The purpose of this study was to compare O2 uptake (VO2) and muscle electromyography activity kinetics during moderate and severe exercise to test the hypothesis of progressive recruitment of fast-twitch fibers in the explanation of the VO2 slow component. After an incremental test to exhaustion, 7 trained cyclists (mean ± SD, 61.4 ± 4.2 ml·min-1·kg-1) performed several square-wave transitions for 6 min at moderate and severe intensities on a bicycle ergometer. The VO2 response and the electrical activity (i.e., median power frequency, MDF) of the quadriceps vastus lateralis and vastus medialis of both lower limbs were measured continuously during exercise. After ~2 to 3 min of exercise onset, MDF values increased similarly during moderate and severe exercise for almost all muscles whereas a VO2 slow component occurred during severe exercise. There was no relationship between the increase of MDF values and the magnitude of the VO2slow component during the severe exercise. These results suggest that the origin of the slow component may not be due to the progressive recruitment of fast-twitch fibers.
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