Résumé
The aim of this study was to locate the breakpoints of cerebral and muscle oxygenation and muscle electrical activity during a ramp exercise in reference to the first and second ventilatory thresholds. Twenty-five cyclists completed a maximal ramp test on an electromagnetically braked cycle-ergometer with a rate of increment of 25 W/min. Expired gazes (breath-by-breath), prefrontal cortex and vastus lateralis (VL) oxygenation [Near-infrared spectroscopy (NIRS)] together with electromyographic (EMG) Root Mean Square (RMS) activity for the VL, rectus femoris (RE), and biceps femoris (BF) muscles were continuously assessed. There was a non-linear increase in both cerebral deoxyhemoglobin (at 56 +/- 13% of the exercise) and oxyhemoglobin (56 +/- 8% of exercise) concomitantly to the first ventilatory threshold (57 +/- 6% of exercise, p > 0.86, Cohen's d < 0.1). Cerebral deoxyhemoglobin further increased (87 +/- 10% of exercise) while oxyhemoglobin reached a plateau/decreased (86 +/- 8% of exercise) after the second ventilatory threshold (81 +/- 6% of exercise, p < 0.05, d > 0.8). We identified one threshold only for muscle parameters with a non-linear decrease in muscle oxyhemoglobin (78 +/- 9% of exercise), attenuation in muscle deoxyhemoglobin (80 +/- 8% of exercise), and increase in EMG activity of VL (89 +/- 5% of exercise), RF (82 +/- 14% of exercise), and BE (85 +/- 9% of exercise). The thresholds in BE and VL EMG activity occurred after the second ventilatory threshold (p < 0.05, d > 0.6). Our results suggest that the metabolic and ventilatory events characterizing this latter cardiopulmonary threshold may affect both cerebral and muscle oxygenation levels, and in turn, muscle recruitment responses.