Oxygen uptake and leg blood flow at the onset of kicking exercise in humans

Abstract

The main hypothesis of this thesis was that changes in the adaptation of leg blood flow (LBF) can impact on oxygen uptake (VO2) at the onset of moderate intensity kicking exercise. LBF and VO2 increase rapidly at the onset of exercise in humans, however, the specific regulatory mechanisms for these processes are unknown. Three studies were conducted in which the time course of change in LBF was determined at the onset of rhythmic leg kicking exercise using Doppler ultrasound. In addition, VO2 responses at the onset of exercise were measured. In the first study (Paper I) LBF kinetics were slowed when muscle perfusion pressure was less in the supine versus upright exercise position. This decrease in LBF kinetics in the supine position was associated with a slowing of VO2 at the onset of exercise. The LBF response was observed to precede the VO2 response, however, the dynamic responses of LBF and VO2 appeared to closely parallel each other at the onset of exercise, particularly in the supine position. In a second study (Paper II), the time course of LBF and VO2 at the onset of kicking exercise was unaffected by a single day of heavy exercise training. On the day of heavy exercise training, bouts of prior high intensity exercise resulted in an increase in the leg vascular conductance during both 0W and 50 W exercise, but did not alter the dynamic response of LBF during transitions between the two work rate levels. In the third study (Paper III), different gas breathing conditions altered the arterial oxygen content, however, adaptations in LBF and O2 extraction resulted in no change in oxygen delivery or in VO2 at the onset of exercise. As part of the third study, the application of near infrared spectroscopy(NIRS) measures of Hb/ Mb O2 saturation were compared to femoral venous O2 saturation (SfvO2) at the onset of exercise in different gas breathing conditions (Paper IV). There was a marked separation between the directly measured SfvO2 and the indirect estimates of Hb/Mb )2 saturation from NIRS. Alterations in the rate of adjustment of LBF to exercising muscle at the onset of moderate intensity large muscle mass, can induce adaptations in the dynamic response of VO2, however, regulatory mechanisms within the exercising muscles assist in maintaining LBF and VO2 at the onset of moderate intensity kicking exercise.