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The purpose of this study was to compare O[2] uptake (VO[2]) 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 VO[2] slow component. After an incremental test to exhaustion, 7 trained cyclists (mean +/- SD, 61.4 +/- 4.2 ml x min[-1] x kg[-1]) performed several square-wave transitions for 6 min at moderate and severe intensities on a bicycle ergometer. The VO[2] 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 approximately 2 to 3 min of exercise onset, MDF values increased similarly during moderate and severe exercise for almost all muscles whereas a VO[2] slow component occurred during severe exercise. There was no relationship between the increase of MDF values and the magnitude of the VO[2]slow 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. [ABSTRACT FROM AUTHOR]

Inconsistencies about dynamic asymmetry between the on- and off-transient responses in VO2 are found in the literature. Therefore the purpose of this study was to examine VO2on- and off-transients during moderate- and heavy-intensity cycling exercise in trained subjects. Ten men underwent an initial incremental test for the estimation of ventilatory threshold (VT) and, on different days, two bouts of square-wave exercise at moderate (VT) intensities. VO2 kinetics in exercise and recovery were better described by a single exponential model (VT). For moderate exercise, we found a symmetry of VO2 kinetics between the on- and off-transients (i.e., fundamental component), consistent with a system manifesting linear control dynamics. For heavy exercise, a slow component superimposed on the fundamental phase was expressed in both the exercise and recovery, with similar parameter estimates. But the on-transient values of the time constant were appreciably faster than the associated off-transient, and independent of the work rate imposed (VT). Our results do not support a dynamically linear system model of VO2 during cycling exercise in the heavy-intensity domain. [ABSTRACT FROM AUTHOR]