| The aim of the present study was to test the hypothesis that compared
to upright posture, slower oxygen uptake (VO2) kinetics
resulting from exercise at the same relative metabolic load in the
supine posture will be associated with increased muscle de-oxygenation
and greater myoelectrical activity. Nine subjects completed one 12-min
heavy-intensity constant-load exercises in each of the supine and
upright postures on an electronically braked cycle ergometer at a
same gain in metabolism per unit increase in work intensity (10.8
± 1.3 vs. 11.8 ± 1.1 mlO2·min-1·W-1
in upright and supine, respectively) on separate days. Breath-by-breath
VO2 kinetics were analyzed with a double exponential model
to characterize the primary and slow component phases. Myoelectrical
activity (RMS) of the vastus lateralis (VL), rectus femoris, and biceps
femoris muscles was recorded at different epochs of the exercise.
Oxygenation of the VL muscle was recorded continuously by near-infrared
spectroscopy. In supine compared with upright cycling, the primary
time constant of VO2 kinetics was significantly increased
(32.7 ± 10.7 s vs. 23.5 ± 6.7 s, respectively) while
the absolute magnitude of VO2 slow component was decreased
(p < 0.05) but not the relative amplitude. VL de-oxygenation was
higher (p < 0.05) in supine cycling throughout the exercising period
whereas RMS values for all muscles did not change appreciably over
time. Our findings suggest that lowered oxygen supply induced by supine
heavy exercise, alters oxidative metabolism dynamics and increases
muscle de-oxygenation. However, cycling supine did not increase markedly
the rate of muscle fatigue.
KEY
WORDS: muscle perfusion, heavy cycling exercises, NIRS, VO2
slow component.
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