Kinetics of pulmonary V . hbox O 2 and femoral artery blood flow and their relationship during repeated bouts of heavy exercise

The mechanism that alters the pulmonary V . hbox O 2 response to heavy-intensity exercise following prior heavy exercise has been frequently ascribed to an improvement in pre-exercise blood flow (BF) or O sub(2) delivery. Interventions to improve O sub(2) delivery have rarely resulted in a similar enhancement of V . hbox O 2 . However, the actual limb blood flow and V . hbox O 2 dynamics in the second bout of repeated exercise remain equivocal. Seven healthy female subjects (21-32years) performed consecutive 6-min (separated by 6min of 10W exercise) bilateral knee extension (KE) exercise in a semisupine position at a work rate halfway between the lactate threshold (LT) and V . hbox O 2 peak. Femoral artery blood flow (FBF) was measured by Doppler ultrasound simultaneously with breath-by-breath V . hbox O 2 ; each protocol being repeated at least four times for precise kinetic characterization. The effective time-constant ( tau ') of the V . hbox O 2 response was reduced following prior exercise (bout 1: 61.0 plus or minus 10.5 vs. bout 2: 51.6 plus or minus 9.0s; mean plus or minus SD; P