, n=13 and n=19), 6) the training program during exercises and training exercises realized at 90, 95, 100 and 115% PVO2max (before and after an aerobic training, n=8) and 7) the inspired oxygen fraction during exercise at 90% PVO2max (FiO2=21% and 16.2%, n=9). Methods All the subjects were specialists in their activity. They realized 1) an incremental test to determine the maximal oxygen uptake (VO2max) and the maximal aerobic power (PVO2max) and 2) constant work rate exercises. All the tests were separated at least by two days and the different constant work rate tests were realized in a random order. To test the effect of the exercise intensity (factor 2) and the training program (factor 6) on the VO2 drop, subjects carried out before and after an aerobic training, constant work rate tests at 90, 95, 100 and 115% PVO2max. To test the effect of the FiO2, acute hypoxic normobar conditions were obtained using an Alti Trainer 200 (S.M.TEC, Geneva, Switzerland). During all the tests, the measurement of gas exchanges were measured using K4b2 (Cosmed, Roma). Results The VO2 drop was significantly better fitted using exponential models followed by a negative linear regression than only exponential models (Figure 1). The VO2 drop was observed in all the 7 conditions tested. However, no factor have a significant effect on the VO2 drop appearance frequency both during the incremental test and the constant work rate test. Looking for the characteristics, the magnitude, duration and slope of drop in the constant work rate (n=43) and incremental test (n=7) were equal to 454 ± 217 mlO2.min-1 i.e. 11 ± 5% VO2max and 389 ± 314 i.e. 9 ± 7% VO2max, 210 ± 149 s i.e. 36 +/- 20% time limit (between 30s and 10min) and 100 ± 59 s (between 30s and 3 min), -213 ± 195 and -314 ± 300 mlO2.min-2. Only the activity (cycling), had an effect on the VO2 drop characteristics by increasing significantly the magnitude (576 ± 292 vs. 304 ± 97 and 280 ± 196 mlO2.min-1) and the slope of the drop (-287 ± 288 vs. -91 ± 53 and -61 ± 35 mlO2.min-2). Discussion/Conclusion The VO2 drop, already observed in several studies1,2,3,4 was not due to methodological problems (VO2 measurements realized with Douglas bag method1, Sensormedics 2900Z3, CPX/D Medical graphics4 and Cosmed K4b2 in this study). No factor have a significant effect on the VO2 drop appearance frequency. Nevertheless, it seems more frequent in adult (n=29/48=60%) compared with children (n=2/11=18%) caused probably by an immature anaerobic metabolism in children and during the constant work rate exercise (n=31/59=53%) compared with the incremental test (n=7/59=12%). The exercise contraction regimen could explain the higher VO2 drop magnitude in cycling than running and swimming by amplifying the slope of the VO2 drop that could decrease the duration of drop (194 ± 162 vs. 266 ± 151 and 302 ± 131, p>0.05). Indeed higher was the slope and higher was the anaerobic metabolism energy necessary to continue the exercise. So the VO2 drop could describe at the end of exercise a "final" oxygen deficit.
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